Best way to represent a fraction in Java

Question

I m trying to work with fractions in Java    I want to implement arithmetic functions   For this  I will first require a way to normalize the functions   I know I can t add 1 6 and 1 2 until I have a common denominator  I will have to add 1 6 and 3 6   A naive approach would have me add 2 12 and 6 12 and then reduce   How can I achieve a common denominator with the least performance penalty   What algorithm is best for this     Version 8  thanks to hstoerr       Improvements include          the equals   method is now consistent with the compareTo   method      final class Fraction extends Number       private int numerator      private int denominator       public Fraction int numerator  int denominator            if denominator    0                throw new IllegalArgumentException  denominator is zero                      if denominator  lt  0                numerator    -1              denominator    -1                    this numerator   numerator          this denominator   denominator             public Fraction int numerator            this numerator   numerator          this denominator   1             public int getNumerator             return this numerator             public int getDenominator             return this denominator             public byte byteValue             return  byte  this doubleValue               public double doubleValue             return   double  numerator    double  denominator              public float floatValue             return  float  this doubleValue               public int intValue             return  int  this doubleValue               public long longValue             return  long  this doubleValue               public short shortValue             return  short  this doubleValue               public boolean equals Fraction frac            return this compareTo frac     0             public int compareTo Fraction frac            long t   this getNumerator     frac getDenominator            long f   frac getNumerator     this getDenominator            int result   0          if t gt f                result   1                    else if f gt t                result   -1                    return result              I have removed all previous versions   My thanks to    Dave Ray cletus duffymo James Milhous Oscar Reyes Jason S Francisco Canedo Outlaw Programmer Beska

User · Answer

I m trying to work with proper fractions in Java    Apache Commons Math has had a Fraction class for quite some time  Most times the answer to   Boy I wish Java had something like X in the core library   can be found under the umbrella of the Apache Commons library

User · Answer

It just so happens that I wrote a BigFraction class not too long ago  for Project Euler problems   It keeps a BigInteger numerator and denominator  so it ll never overflow   But it ll be a tad slow for a lot of operations that you know will never overflow   anyway  use it if you want it   I ve been dying to show this off somehow      Edit  Latest and greatest version of this code  including unit tests is now hosted on GitHub and also available via Maven Central  I m leaving my original code here so that this answer isn t just a link       import java math            Arbitrary-precision fractions  utilizing BigIntegers for numerator and    denominator   Fraction is always kept in lowest terms   Fraction is    immutable  and guaranteed not to have a null numerator or denominator     Denominator will always be positive  so sign is carried by numerator     and a zero-denominator is impossible       public final class BigFraction extends Number implements Comparable lt BigFraction gt      private static final long serialVersionUID   1L    because Number is Serializable   private final BigInteger numerator    private final BigInteger denominator     public final static BigFraction ZERO   new BigFraction BigInteger ZERO  BigInteger ONE  true     public final static BigFraction ONE   new BigFraction BigInteger ONE  BigInteger ONE  true               Constructs a BigFraction with given numerator and denominator   Fraction      will be reduced to lowest terms   If fraction is negative  negative sign will      be carried on numerator  regardless of how the values were passed in          public BigFraction BigInteger numerator  BigInteger denominator          if numerator    null        throw new IllegalArgumentException  Numerator is null        if denominator    null        throw new IllegalArgumentException  Denominator is null        if denominator equals BigInteger ZERO         throw new ArithmeticException  Divide by zero            only numerator should be negative      if denominator signum    lt  0              numerator   numerator negate          denominator   denominator negate                 create a reduced fraction     BigInteger gcd   numerator gcd denominator       this numerator   numerator divide gcd       this denominator   denominator divide gcd                   Constructs a BigFraction from a whole number          public BigFraction BigInteger numerator          this numerator  BigInteger ONE  true          public BigFraction long numerator  long denominator          this BigInteger valueOf numerator   BigInteger valueOf denominator           public BigFraction long numerator          this BigInteger valueOf numerator   BigInteger ONE  true                   Constructs a BigFraction from a floating-point number             Warning  round-off error in IEEE floating point numbers can result      in answers that are unexpected   For example            System out println new BigFraction 1 1        will print           2476979795053773 2251799813685248            This is because 1 1 cannot be expressed exactly in binary form   The      given fraction is exactly equal to the internal representation of      the double-precision floating-point number    Which  for 1 1  is        -1  0   2 0    1   0x199999999999aL   0x10000000000000L               NOTE  In many cases  BigFraction Double toString d   may give a result      closer to what the user expects          public BigFraction double d          if Double isInfinite d         throw new IllegalArgumentException  double val is infinite        if Double isNaN d         throw new IllegalArgumentException  double val is NaN           special case - math below won t work right for 0 0 or -0 0     if d    0              numerator   BigInteger ZERO        denominator   BigInteger ONE        return             final long bits   Double doubleToLongBits d       final int sign    int  bits  gt  gt  63   amp  0x1      final int exponent     int  bits  gt  gt  52   amp  0x7ff  - 0x3ff      final long mantissa   bits  amp  0xfffffffffffffL         number is  -1  sign   2  exponent    1 mantissa     BigInteger tmpNumerator   BigInteger valueOf sign  0   1   -1       BigInteger tmpDenominator   BigInteger ONE         use shortcut  2 x    1  lt  lt  x   if x is negative  shift the denominator     if exponent  gt   0        tmpNumerator   tmpNumerator multiply BigInteger ONE shiftLeft exponent        else       tmpDenominator   tmpDenominator multiply BigInteger ONE shiftLeft -exponent           1 mantissa    1   mantissa 2 52     2 52   mantissa  2 52     tmpDenominator   tmpDenominator multiply BigInteger valueOf 0x10000000000000L        tmpNumerator   tmpNumerator multiply BigInteger valueOf 0x10000000000000L   mantissa         BigInteger gcd   tmpNumerator gcd tmpDenominator       numerator   tmpNumerator divide gcd       denominator   tmpDenominator divide gcd                   Constructs a BigFraction from two floating-point numbers             Warning  round-off error in IEEE floating point numbers can result      in answers that are unexpected   See BigFraction double  for more      information             NOTE  In many cases  BigFraction Double toString numerator          Double toString denominator        may give a result closer to what the user expects          public BigFraction double numerator  double denominator          if denominator    0        throw new ArithmeticException  Divide by zero          BigFraction tmp   new BigFraction numerator  divide new BigFraction denominator        this numerator   tmp numerator      this denominator   tmp denominator                  Constructs a new BigFraction from the given BigDecimal object          public BigFraction BigDecimal d          this d scale    lt  0   d unscaledValue   multiply BigInteger TEN pow -d scale       d unscaledValue             d scale    lt  0   BigInteger ONE                                               BigInteger TEN pow d scale             public BigFraction BigDecimal numerator  BigDecimal denominator          if denominator equals BigDecimal ZERO         throw new ArithmeticException  Divide by zero          BigFraction tmp   new BigFraction numerator  divide new BigFraction denominator        this numerator   tmp numerator      this denominator   tmp denominator                  Constructs a BigFraction from a String   Expected format is numerator denominator       but  denominator part is optional   Either numerator or denominator may be a floating-      point decimal number  which in the same format as a parameter to the       lt code gt BigDecimal String  lt  code gt  constructor              throws NumberFormatException  if the string cannot be properly parsed          public BigFraction String s          int slashPos   s indexOf           if slashPos  lt  0              BigFraction res   new BigFraction new BigDecimal s          this numerator   res numerator        this denominator   res denominator            else             BigDecimal num   new BigDecimal s substring 0  slashPos          BigDecimal den   new BigDecimal s substring slashPos 1  s length            BigFraction res   new BigFraction num  den         this numerator   res numerator        this denominator   res denominator                        Returns this   f          public BigFraction add BigFraction f          if f    null        throw new IllegalArgumentException  Null argument           n1 d1   n2 d2    n1 d2   d1 n2   d1 d2       return new BigFraction numerator multiply f denominator  add denominator multiply f numerator                               denominator multiply f denominator                    Returns this   b          public BigFraction add BigInteger b          if b    null        throw new IllegalArgumentException  Null argument           n1 d1   n2    n1   d1 n2  d1     return new BigFraction numerator add denominator multiply b                               denominator  true                   Returns this   n          public BigFraction add long n          return add BigInteger valueOf n                    Returns this - f          public BigFraction subtract BigFraction f          if f    null        throw new IllegalArgumentException  Null argument         return new BigFraction numerator multiply f denominator  subtract denominator multiply f numerator                               denominator multiply f denominator                    Returns this - b          public BigFraction subtract BigInteger b          if b    null        throw new IllegalArgumentException  Null argument         return new BigFraction numerator subtract denominator multiply b                               denominator  true                   Returns this - n          public BigFraction subtract long n          return subtract BigInteger valueOf n                    Returns this   f          public BigFraction multiply BigFraction f          if f    null        throw new IllegalArgumentException  Null argument         return new BigFraction numerator multiply f numerator   denominator multiply f denominator                    Returns this   b          public BigFraction multiply BigInteger b          if b    null        throw new IllegalArgumentException  Null argument         return new BigFraction numerator multiply b   denominator                   Returns this   n          public BigFraction multiply long n          return multiply BigInteger valueOf n                    Returns this   f          public BigFraction divide BigFraction f          if f    null        throw new IllegalArgumentException  Null argument         if f numerator equals BigInteger ZERO         throw new ArithmeticException  Divide by zero         return new BigFraction numerator multiply f denominator   denominator multiply f numerator                    Returns this   b          public BigFraction divide BigInteger b          if b    null        throw new IllegalArgumentException  Null argument         if b equals BigInteger ZERO         throw new ArithmeticException  Divide by zero         return new BigFraction numerator  denominator multiply b                    Returns this   n          public BigFraction divide long n          return divide BigInteger valueOf n                    Returns this exponent          public BigFraction pow int exponent          if exponent    0        return BigFraction ONE      else if  exponent    1        return this      else if  exponent  lt  0        return new BigFraction denominator pow -exponent   numerator pow -exponent   true       else       return new BigFraction numerator pow exponent   denominator pow exponent   true                   Returns 1 this          public BigFraction reciprocal           if this numerator equals BigInteger ZERO         throw new ArithmeticException  Divide by zero         return new BigFraction denominator  numerator  true                   Returns the complement of this fraction  which is equal to 1 - this       Useful for probabilities statistics           public BigFraction complement           return new BigFraction denominator subtract numerator   denominator  true                   Returns -this          public BigFraction negate           return new BigFraction numerator negate    denominator  true                   Returns -1  0  or 1  representing the sign of this fraction          public int signum           return numerator signum                    Returns the absolute value of this          public BigFraction abs           return  signum    lt  0   negate     this                   Returns a string representation of this  in the form      numerator denominator          public String toString           return numerator toString           denominator toString                    Returns if this object is equal to another object          public boolean equals Object o          if   o instanceof BigFraction         return false       BigFraction f    BigFraction o      return numerator equals f numerator   amp  amp  denominator equals f denominator                   Returns a hash code for this object          public int hashCode             using the method generated by Eclipse  but streamlined a bit       return  31   numerator hashCode    31   denominator hashCode                    Returns a negative  zero  or positive number  indicating if this object      is less than  equal to  or greater than f  respectively          public int compareTo BigFraction f          if f    null        throw new IllegalArgumentException  Null argument           easy case  this and f have different signs     if signum      f signum          return signum   - f signum           next easy case  this and f have the same denominator     if denominator equals f denominator         return numerator compareTo f numerator          not an easy case  so first make the denominators equal then compare the numerators      return numerator multiply f denominator  compareTo denominator multiply f numerator                    Returns the smaller of this and f          public BigFraction min BigFraction f          if f    null        throw new IllegalArgumentException  Null argument         return  this compareTo f   lt   0   this   f                   Returns the maximum of this and f          public BigFraction max BigFraction f          if f    null        throw new IllegalArgumentException  Null argument         return  this compareTo f   gt   0   this   f                   Returns a positive BigFraction  greater than or equal to zero  and less than one          public static BigFraction random           return new BigFraction Math random            public final BigInteger getNumerator     return numerator      public final BigInteger getDenominator     return denominator         implementation of Number class   may cause overflow    public byte   byteValue       return  byte  Math max Byte MIN VALUE     Math min Byte MAX VALUE     longValue          public short  shortValue      return  short Math max Short MIN VALUE    Math min Short MAX VALUE    longValue          public int    intValue        return  int   Math max Integer MIN VALUE  Math min Integer MAX VALUE  longValue          public long   longValue       return Math round doubleValue         public float  floatValue      return  float doubleValue        public double doubleValue     return toBigDecimal 18  doubleValue                  Returns a BigDecimal representation of this fraction   If possible  the      returned value will be exactly equal to the fraction   If not  the BigDecimal      will have a scale large enough to hold the same number of significant figures      as both numerator and denominator  or the equivalent of a double-precision      number  whichever is more          public BigDecimal toBigDecimal             Implementation note   A fraction can be represented exactly in base-10 iff its       denominator is of the form 2 a   5 b  where a and b are nonnegative integers         In other words  if there are no prime factors of the denominator except for       2 and 5  or if the denominator is 1    So to determine if this denominator is       of this form  continually divide by 2 to get the number of 2 s  and then       continually divide by 5 to get the number of 5 s   Afterward  if the denominator       is 1 then there are no other prime factors         Note  number of 2 s is given by the number of trailing 0 bits in the number     int twos   denominator getLowestSetBit        BigInteger tmpDen   denominator shiftRight twos      x   2 n     x  gt  gt  n      final BigInteger FIVE   BigInteger valueOf 5       int fives   0      BigInteger   divMod   null         while tmpDen   5    0    fives    tmpDen    5        while BigInteger ZERO equals  divMod   tmpDen divideAndRemainder FIVE   1                fives          tmpDen   divMod 0              if BigInteger ONE equals tmpDen                 This fraction will terminate in base 10  so it can be represented exactly as         a BigDecimal   We would now like to make the fraction of the form         unscaled   10 scale   We know that 2 x   5 x   10 x  and our denominator is         in the form 2 twos   5 fives   So use max twos  fives  as the scale  and         multiply the numerator and deminator by the appropriate number of 2 s or 5 s         such that the denominator is of the form 2 scale   5 scale    Of course  we         only have to actually multiply the numerator  since all we need for the         BigDecimal constructor is the scale        BigInteger unscaled   numerator        int scale   Math max twos  fives          if twos  lt  fives          unscaled   unscaled shiftLeft fives - twos     x   2 n     x  lt  lt  n       else if  fives  lt  twos          unscaled   unscaled multiply FIVE pow twos - fives           return new BigDecimal unscaled  scale                else  this number will repeat infinitely in base-10   So try to figure out       a good number of significant digits   Start with the number of digits required       to represent the numerator and denominator in base-10  which is given by       bitLength   log 2  10     bitLenth is the number of digits in base-2       final double LG10   3 321928094887362    Precomputed ln 10  ln 2   a k a  log 2  10      int precision   Math max numerator bitLength    denominator bitLength         precision    int Math ceil precision   LG10          If the precision is less than 18 digits  use 18 digits so that the number       will be at least as accurate as a cast to a double   For example  with       the fraction 1 3  precision will be 1  giving a result of 0 3   This is       quite a bit different from what a user would expect      if precision  lt  18        precision   18       return toBigDecimal precision                   Returns a BigDecimal representation of this fraction  with a given precision        param precision  the number of significant figures to be used in the result          public BigDecimal toBigDecimal int precision          return new BigDecimal numerator  divide new BigDecimal denominator   new MathContext precision  RoundingMode HALF EVEN             --------------------------------------------------------------------------       PRIVATE FUNCTIONS     --------------------------------------------------------------------------             Private constructor  used when you can be certain that the fraction is already in      lowest terms   No check is done to reduce numerator denominator   A check is still      done to maintain a positive denominator              param throwaway  unused variable  only here to signal to the compiler that this                        constructor should be used          private BigFraction BigInteger numerator  BigInteger denominator  boolean throwaway          if denominator signum    lt  0              this numerator   numerator negate          this denominator   denominator negate              else             this numerator   numerator        this denominator   denominator

User · Answer

One very minor improvement could potentially be to save the double value that you're computing so that you only compute it on the first access. This won't be a big win unless you're accessing this number a lot, but it's not overly difficult to do, either.

One additional point might be the error checking you do in the denominator...you automatically change 0 to 1. Not sure if this is correct for your particular application, but in general if someone is trying to divide by 0, something is very wrong. I'd let this throw an exception (a specialized exception if you feel it's needed) rather than change the value in a seemingly arbitrary way that isn't known to the user.

In constrast with some other comments, about adding methods to add subtract, etc...since you didn't mention needing them, I'm assuming you don't. And unless you're building a library that is really going to be used in many places or by other people, go with YAGNI (you ain't going to need it, so it shouldn't be there.)

User · Answer

It s kinda pointless without arithmetic methods like add   and multiply    etc  You should definitely override equals   and hashCode    You should either add a method to normalize the fraction  or do it automatically  Think about whether you want 1 2 and 2 4 to be considered the same or not - this has implications for the equals    hashCode   and compareTo   methods

User · Answer

One very minor improvement could potentially be to save the double value that you're computing so that you only compute it on the first access. This won't be a big win unless you're accessing this number a lot, but it's not overly difficult to do, either.

One additional point might be the error checking you do in the denominator...you automatically change 0 to 1. Not sure if this is correct for your particular application, but in general if someone is trying to divide by 0, something is very wrong. I'd let this throw an exception (a specialized exception if you feel it's needed) rather than change the value in a seemingly arbitrary way that isn't known to the user.

In constrast with some other comments, about adding methods to add subtract, etc...since you didn't mention needing them, I'm assuming you don't. And unless you're building a library that is really going to be used in many places or by other people, go with YAGNI (you ain't going to need it, so it shouldn't be there.)

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Make it immutable  Make it canonical  meaning 6 4 becomes 3 2  greatest common divisor algorithm is useful for this   Call it Rational  since what you re representing is a rational number  You could use BigInteger to store arbitrarilyy-precise values   If not that then long  which has an easier implementation  Make the denominator always positive   Sign should be carried by the numerator  Extend Number  Implement Comparable lt T gt   Implement equals   and hashCode    Add factory method for a number represented by a String  Add some convenience factory methods  Add a toString    and Make it Serializable    In fact  try this on for size   It runs but may have some issues   public class BigRational extends Number implements Comparable lt BigRational gt   Serializable       public final static BigRational ZERO   new BigRational BigInteger ZERO  BigInteger ONE       private final static long serialVersionUID   1099377265582986378L       private final BigInteger numerator  denominator       private BigRational BigInteger numerator  BigInteger denominator            this numerator   numerator          this denominator   denominator             private static BigRational canonical BigInteger numerator  BigInteger denominator  boolean checkGcd            if  denominator signum      0                throw new IllegalArgumentException  denominator is zero                      if  numerator signum      0                return ZERO                    if  denominator signum    lt  0                numerator   numerator negate                denominator   denominator negate                      if  checkGcd                BigInteger gcd   numerator gcd denominator               if   gcd equals BigInteger ONE                     numerator   numerator divide gcd                   denominator   denominator divide gcd                                   return new BigRational numerator  denominator              public static BigRational getInstance BigInteger numerator  BigInteger denominator            return canonical numerator  denominator  true              public static BigRational getInstance long numerator  long denominator            return canonical new BigInteger      numerator   new BigInteger      denominator   true              public static BigRational getInstance String numerator  String denominator            return canonical new BigInteger numerator   new BigInteger denominator   true              public static BigRational valueOf String s            Pattern p   Pattern compile   -   d         d     0    e -   d                Matcher m   p matcher s           if   m matches                  throw new IllegalArgumentException  Unknown format      s                               this translates 23 123e5 to 25 123   1000   10 5   2 512 300   1  GCD          String whole   m group 1           String decimal   m group 2           String exponent   m group 3           String n   whole              23 123   gt  23123         if  decimal    null                n    decimal                    BigInteger numerator   new BigInteger n               exponent is an int because BigInteger pow   takes an int argument            it gets more difficult if exponent needs to be outside  -2 billion 2 billion          int exp   exponent    null   0   Integer valueOf exponent           int decimalPlaces   decimal    null   0   decimal length            exp -  decimalPlaces          BigInteger denominator          if  exp  lt  0                denominator   BigInteger TEN pow -exp             else               numerator   numerator multiply BigInteger TEN pow exp                denominator   BigInteger ONE                        done         return canonical numerator  denominator  true                 Comparable     public int compareTo BigRational o               note  this is a bit of cheat  relying on BigInteger compareTo   returning            -1  0 or 1   For the more general contract of compareTo    you d need to do            more checking         if  numerator signum      o numerator signum                  return numerator signum   - o numerator signum              else                  oddly BigInteger has gcd   but no lcm               BigInteger i1   numerator multiply o denominator               BigInteger i2   o numerator multiply denominator               return i1 compareTo i2      expensive                       public BigRational add BigRational o            if  o numerator signum      0                return this            else if  numerator signum      0                return o            else if  denominator equals o denominator                 return new BigRational numerator add o numerator   denominator             else               return canonical numerator multiply o denominator  add o numerator multiply denominator    denominator multiply o denominator   true                         public BigRational multiply BigRational o            if  numerator signum      0    o numerator signum      0                return ZERO            else if  numerator equals o denominator                 return canonical o numerator  denominator  true             else if  o numerator equals denominator                 return canonical numerator  o denominator  true             else if  numerator negate   equals o denominator                 return canonical o numerator negate    denominator  true             else if  o numerator negate   equals denominator                 return canonical numerator negate    o denominator  true             else               return canonical numerator multiply o numerator   denominator multiply o denominator   true                        public BigInteger getNumerator     return numerator        public BigInteger getDenominator     return denominator        public boolean isInteger     return numerator signum      0    denominator equals BigInteger ONE         public BigRational negate     return new BigRational numerator negate    denominator         public BigRational invert     return canonical denominator  numerator  false         public BigRational abs     return numerator signum    lt  0   negate     this        public BigRational pow int exp    return canonical numerator pow exp   denominator pow exp   true         public BigRational subtract BigRational o    return add o negate           public BigRational divide BigRational o    return multiply o invert           public BigRational min BigRational o    return compareTo o   lt   0   this   o        public BigRational max BigRational o    return compareTo o   gt   0   this   o         public BigDecimal toBigDecimal int scale  RoundingMode roundingMode            return isInteger     new BigDecimal numerator    new BigDecimal numerator  divide new BigDecimal denominator   scale  roundingMode                 Number     public int intValue     return isInteger     numerator intValue     numerator divide denominator  intValue          public long longValue     return isInteger     numerator longValue     numerator divide denominator  longValue          public float floatValue     return  float doubleValue          public double doubleValue     return isInteger     numerator doubleValue     numerator doubleValue     denominator doubleValue            Override     public String toString     return isInteger     String format    d   numerator    String format    d     d   numerator  denominator           Override     public boolean equals Object o            if  this    o  return true          if  o    null    getClass      o getClass    return false           BigRational that    BigRational  o           if  denominator    null    denominator equals that denominator    that denominator    null  return false          if  numerator    null    numerator equals that numerator    that numerator    null  return false           return true              Override     public int hashCode             int result   numerator    null   numerator hashCode     0          result   31   result    denominator    null   denominator hashCode     0           return result             public static void main String args              BigRational r1   BigRational valueOf  3 14e4            BigRational r2   BigRational getInstance 111  7           dump  r1   r1           dump  r2   r2           dump  r1   r2   r1 add r2            dump  r1 - r2   r1 subtract r2            dump  r1   r2   r1 multiply r2            dump  r1   r2   r1 divide r2            dump  r2   2   r2 pow 2               public static void dump String name  BigRational r            System out printf   s    s n   name  r           System out printf   s negate      s n   name  r negate             System out printf   s invert      s n   name  r invert             System out printf   s intValue       d n   name  r intValue             System out printf   s longValue       d n   name  r longValue             System out printf   s floatValue       f n   name  r floatValue             System out printf   s doubleValue       f n   name  r doubleValue             System out println              Output is   r1   31 400 r1 negate     -31 400 r1 invert     1   31 400 r1 intValue     31 400 r1 longValue     31 400 r1 floatValue     31 400 000000 r1 doubleValue     31 400 000000  r2   111   7 r2 negate     -111   7 r2 invert     7   111 r2 intValue     15 r2 longValue     15 r2 floatValue     15 857142 r2 doubleValue     15 857143  r1   r2   219 911   7 r1   r2 negate     -219 911   7 r1   r2 invert     7   219 911 r1   r2 intValue     31 415 r1   r2 longValue     31 415 r1   r2 floatValue     31 415 857422 r1   r2 doubleValue     31 415 857143  r1 - r2   219 689   7 r1 - r2 negate     -219 689   7 r1 - r2 invert     7   219 689 r1 - r2 intValue     31 384 r1 - r2 longValue     31 384 r1 - r2 floatValue     31 384 142578 r1 - r2 doubleValue     31 384 142857  r1   r2   3 485 400   7 r1   r2 negate     -3 485 400   7 r1   r2 invert     7   3 485 400 r1   r2 intValue     497 914 r1   r2 longValue     497 914 r1   r2 floatValue     497 914 281250 r1   r2 doubleValue     497 914 285714  r1   r2   219 800   111 r1   r2 negate     -219 800   111 r1   r2 invert     111   219 800 r1   r2 intValue     1 980 r1   r2 longValue     1 980 r1   r2 floatValue     1 980 180176 r1   r2 doubleValue     1 980 180180  r2   2   12 321   49 r2   2 negate     -12 321   49 r2   2 invert     49   12 321 r2   2 intValue     251 r2   2 longValue     251 r2   2 floatValue     251 448975 r2   2 doubleValue     251 448980

User · Answer

Please make it an immutable type  The value of a fraction doesn t change - a half doesn t become a third  for example  Instead of setDenominator  you could have withDenominator which returns a new fraction which has the same numerator but the specified denominator   Life is much easier with immutable types   Overriding equals and hashcode would be sensible too  so it can be used in maps and sets  Outlaw Programmer s points about arithmetic operators and string formatting are good too   As a general guide  have a look at BigInteger and BigDecimal  They re not doing the same thing  but they re similar enough to give you good ideas

User · Answer

There are several ways to improve this or any value type:

Make your class immutable, including making numerator and denominator final
Automatically convert fractions to a canonical form, e.g. 2/4 -> 1/2
Implement toString()
Implement "public static Fraction valueOf(String s)" to convert from strings to fractions. Implement similar factory methods for converting from int, double, etc.
Implement addition, multiplication, etc
Add constructor from whole numbers
Override equals/hashCode
Consider making Fraction an interface with an implementation that switches to BigInteger as necessary
Consider sub-classing Number
Consider including named constants for common values like 0 and 1
Consider making it serializable
Test for division by zero
Document your API

Basically, take a look at the API for other value classes like Double, Integer and do what they do :)

User · Answer

If you re feeling adventurous  take a look at JScience  It has a Rational class that represents fractions

User · Answer

I cleaned up cletus  answer    Added Javadoc for all methods  Added checks for method preconditions  Replaced custom parsing in valueOf String  with the BigInteger String  which is both more flexible and faster      import com google common base Splitter  import java math BigDecimal  import java math BigInteger  import java math RoundingMode  import java util List  import java util Objects  import org bitbucket cowwoc preconditions Preconditions          A rational fraction  represented by   code numerator   denominator       lt p gt     This implementation is based on  lt a    href  https   stackoverflow com a 474577 14731  gt https   stackoverflow com a 474577 14731 lt  a gt      lt p gt      author Gili Tzabari     public final class BigRational extends Number implements Comparable lt BigRational gt        private static final long serialVersionUID   0L      public static final BigRational ZERO   new BigRational BigInteger ZERO  BigInteger ONE       public static final BigRational ONE   new BigRational BigInteger ONE  BigInteger ONE                   Ensures the fraction the denominator is positive and optionally divides the numerator and        denominator by the greatest common factor          lt p gt          param numerator   a numerator         param denominator a denominator         param checkGcd    true if the numerator and denominator should be divided by the greatest                           common factor         return the canonical representation of the rational fraction             private static BigRational canonical BigInteger numerator  BigInteger denominator          boolean checkGcd                assert  numerator    null           assert  denominator    null           if  denominator signum      0              throw new IllegalArgumentException  denominator is zero            if  numerator signum      0              return ZERO          BigInteger newNumerator   numerator          BigInteger newDenominator   denominator          if  newDenominator signum    lt  0                        newNumerator   newNumerator negate                newDenominator   newDenominator negate                      if  checkGcd                        BigInteger gcd   newNumerator gcd newDenominator               if   gcd equals BigInteger ONE                                 newNumerator   newNumerator divide gcd                   newDenominator   newDenominator divide gcd                                   return new BigRational newNumerator  newDenominator                          param numerator   a numerator         param denominator a denominator         return a BigRational having value   code numerator   denominator          throws NullPointerException if numerator or denominator are null             public static BigRational valueOf BigInteger numerator  BigInteger denominator                Preconditions requireThat numerator   numerator   isNotNull            Preconditions requireThat denominator   denominator   isNotNull            return canonical numerator  denominator  true                          param numerator   a numerator         param denominator a denominator         return a BigRational having value   code numerator   denominator              public static BigRational valueOf long numerator  long denominator                BigInteger bigNumerator   BigInteger valueOf numerator           BigInteger bigDenominator   BigInteger valueOf denominator           return canonical bigNumerator  bigDenominator  true                          param value the parameter value         param name  the parameter name         return the BigInteger representation of the parameter         throws NumberFormatException if value is not a valid representation of BigInteger             private static BigInteger requireBigInteger String value  String name          throws NumberFormatException               try                       return new BigInteger value                     catch  NumberFormatException e                        throw  NumberFormatException  new NumberFormatException  Invalid     name          value                   initCause e                                    param numerator   a numerator         param denominator a denominator         return a BigRational having value   code numerator   denominator          throws NullPointerException     if numerator or denominator are null         throws IllegalArgumentException if numerator or denominator are empty         throws NumberFormatException    if numerator or denominator are not a valid representation of                                         BigDecimal             public static BigRational valueOf String numerator  String denominator          throws NullPointerException  IllegalArgumentException  NumberFormatException               Preconditions requireThat numerator   numerator   isNotNull   isNotEmpty            Preconditions requireThat denominator   denominator   isNotNull   isNotEmpty            BigInteger bigNumerator   requireBigInteger numerator   numerator            BigInteger bigDenominator   requireBigInteger denominator   denominator            return canonical bigNumerator  bigDenominator  true                          param value a string representation of a rational fraction  e g   12 34e5  or  3 4           return a BigRational representation of the String         throws NullPointerException     if value is null         throws IllegalArgumentException if value is empty         throws NumberFormatException    if numerator or denominator are not a valid representation of                                         BigDecimal             public static BigRational valueOf String value          throws NullPointerException  IllegalArgumentException  NumberFormatException               Preconditions requireThat value   value   isNotNull   isNotEmpty            List lt String gt  fractionParts   Splitter on      splitToList value           if  fractionParts size      1              return valueOfRational value           if  fractionParts size      2              return BigRational valueOf fractionParts get 0   fractionParts get 1            throw new IllegalArgumentException  Too many slashes      value                          param value a string representation of a rational fraction  e g   12 34e5           return a BigRational representation of the String         throws NullPointerException     if value is null         throws IllegalArgumentException if value is empty         throws NumberFormatException    if numerator or denominator are not a valid representation of                                         BigDecimal             private static BigRational valueOfRational String value          throws NullPointerException  IllegalArgumentException  NumberFormatException               Preconditions requireThat value   value   isNotNull   isNotEmpty            BigDecimal bigDecimal   new BigDecimal value           int scale   bigDecimal scale            BigInteger numerator   bigDecimal unscaledValue            BigInteger denominator          if  scale  gt  0              denominator   BigInteger TEN pow scale           else                       numerator   numerator multiply BigInteger TEN pow -scale                denominator   BigInteger ONE                     return canonical numerator  denominator  true              private final BigInteger numerator      private final BigInteger denominator                   param numerator   the numerator         param denominator the denominator         throws NullPointerException if numerator or denominator are null             private BigRational BigInteger numerator  BigInteger denominator                Preconditions requireThat numerator   numerator   isNotNull            Preconditions requireThat denominator   denominator   isNotNull            this numerator   numerator          this denominator   denominator                         return the numerator             public BigInteger getNumerator                 return numerator                         return the denominator             public BigInteger getDenominator                 return denominator              Override      SuppressWarnings  AccessingNonPublicFieldOfAnotherObject       public int compareTo BigRational other                Preconditions requireThat other   other   isNotNull                canonical   ensures denominator is positive         if  numerator signum      other numerator signum                return numerator signum   - other numerator signum                Set the denominator to a common multiple before comparing the numerators         BigInteger first   numerator multiply other denominator           BigInteger second   other numerator multiply denominator           return first compareTo second                          param other another rational fraction         return the result of adding this object to   code other          throws NullPointerException if other is null              SuppressWarnings  AccessingNonPublicFieldOfAnotherObject       public BigRational add BigRational other                Preconditions requireThat other   other   isNotNull            if  other numerator signum      0              return this          if  numerator signum      0              return other          if  denominator equals other denominator               return new BigRational numerator add other numerator   denominator           return canonical numerator multiply other denominator               add other numerator multiply denominator                denominator multiply other denominator   true                          param other another rational fraction         return the result of subtracting   code other  from this object         throws NullPointerException if other is null              SuppressWarnings  AccessingNonPublicFieldOfAnotherObject       public BigRational subtract BigRational other                return add other negate                            param other another rational fraction         return the result of multiplying this object by   code other          throws NullPointerException if other is null              SuppressWarnings  AccessingNonPublicFieldOfAnotherObject       public BigRational multiply BigRational other                Preconditions requireThat other   other   isNotNull            if  numerator signum      0    other numerator signum      0              return ZERO          if  numerator equals other denominator               return canonical other numerator  denominator  true           if  other numerator equals denominator               return canonical numerator  other denominator  true           if  numerator negate   equals other denominator               return canonical other numerator negate    denominator  true           if  other numerator negate   equals denominator               return canonical numerator negate    other denominator  true           return canonical numerator multiply other numerator   denominator multiply other denominator               true                          param other another rational fraction         return the result of dividing this object by   code other          throws NullPointerException if other is null             public BigRational divide BigRational other                return multiply other invert                            return true if the object is a whole number             public boolean isInteger                 return numerator signum      0    denominator equals BigInteger ONE                         Returns a BigRational whose value is  -this           lt p gt          return -this             public BigRational negate                 return new BigRational numerator negate    denominator                          return a rational fraction with the numerator and denominator swapped             public BigRational invert                 return canonical denominator  numerator  false                          return the absolute value of this   code BigRational              public BigRational abs                 if  numerator signum    lt  0              return negate            return this                         param exponent exponent to which both numerator and denominator is to be raised          return a BigRational whose value is  this lt sup gt exponent lt  sup gt                public BigRational pow int exponent                return canonical numerator pow exponent   denominator pow exponent   true                          param other another rational fraction         return the minimum of this object and the other fraction             public BigRational min BigRational other                if  compareTo other   lt   0              return this          return other                         param other another rational fraction         return the maximum of this object and the other fraction             public BigRational max BigRational other                if  compareTo other   gt   0              return this          return other                         param scale        scale of the BigDecimal quotient to be returned         param roundingMode the rounding mode to apply         return a BigDecimal representation of this object         throws NullPointerException if roundingMode is null             public BigDecimal toBigDecimal int scale  RoundingMode roundingMode                Preconditions requireThat roundingMode   roundingMode   isNotNull            if  isInteger                return new BigDecimal numerator           return new BigDecimal numerator  divide new BigDecimal denominator   scale  roundingMode               Override     public int intValue                 return  int  longValue                Override     public long longValue                 if  isInteger                return numerator longValue            return numerator divide denominator  longValue                Override     public float floatValue                 return  float  doubleValue                Override     public double doubleValue                 if  isInteger                return numerator doubleValue            return numerator doubleValue     denominator doubleValue                Override      SuppressWarnings  AccessingNonPublicFieldOfAnotherObject       public boolean equals Object o                if  this    o              return true          if    o instanceof BigRational               return false          BigRational other    BigRational  o           return numerator equals other denominator   amp  amp  Objects equals denominator  other denominator               Override     public int hashCode                 return Objects hash numerator  denominator                         Returns the String representation    code numerator   denominator                Override     public String toString                 if  isInteger                return String format    d   numerator           return String format    d     d   numerator  denominator

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One very minor improvement could potentially be to save the double value that you're computing so that you only compute it on the first access. This won't be a big win unless you're accessing this number a lot, but it's not overly difficult to do, either.

One additional point might be the error checking you do in the denominator...you automatically change 0 to 1. Not sure if this is correct for your particular application, but in general if someone is trying to divide by 0, something is very wrong. I'd let this throw an exception (a specialized exception if you feel it's needed) rather than change the value in a seemingly arbitrary way that isn't known to the user.

In constrast with some other comments, about adding methods to add subtract, etc...since you didn't mention needing them, I'm assuming you don't. And unless you're building a library that is really going to be used in many places or by other people, go with YAGNI (you ain't going to need it, so it shouldn't be there.)

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You have a compareTo function already     I would implement the Comparable interface   May not really matter for whatever you re going to do with it though

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Initial remark   Never write this   if   condition   statement    This is much better   if   condition     statement      Just create to create a good habit   By making the class immutable as suggested  you can also take advantage of the double to perform the equals and hashCode and compareTo operations   Here s my quick dirty version   public final class Fraction implements Comparable        private final int numerator      private final int denominator      private final Double internal       public static Fraction createFraction  int numerator  int denominator              return new Fraction  numerator  denominator               private Fraction int numerator  int denominator            this numerator     numerator          this denominator   denominator          this internal     double  numerator    double  denominator               public int getNumerator             return this numerator             public int getDenominator             return this denominator              private double doubleValue             return internal             public int compareTo  Object o             if   o instanceof Fraction                  return internal compareTo    Fraction o  internal                      return 1             public boolean equals  Object o               if   o instanceof Fraction                    return this internal equals    Fraction o  internal                           return false             public int hashCode              return internal hashCode                 public String toString              return String format   d  d   numerator  denominator               public static void main  String    args              System out println  Fraction createFraction  1   2               System out println  Fraction createFraction  1   2   hashCode               System out println  Fraction createFraction  1   2   compareTo  Fraction createFraction 2 4                System out println  Fraction createFraction  1   2   equals  Fraction createFraction 4 8                System out println  Fraction createFraction  3   9   equals  Fraction createFraction 1 3                          About the static factory method  it may be useful later  if you subclass the Fraction to handle more complex things  or if you decide to use a pool for the most frequently used objects    It may not be the case  I just wanted to point it out       See Effective Java first item

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For industry-grade Fraction Rational implementation  I would implement it so it can represent NaN  positive infinity  negative infinity  and optionally negative zero with operational semantics exactly the same as the IEEE 754 standard states for floating point arithmetics  it also eases the conversion to from floating point values   Plus  since comparison to zero  one  and the special values above only needs simple  but combined comparison of the numerator and denominator against 0 and 1 - i would add several isXXX and compareToXXX methods for ease of use  eg  eq0   would use numerator    0  amp  amp  denominator    0 behind the scenes instead of letting the client to compare against a zero valued instance   Some statically predefined values  ZERO  ONE  TWO  TEN  ONE TENTH  NAN  etc   are also useful  since they appear at several places as constant values  This is the best way IMHO

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I ll third or fifth or whatever the recommendation for making your fraction immutable   I d also recommend that you have it extend the Number class   I d probably look at the Double class  since you re probably going to want to implement many of the same methods     You should probably also implement Comparable and Serializable since this behavior will probably be expected   Thus  you will need to implement compareTo     You will also need to override equals   and I cannot stress strongly enough that you also override hashCode     This might be one of the few cases though where you don t want compareTo   and equals   to be consistent since fractions reducable to each other are not necessarily equal

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It just so happens that I wrote a BigFraction class not too long ago  for Project Euler problems   It keeps a BigInteger numerator and denominator  so it ll never overflow   But it ll be a tad slow for a lot of operations that you know will never overflow   anyway  use it if you want it   I ve been dying to show this off somehow      Edit  Latest and greatest version of this code  including unit tests is now hosted on GitHub and also available via Maven Central  I m leaving my original code here so that this answer isn t just a link       import java math            Arbitrary-precision fractions  utilizing BigIntegers for numerator and    denominator   Fraction is always kept in lowest terms   Fraction is    immutable  and guaranteed not to have a null numerator or denominator     Denominator will always be positive  so sign is carried by numerator     and a zero-denominator is impossible       public final class BigFraction extends Number implements Comparable lt BigFraction gt      private static final long serialVersionUID   1L    because Number is Serializable   private final BigInteger numerator    private final BigInteger denominator     public final static BigFraction ZERO   new BigFraction BigInteger ZERO  BigInteger ONE  true     public final static BigFraction ONE   new BigFraction BigInteger ONE  BigInteger ONE  true               Constructs a BigFraction with given numerator and denominator   Fraction      will be reduced to lowest terms   If fraction is negative  negative sign will      be carried on numerator  regardless of how the values were passed in          public BigFraction BigInteger numerator  BigInteger denominator          if numerator    null        throw new IllegalArgumentException  Numerator is null        if denominator    null        throw new IllegalArgumentException  Denominator is null        if denominator equals BigInteger ZERO         throw new ArithmeticException  Divide by zero            only numerator should be negative      if denominator signum    lt  0              numerator   numerator negate          denominator   denominator negate                 create a reduced fraction     BigInteger gcd   numerator gcd denominator       this numerator   numerator divide gcd       this denominator   denominator divide gcd                   Constructs a BigFraction from a whole number          public BigFraction BigInteger numerator          this numerator  BigInteger ONE  true          public BigFraction long numerator  long denominator          this BigInteger valueOf numerator   BigInteger valueOf denominator           public BigFraction long numerator          this BigInteger valueOf numerator   BigInteger ONE  true                   Constructs a BigFraction from a floating-point number             Warning  round-off error in IEEE floating point numbers can result      in answers that are unexpected   For example            System out println new BigFraction 1 1        will print           2476979795053773 2251799813685248            This is because 1 1 cannot be expressed exactly in binary form   The      given fraction is exactly equal to the internal representation of      the double-precision floating-point number    Which  for 1 1  is        -1  0   2 0    1   0x199999999999aL   0x10000000000000L               NOTE  In many cases  BigFraction Double toString d   may give a result      closer to what the user expects          public BigFraction double d          if Double isInfinite d         throw new IllegalArgumentException  double val is infinite        if Double isNaN d         throw new IllegalArgumentException  double val is NaN           special case - math below won t work right for 0 0 or -0 0     if d    0              numerator   BigInteger ZERO        denominator   BigInteger ONE        return             final long bits   Double doubleToLongBits d       final int sign    int  bits  gt  gt  63   amp  0x1      final int exponent     int  bits  gt  gt  52   amp  0x7ff  - 0x3ff      final long mantissa   bits  amp  0xfffffffffffffL         number is  -1  sign   2  exponent    1 mantissa     BigInteger tmpNumerator   BigInteger valueOf sign  0   1   -1       BigInteger tmpDenominator   BigInteger ONE         use shortcut  2 x    1  lt  lt  x   if x is negative  shift the denominator     if exponent  gt   0        tmpNumerator   tmpNumerator multiply BigInteger ONE shiftLeft exponent        else       tmpDenominator   tmpDenominator multiply BigInteger ONE shiftLeft -exponent           1 mantissa    1   mantissa 2 52     2 52   mantissa  2 52     tmpDenominator   tmpDenominator multiply BigInteger valueOf 0x10000000000000L        tmpNumerator   tmpNumerator multiply BigInteger valueOf 0x10000000000000L   mantissa         BigInteger gcd   tmpNumerator gcd tmpDenominator       numerator   tmpNumerator divide gcd       denominator   tmpDenominator divide gcd                   Constructs a BigFraction from two floating-point numbers             Warning  round-off error in IEEE floating point numbers can result      in answers that are unexpected   See BigFraction double  for more      information             NOTE  In many cases  BigFraction Double toString numerator          Double toString denominator        may give a result closer to what the user expects          public BigFraction double numerator  double denominator          if denominator    0        throw new ArithmeticException  Divide by zero          BigFraction tmp   new BigFraction numerator  divide new BigFraction denominator        this numerator   tmp numerator      this denominator   tmp denominator                  Constructs a new BigFraction from the given BigDecimal object          public BigFraction BigDecimal d          this d scale    lt  0   d unscaledValue   multiply BigInteger TEN pow -d scale       d unscaledValue             d scale    lt  0   BigInteger ONE                                               BigInteger TEN pow d scale             public BigFraction BigDecimal numerator  BigDecimal denominator          if denominator equals BigDecimal ZERO         throw new ArithmeticException  Divide by zero          BigFraction tmp   new BigFraction numerator  divide new BigFraction denominator        this numerator   tmp numerator      this denominator   tmp denominator                  Constructs a BigFraction from a String   Expected format is numerator denominator       but  denominator part is optional   Either numerator or denominator may be a floating-      point decimal number  which in the same format as a parameter to the       lt code gt BigDecimal String  lt  code gt  constructor              throws NumberFormatException  if the string cannot be properly parsed          public BigFraction String s          int slashPos   s indexOf           if slashPos  lt  0              BigFraction res   new BigFraction new BigDecimal s          this numerator   res numerator        this denominator   res denominator            else             BigDecimal num   new BigDecimal s substring 0  slashPos          BigDecimal den   new BigDecimal s substring slashPos 1  s length            BigFraction res   new BigFraction num  den         this numerator   res numerator        this denominator   res denominator                        Returns this   f          public BigFraction add BigFraction f          if f    null        throw new IllegalArgumentException  Null argument           n1 d1   n2 d2    n1 d2   d1 n2   d1 d2       return new BigFraction numerator multiply f denominator  add denominator multiply f numerator                               denominator multiply f denominator                    Returns this   b          public BigFraction add BigInteger b          if b    null        throw new IllegalArgumentException  Null argument           n1 d1   n2    n1   d1 n2  d1     return new BigFraction numerator add denominator multiply b                               denominator  true                   Returns this   n          public BigFraction add long n          return add BigInteger valueOf n                    Returns this - f          public BigFraction subtract BigFraction f          if f    null        throw new IllegalArgumentException  Null argument         return new BigFraction numerator multiply f denominator  subtract denominator multiply f numerator                               denominator multiply f denominator                    Returns this - b          public BigFraction subtract BigInteger b          if b    null        throw new IllegalArgumentException  Null argument         return new BigFraction numerator subtract denominator multiply b                               denominator  true                   Returns this - n          public BigFraction subtract long n          return subtract BigInteger valueOf n                    Returns this   f          public BigFraction multiply BigFraction f          if f    null        throw new IllegalArgumentException  Null argument         return new BigFraction numerator multiply f numerator   denominator multiply f denominator                    Returns this   b          public BigFraction multiply BigInteger b          if b    null        throw new IllegalArgumentException  Null argument         return new BigFraction numerator multiply b   denominator                   Returns this   n          public BigFraction multiply long n          return multiply BigInteger valueOf n                    Returns this   f          public BigFraction divide BigFraction f          if f    null        throw new IllegalArgumentException  Null argument         if f numerator equals BigInteger ZERO         throw new ArithmeticException  Divide by zero         return new BigFraction numerator multiply f denominator   denominator multiply f numerator                    Returns this   b          public BigFraction divide BigInteger b          if b    null        throw new IllegalArgumentException  Null argument         if b equals BigInteger ZERO         throw new ArithmeticException  Divide by zero         return new BigFraction numerator  denominator multiply b                    Returns this   n          public BigFraction divide long n          return divide BigInteger valueOf n                    Returns this exponent          public BigFraction pow int exponent          if exponent    0        return BigFraction ONE      else if  exponent    1        return this      else if  exponent  lt  0        return new BigFraction denominator pow -exponent   numerator pow -exponent   true       else       return new BigFraction numerator pow exponent   denominator pow exponent   true                   Returns 1 this          public BigFraction reciprocal           if this numerator equals BigInteger ZERO         throw new ArithmeticException  Divide by zero         return new BigFraction denominator  numerator  true                   Returns the complement of this fraction  which is equal to 1 - this       Useful for probabilities statistics           public BigFraction complement           return new BigFraction denominator subtract numerator   denominator  true                   Returns -this          public BigFraction negate           return new BigFraction numerator negate    denominator  true                   Returns -1  0  or 1  representing the sign of this fraction          public int signum           return numerator signum                    Returns the absolute value of this          public BigFraction abs           return  signum    lt  0   negate     this                   Returns a string representation of this  in the form      numerator denominator          public String toString           return numerator toString           denominator toString                    Returns if this object is equal to another object          public boolean equals Object o          if   o instanceof BigFraction         return false       BigFraction f    BigFraction o      return numerator equals f numerator   amp  amp  denominator equals f denominator                   Returns a hash code for this object          public int hashCode             using the method generated by Eclipse  but streamlined a bit       return  31   numerator hashCode    31   denominator hashCode                    Returns a negative  zero  or positive number  indicating if this object      is less than  equal to  or greater than f  respectively          public int compareTo BigFraction f          if f    null        throw new IllegalArgumentException  Null argument           easy case  this and f have different signs     if signum      f signum          return signum   - f signum           next easy case  this and f have the same denominator     if denominator equals f denominator         return numerator compareTo f numerator          not an easy case  so first make the denominators equal then compare the numerators      return numerator multiply f denominator  compareTo denominator multiply f numerator                    Returns the smaller of this and f          public BigFraction min BigFraction f          if f    null        throw new IllegalArgumentException  Null argument         return  this compareTo f   lt   0   this   f                   Returns the maximum of this and f          public BigFraction max BigFraction f          if f    null        throw new IllegalArgumentException  Null argument         return  this compareTo f   gt   0   this   f                   Returns a positive BigFraction  greater than or equal to zero  and less than one          public static BigFraction random           return new BigFraction Math random            public final BigInteger getNumerator     return numerator      public final BigInteger getDenominator     return denominator         implementation of Number class   may cause overflow    public byte   byteValue       return  byte  Math max Byte MIN VALUE     Math min Byte MAX VALUE     longValue          public short  shortValue      return  short Math max Short MIN VALUE    Math min Short MAX VALUE    longValue          public int    intValue        return  int   Math max Integer MIN VALUE  Math min Integer MAX VALUE  longValue          public long   longValue       return Math round doubleValue         public float  floatValue      return  float doubleValue        public double doubleValue     return toBigDecimal 18  doubleValue                  Returns a BigDecimal representation of this fraction   If possible  the      returned value will be exactly equal to the fraction   If not  the BigDecimal      will have a scale large enough to hold the same number of significant figures      as both numerator and denominator  or the equivalent of a double-precision      number  whichever is more          public BigDecimal toBigDecimal             Implementation note   A fraction can be represented exactly in base-10 iff its       denominator is of the form 2 a   5 b  where a and b are nonnegative integers         In other words  if there are no prime factors of the denominator except for       2 and 5  or if the denominator is 1    So to determine if this denominator is       of this form  continually divide by 2 to get the number of 2 s  and then       continually divide by 5 to get the number of 5 s   Afterward  if the denominator       is 1 then there are no other prime factors         Note  number of 2 s is given by the number of trailing 0 bits in the number     int twos   denominator getLowestSetBit        BigInteger tmpDen   denominator shiftRight twos      x   2 n     x  gt  gt  n      final BigInteger FIVE   BigInteger valueOf 5       int fives   0      BigInteger   divMod   null         while tmpDen   5    0    fives    tmpDen    5        while BigInteger ZERO equals  divMod   tmpDen divideAndRemainder FIVE   1                fives          tmpDen   divMod 0              if BigInteger ONE equals tmpDen                 This fraction will terminate in base 10  so it can be represented exactly as         a BigDecimal   We would now like to make the fraction of the form         unscaled   10 scale   We know that 2 x   5 x   10 x  and our denominator is         in the form 2 twos   5 fives   So use max twos  fives  as the scale  and         multiply the numerator and deminator by the appropriate number of 2 s or 5 s         such that the denominator is of the form 2 scale   5 scale    Of course  we         only have to actually multiply the numerator  since all we need for the         BigDecimal constructor is the scale        BigInteger unscaled   numerator        int scale   Math max twos  fives          if twos  lt  fives          unscaled   unscaled shiftLeft fives - twos     x   2 n     x  lt  lt  n       else if  fives  lt  twos          unscaled   unscaled multiply FIVE pow twos - fives           return new BigDecimal unscaled  scale                else  this number will repeat infinitely in base-10   So try to figure out       a good number of significant digits   Start with the number of digits required       to represent the numerator and denominator in base-10  which is given by       bitLength   log 2  10     bitLenth is the number of digits in base-2       final double LG10   3 321928094887362    Precomputed ln 10  ln 2   a k a  log 2  10      int precision   Math max numerator bitLength    denominator bitLength         precision    int Math ceil precision   LG10          If the precision is less than 18 digits  use 18 digits so that the number       will be at least as accurate as a cast to a double   For example  with       the fraction 1 3  precision will be 1  giving a result of 0 3   This is       quite a bit different from what a user would expect      if precision  lt  18        precision   18       return toBigDecimal precision                   Returns a BigDecimal representation of this fraction  with a given precision        param precision  the number of significant figures to be used in the result          public BigDecimal toBigDecimal int precision          return new BigDecimal numerator  divide new BigDecimal denominator   new MathContext precision  RoundingMode HALF EVEN             --------------------------------------------------------------------------       PRIVATE FUNCTIONS     --------------------------------------------------------------------------             Private constructor  used when you can be certain that the fraction is already in      lowest terms   No check is done to reduce numerator denominator   A check is still      done to maintain a positive denominator              param throwaway  unused variable  only here to signal to the compiler that this                        constructor should be used          private BigFraction BigInteger numerator  BigInteger denominator  boolean throwaway          if denominator signum    lt  0              this numerator   numerator negate          this denominator   denominator negate              else             this numerator   numerator        this denominator   denominator

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Timothy Budd has a fine implementation of a Rational class in his "Data Structures in C++". Different language, of course, but it ports over to Java very nicely.

I'd recommend more constructors. A default constructor would have numerator 0, denominator 1. A single arg constructor would assume a denominator of 1. Think how your users might use this class.

No check for zero denominator? Programming by contract would have you add it.

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Timothy Budd has a fine implementation of a Rational class in his "Data Structures in C++". Different language, of course, but it ports over to Java very nicely.

I'd recommend more constructors. A default constructor would have numerator 0, denominator 1. A single arg constructor would assume a denominator of 1. Think how your users might use this class.

No check for zero denominator? Programming by contract would have you add it.

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Initial remark   Never write this   if   condition   statement    This is much better   if   condition     statement      Just create to create a good habit   By making the class immutable as suggested  you can also take advantage of the double to perform the equals and hashCode and compareTo operations   Here s my quick dirty version   public final class Fraction implements Comparable        private final int numerator      private final int denominator      private final Double internal       public static Fraction createFraction  int numerator  int denominator              return new Fraction  numerator  denominator               private Fraction int numerator  int denominator            this numerator     numerator          this denominator   denominator          this internal     double  numerator    double  denominator               public int getNumerator             return this numerator             public int getDenominator             return this denominator              private double doubleValue             return internal             public int compareTo  Object o             if   o instanceof Fraction                  return internal compareTo    Fraction o  internal                      return 1             public boolean equals  Object o               if   o instanceof Fraction                    return this internal equals    Fraction o  internal                           return false             public int hashCode              return internal hashCode                 public String toString              return String format   d  d   numerator  denominator               public static void main  String    args              System out println  Fraction createFraction  1   2               System out println  Fraction createFraction  1   2   hashCode               System out println  Fraction createFraction  1   2   compareTo  Fraction createFraction 2 4                System out println  Fraction createFraction  1   2   equals  Fraction createFraction 4 8                System out println  Fraction createFraction  3   9   equals  Fraction createFraction 1 3                          About the static factory method  it may be useful later  if you subclass the Fraction to handle more complex things  or if you decide to use a pool for the most frequently used objects    It may not be the case  I just wanted to point it out       See Effective Java first item

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I ll third or fifth or whatever the recommendation for making your fraction immutable   I d also recommend that you have it extend the Number class   I d probably look at the Double class  since you re probably going to want to implement many of the same methods     You should probably also implement Comparable and Serializable since this behavior will probably be expected   Thus  you will need to implement compareTo     You will also need to override equals   and I cannot stress strongly enough that you also override hashCode     This might be one of the few cases though where you don t want compareTo   and equals   to be consistent since fractions reducable to each other are not necessarily equal

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One very minor improvement could potentially be to save the double value that you're computing so that you only compute it on the first access. This won't be a big win unless you're accessing this number a lot, but it's not overly difficult to do, either.

One additional point might be the error checking you do in the denominator...you automatically change 0 to 1. Not sure if this is correct for your particular application, but in general if someone is trying to divide by 0, something is very wrong. I'd let this throw an exception (a specialized exception if you feel it's needed) rather than change the value in a seemingly arbitrary way that isn't known to the user.

In constrast with some other comments, about adding methods to add subtract, etc...since you didn't mention needing them, I'm assuming you don't. And unless you're building a library that is really going to be used in many places or by other people, go with YAGNI (you ain't going to need it, so it shouldn't be there.)

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If you multiply the numerator and denominator of one Fraction with the denominator of the other and vice versa  you end up with two fractions  that are still the same values  with the same denominator and you can compare the numerators directly  Therefore you wouldn t need to calculate the double value   public int compareTo Fraction frac        int t   this numerator   frac getDenominator        int f   frac getNumerator     this denominator      if t gt f  return 1      if f gt t  return -1      return 0

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Specifically  Is there a better way to handle being passed a zero denominator  Setting the denominator to 1 is feels mighty arbitrary  How can I do this right    I would say throw a ArithmeticException for divide by zero  since that s really what s happening   public Fraction int numerator  int denominator        if denominator    0          throw new ArithmeticException  Divide by zero         this numerator   numerator      this denominator   denominator      Instead of  Divide by zero    you might want to make the message say  Divide by zero  Denominator for Fraction is zero

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If you multiply the numerator and denominator of one Fraction with the denominator of the other and vice versa  you end up with two fractions  that are still the same values  with the same denominator and you can compare the numerators directly  Therefore you wouldn t need to calculate the double value   public int compareTo Fraction frac        int t   this numerator   frac getDenominator        int f   frac getNumerator     this denominator      if t gt f  return 1      if f gt t  return -1      return 0

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This function simplify using the eucledian algorithm is quite useful when defining fractions   public Fraction simplify           int safe       int h  Math max numerator  denominator        int h2   Math min denominator  numerator         if  h    0             return new Fraction 1 1                while  h gt h2  amp  amp  h2 gt 0              h   h - h2            if  h gt h2                  safe   h                h   h2                h2   safe                            return new Fraction numerator h denominator h

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Please make it an immutable type  The value of a fraction doesn t change - a half doesn t become a third  for example  Instead of setDenominator  you could have withDenominator which returns a new fraction which has the same numerator but the specified denominator   Life is much easier with immutable types   Overriding equals and hashcode would be sensible too  so it can be used in maps and sets  Outlaw Programmer s points about arithmetic operators and string formatting are good too   As a general guide  have a look at BigInteger and BigDecimal  They re not doing the same thing  but they re similar enough to give you good ideas

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If you re feeling adventurous  take a look at JScience  It has a Rational class that represents fractions

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how I would improve that code    a constructor based on String Fraction String s    expect  number number  a copy constructor Fraction Fraction copy  override the clone method implements the equals  toString and hashcode methods implements the interface java io Serializable  Comparable a method  double getDoubleValue    a method add divide etc    I would make that class as immutable  no setters

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Please make it an immutable type  The value of a fraction doesn t change - a half doesn t become a third  for example  Instead of setDenominator  you could have withDenominator which returns a new fraction which has the same numerator but the specified denominator   Life is much easier with immutable types   Overriding equals and hashcode would be sensible too  so it can be used in maps and sets  Outlaw Programmer s points about arithmetic operators and string formatting are good too   As a general guide  have a look at BigInteger and BigDecimal  They re not doing the same thing  but they re similar enough to give you good ideas

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Well  for one  I d get rid of the setters and make Fractions immutable   You ll probably also want methods to add  subtract  etc   and maybe some way to get the representation in various String formats   EDIT  I d probably mark the fields as  final  to signal my intent but I guess it s not a big deal

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This function simplify using the eucledian algorithm is quite useful when defining fractions   public Fraction simplify           int safe       int h  Math max numerator  denominator        int h2   Math min denominator  numerator         if  h    0             return new Fraction 1 1                while  h gt h2  amp  amp  h2 gt 0              h   h - h2            if  h gt h2                  safe   h                h   h2                h2   safe                            return new Fraction numerator h denominator h

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Use Rational class from JScience library  It s the best thing for fractional arithmetic I seen in Java

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I ll third or fifth or whatever the recommendation for making your fraction immutable   I d also recommend that you have it extend the Number class   I d probably look at the Double class  since you re probably going to want to implement many of the same methods     You should probably also implement Comparable and Serializable since this behavior will probably be expected   Thus  you will need to implement compareTo     You will also need to override equals   and I cannot stress strongly enough that you also override hashCode     This might be one of the few cases though where you don t want compareTo   and equals   to be consistent since fractions reducable to each other are not necessarily equal

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Make it immutable  Make it canonical  meaning 6 4 becomes 3 2  greatest common divisor algorithm is useful for this   Call it Rational  since what you re representing is a rational number  You could use BigInteger to store arbitrarilyy-precise values   If not that then long  which has an easier implementation  Make the denominator always positive   Sign should be carried by the numerator  Extend Number  Implement Comparable lt T gt   Implement equals   and hashCode    Add factory method for a number represented by a String  Add some convenience factory methods  Add a toString    and Make it Serializable    In fact  try this on for size   It runs but may have some issues   public class BigRational extends Number implements Comparable lt BigRational gt   Serializable       public final static BigRational ZERO   new BigRational BigInteger ZERO  BigInteger ONE       private final static long serialVersionUID   1099377265582986378L       private final BigInteger numerator  denominator       private BigRational BigInteger numerator  BigInteger denominator            this numerator   numerator          this denominator   denominator             private static BigRational canonical BigInteger numerator  BigInteger denominator  boolean checkGcd            if  denominator signum      0                throw new IllegalArgumentException  denominator is zero                      if  numerator signum      0                return ZERO                    if  denominator signum    lt  0                numerator   numerator negate                denominator   denominator negate                      if  checkGcd                BigInteger gcd   numerator gcd denominator               if   gcd equals BigInteger ONE                     numerator   numerator divide gcd                   denominator   denominator divide gcd                                   return new BigRational numerator  denominator              public static BigRational getInstance BigInteger numerator  BigInteger denominator            return canonical numerator  denominator  true              public static BigRational getInstance long numerator  long denominator            return canonical new BigInteger      numerator   new BigInteger      denominator   true              public static BigRational getInstance String numerator  String denominator            return canonical new BigInteger numerator   new BigInteger denominator   true              public static BigRational valueOf String s            Pattern p   Pattern compile   -   d         d     0    e -   d                Matcher m   p matcher s           if   m matches                  throw new IllegalArgumentException  Unknown format      s                               this translates 23 123e5 to 25 123   1000   10 5   2 512 300   1  GCD          String whole   m group 1           String decimal   m group 2           String exponent   m group 3           String n   whole              23 123   gt  23123         if  decimal    null                n    decimal                    BigInteger numerator   new BigInteger n               exponent is an int because BigInteger pow   takes an int argument            it gets more difficult if exponent needs to be outside  -2 billion 2 billion          int exp   exponent    null   0   Integer valueOf exponent           int decimalPlaces   decimal    null   0   decimal length            exp -  decimalPlaces          BigInteger denominator          if  exp  lt  0                denominator   BigInteger TEN pow -exp             else               numerator   numerator multiply BigInteger TEN pow exp                denominator   BigInteger ONE                        done         return canonical numerator  denominator  true                 Comparable     public int compareTo BigRational o               note  this is a bit of cheat  relying on BigInteger compareTo   returning            -1  0 or 1   For the more general contract of compareTo    you d need to do            more checking         if  numerator signum      o numerator signum                  return numerator signum   - o numerator signum              else                  oddly BigInteger has gcd   but no lcm               BigInteger i1   numerator multiply o denominator               BigInteger i2   o numerator multiply denominator               return i1 compareTo i2      expensive                       public BigRational add BigRational o            if  o numerator signum      0                return this            else if  numerator signum      0                return o            else if  denominator equals o denominator                 return new BigRational numerator add o numerator   denominator             else               return canonical numerator multiply o denominator  add o numerator multiply denominator    denominator multiply o denominator   true                         public BigRational multiply BigRational o            if  numerator signum      0    o numerator signum      0                return ZERO            else if  numerator equals o denominator                 return canonical o numerator  denominator  true             else if  o numerator equals denominator                 return canonical numerator  o denominator  true             else if  numerator negate   equals o denominator                 return canonical o numerator negate    denominator  true             else if  o numerator negate   equals denominator                 return canonical numerator negate    o denominator  true             else               return canonical numerator multiply o numerator   denominator multiply o denominator   true                        public BigInteger getNumerator     return numerator        public BigInteger getDenominator     return denominator        public boolean isInteger     return numerator signum      0    denominator equals BigInteger ONE         public BigRational negate     return new BigRational numerator negate    denominator         public BigRational invert     return canonical denominator  numerator  false         public BigRational abs     return numerator signum    lt  0   negate     this        public BigRational pow int exp    return canonical numerator pow exp   denominator pow exp   true         public BigRational subtract BigRational o    return add o negate           public BigRational divide BigRational o    return multiply o invert           public BigRational min BigRational o    return compareTo o   lt   0   this   o        public BigRational max BigRational o    return compareTo o   gt   0   this   o         public BigDecimal toBigDecimal int scale  RoundingMode roundingMode            return isInteger     new BigDecimal numerator    new BigDecimal numerator  divide new BigDecimal denominator   scale  roundingMode                 Number     public int intValue     return isInteger     numerator intValue     numerator divide denominator  intValue          public long longValue     return isInteger     numerator longValue     numerator divide denominator  longValue          public float floatValue     return  float doubleValue          public double doubleValue     return isInteger     numerator doubleValue     numerator doubleValue     denominator doubleValue            Override     public String toString     return isInteger     String format    d   numerator    String format    d     d   numerator  denominator           Override     public boolean equals Object o            if  this    o  return true          if  o    null    getClass      o getClass    return false           BigRational that    BigRational  o           if  denominator    null    denominator equals that denominator    that denominator    null  return false          if  numerator    null    numerator equals that numerator    that numerator    null  return false           return true              Override     public int hashCode             int result   numerator    null   numerator hashCode     0          result   31   result    denominator    null   denominator hashCode     0           return result             public static void main String args              BigRational r1   BigRational valueOf  3 14e4            BigRational r2   BigRational getInstance 111  7           dump  r1   r1           dump  r2   r2           dump  r1   r2   r1 add r2            dump  r1 - r2   r1 subtract r2            dump  r1   r2   r1 multiply r2            dump  r1   r2   r1 divide r2            dump  r2   2   r2 pow 2               public static void dump String name  BigRational r            System out printf   s    s n   name  r           System out printf   s negate      s n   name  r negate             System out printf   s invert      s n   name  r invert             System out printf   s intValue       d n   name  r intValue             System out printf   s longValue       d n   name  r longValue             System out printf   s floatValue       f n   name  r floatValue             System out printf   s doubleValue       f n   name  r doubleValue             System out println              Output is   r1   31 400 r1 negate     -31 400 r1 invert     1   31 400 r1 intValue     31 400 r1 longValue     31 400 r1 floatValue     31 400 000000 r1 doubleValue     31 400 000000  r2   111   7 r2 negate     -111   7 r2 invert     7   111 r2 intValue     15 r2 longValue     15 r2 floatValue     15 857142 r2 doubleValue     15 857143  r1   r2   219 911   7 r1   r2 negate     -219 911   7 r1   r2 invert     7   219 911 r1   r2 intValue     31 415 r1   r2 longValue     31 415 r1   r2 floatValue     31 415 857422 r1   r2 doubleValue     31 415 857143  r1 - r2   219 689   7 r1 - r2 negate     -219 689   7 r1 - r2 invert     7   219 689 r1 - r2 intValue     31 384 r1 - r2 longValue     31 384 r1 - r2 floatValue     31 384 142578 r1 - r2 doubleValue     31 384 142857  r1   r2   3 485 400   7 r1   r2 negate     -3 485 400   7 r1   r2 invert     7   3 485 400 r1   r2 intValue     497 914 r1   r2 longValue     497 914 r1   r2 floatValue     497 914 281250 r1   r2 doubleValue     497 914 285714  r1   r2   219 800   111 r1   r2 negate     -219 800   111 r1   r2 invert     111   219 800 r1   r2 intValue     1 980 r1   r2 longValue     1 980 r1   r2 floatValue     1 980 180176 r1   r2 doubleValue     1 980 180180  r2   2   12 321   49 r2   2 negate     -12 321   49 r2   2 invert     49   12 321 r2   2 intValue     251 r2   2 longValue     251 r2   2 floatValue     251 448975 r2   2 doubleValue     251 448980

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Once you ve created a fraction object why would you want to allow other objects to set the numerator or the denominator   I would think these should be read only   It makes the object immutable     Also   setting the denominator to zero should throw an invalid argument exception  I don t know what it is in Java

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If you multiply the numerator and denominator of one Fraction with the denominator of the other and vice versa  you end up with two fractions  that are still the same values  with the same denominator and you can compare the numerators directly  Therefore you wouldn t need to calculate the double value   public int compareTo Fraction frac        int t   this numerator   frac getDenominator        int f   frac getNumerator     this denominator      if t gt f  return 1      if f gt t  return -1      return 0

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There are several ways to improve this or any value type:

Make your class immutable, including making numerator and denominator final
Automatically convert fractions to a canonical form, e.g. 2/4 -> 1/2
Implement toString()
Implement "public static Fraction valueOf(String s)" to convert from strings to fractions. Implement similar factory methods for converting from int, double, etc.
Implement addition, multiplication, etc
Add constructor from whole numbers
Override equals/hashCode
Consider making Fraction an interface with an implementation that switches to BigInteger as necessary
Consider sub-classing Number
Consider including named constants for common values like 0 and 1
Consider making it serializable
Test for division by zero
Document your API

Basically, take a look at the API for other value classes like Double, Integer and do what they do :)

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I will need to order them from smallest to largest, so eventually I will need to represent them as a double also

Not strictly necessary. (In fact if you want to handle equality correctly, don't rely on double to work properly.) If b*d is positive, a/b < c/d if ad < bc. If there are negative integers involved, that can be handled appropriately...

I might rewrite as:

public int compareTo(Fraction frac)
{
    // we are comparing this=a/b with frac=c/d 
    // by multiplying both sides by bd.
    // If bd is positive, then a/b < c/d <=> ad < bc.
    // If bd is negative, then a/b < c/d <=> ad > bc.
    // If bd is 0, then you've got other problems (either b=0 or d=0)
    int d = frac.getDenominator();
    long ad = (long)this.numerator * d;
    long bc = (long)this.denominator * frac.getNumerator();
    long diff = ((long)d*this.denominator > 0) ? (ad-bc) : (bc-ad);
    return (diff > 0 ? 1 : (diff < 0 ? -1 : 0));
}

The use of long here is to ensure there's not an overflow if you multiply two large ints. handle If you can guarantee that the denominator is always nonnegative (if it's negative, just negate both numerator and denominator), then you can get rid of having to check whether b*d is positive and save a few steps. I'm not sure what behavior you're looking for with zero denominator.

Not sure how performance compares to using doubles to compare. (that is, if you care about performance that much) Here's a test method I used to check. (Appears to work properly.)

public static void main(String[] args)
{
    int a = Integer.parseInt(args[0]);
    int b = Integer.parseInt(args[1]);
    int c = Integer.parseInt(args[2]);
    int d = Integer.parseInt(args[3]);
    Fraction f1 = new Fraction(a,b); 
    Fraction f2 = new Fraction(c,d);
    int rel = f1.compareTo(f2);
    String relstr = "<=>";
    System.out.println(a+"/"+b+" "+relstr.charAt(rel+1)+" "+c+"/"+d);
}

(p.s. you might consider restructuring to implement Comparable or Comparator for your class.)

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I will need to order them from smallest to largest, so eventually I will need to represent them as a double also

Not strictly necessary. (In fact if you want to handle equality correctly, don't rely on double to work properly.) If b*d is positive, a/b < c/d if ad < bc. If there are negative integers involved, that can be handled appropriately...

I might rewrite as:

public int compareTo(Fraction frac)
{
    // we are comparing this=a/b with frac=c/d 
    // by multiplying both sides by bd.
    // If bd is positive, then a/b < c/d <=> ad < bc.
    // If bd is negative, then a/b < c/d <=> ad > bc.
    // If bd is 0, then you've got other problems (either b=0 or d=0)
    int d = frac.getDenominator();
    long ad = (long)this.numerator * d;
    long bc = (long)this.denominator * frac.getNumerator();
    long diff = ((long)d*this.denominator > 0) ? (ad-bc) : (bc-ad);
    return (diff > 0 ? 1 : (diff < 0 ? -1 : 0));
}

The use of long here is to ensure there's not an overflow if you multiply two large ints. handle If you can guarantee that the denominator is always nonnegative (if it's negative, just negate both numerator and denominator), then you can get rid of having to check whether b*d is positive and save a few steps. I'm not sure what behavior you're looking for with zero denominator.

Not sure how performance compares to using doubles to compare. (that is, if you care about performance that much) Here's a test method I used to check. (Appears to work properly.)

public static void main(String[] args)
{
    int a = Integer.parseInt(args[0]);
    int b = Integer.parseInt(args[1]);
    int c = Integer.parseInt(args[2]);
    int d = Integer.parseInt(args[3]);
    Fraction f1 = new Fraction(a,b); 
    Fraction f2 = new Fraction(c,d);
    int rel = f1.compareTo(f2);
    String relstr = "<=>";
    System.out.println(a+"/"+b+" "+relstr.charAt(rel+1)+" "+c+"/"+d);
}

(p.s. you might consider restructuring to implement Comparable or Comparator for your class.)

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Class Fraction        public class Fraction           private int num                numerator          private int denom              denominator             default constructor         public Fraction                 constructor         public Fraction  int a  int b                 num   a              if   b    0                   throw new ZeroDenomException                else                 denom   b                       return string representation of ComplexNumber          Override         public String toString                 return        num           denom                              the addition operation         public Fraction add Fraction x               return new Fraction                      x num   denom   x denom   num  x denom   denom                         the multiplication operation         public Fraction multiply Fraction x                return new Fraction x num   num  x denom   denom                  The main program       static void main String   args       Scanner input   new Scanner System in       System out println  Enter numerator and denominator of first fraction        int num1  input nextInt        int denom1  input nextInt        Fraction x   new Fraction num1  denom1       System out println  Enter numerator and denominator of second fraction        int num2  input nextInt        int denom2  input nextInt        Fraction y   new Fraction num2  denom2       Fraction result   new Fraction        System out println  Enter required operation  A  Add   M  Multiply         char op   input next   charAt 0       if op     A             result   x add y           System out println x           y           result

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how I would improve that code    a constructor based on String Fraction String s    expect  number number  a copy constructor Fraction Fraction copy  override the clone method implements the equals  toString and hashcode methods implements the interface java io Serializable  Comparable a method  double getDoubleValue    a method add divide etc    I would make that class as immutable  no setters

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Initial remark   Never write this   if   condition   statement    This is much better   if   condition     statement      Just create to create a good habit   By making the class immutable as suggested  you can also take advantage of the double to perform the equals and hashCode and compareTo operations   Here s my quick dirty version   public final class Fraction implements Comparable        private final int numerator      private final int denominator      private final Double internal       public static Fraction createFraction  int numerator  int denominator              return new Fraction  numerator  denominator               private Fraction int numerator  int denominator            this numerator     numerator          this denominator   denominator          this internal     double  numerator    double  denominator               public int getNumerator             return this numerator             public int getDenominator             return this denominator              private double doubleValue             return internal             public int compareTo  Object o             if   o instanceof Fraction                  return internal compareTo    Fraction o  internal                      return 1             public boolean equals  Object o               if   o instanceof Fraction                    return this internal equals    Fraction o  internal                           return false             public int hashCode              return internal hashCode                 public String toString              return String format   d  d   numerator  denominator               public static void main  String    args              System out println  Fraction createFraction  1   2               System out println  Fraction createFraction  1   2   hashCode               System out println  Fraction createFraction  1   2   compareTo  Fraction createFraction 2 4                System out println  Fraction createFraction  1   2   equals  Fraction createFraction 4 8                System out println  Fraction createFraction  3   9   equals  Fraction createFraction 1 3                          About the static factory method  it may be useful later  if you subclass the Fraction to handle more complex things  or if you decide to use a pool for the most frequently used objects    It may not be the case  I just wanted to point it out       See Effective Java first item

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A clean up practice that I like is to only have only one return      public int compareTo Fraction frac            int result   0         double t   this doubleValue            double f   frac doubleValue            if t gt f              result   1          else if f gt t              result -1          return result

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A clean up practice that I like is to only have only one return      public int compareTo Fraction frac            int result   0         double t   this doubleValue            double f   frac doubleValue            if t gt f              result   1          else if f gt t              result -1          return result

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Timothy Budd has a fine implementation of a Rational class in his "Data Structures in C++". Different language, of course, but it ports over to Java very nicely.

I'd recommend more constructors. A default constructor would have numerator 0, denominator 1. A single arg constructor would assume a denominator of 1. Think how your users might use this class.

No check for zero denominator? Programming by contract would have you add it.

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Specifically  Is there a better way to handle being passed a zero denominator  Setting the denominator to 1 is feels mighty arbitrary  How can I do this right    I would say throw a ArithmeticException for divide by zero  since that s really what s happening   public Fraction int numerator  int denominator        if denominator    0          throw new ArithmeticException  Divide by zero         this numerator   numerator      this denominator   denominator      Instead of  Divide by zero    you might want to make the message say  Divide by zero  Denominator for Fraction is zero

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I m trying to work with proper fractions in Java    Apache Commons Math has had a Fraction class for quite some time  Most times the answer to   Boy I wish Java had something like X in the core library   can be found under the umbrella of the Apache Commons library

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You have a compareTo function already     I would implement the Comparable interface   May not really matter for whatever you re going to do with it though

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Make it immutable  Make it canonical  meaning 6 4 becomes 3 2  greatest common divisor algorithm is useful for this   Call it Rational  since what you re representing is a rational number  You could use BigInteger to store arbitrarilyy-precise values   If not that then long  which has an easier implementation  Make the denominator always positive   Sign should be carried by the numerator  Extend Number  Implement Comparable lt T gt   Implement equals   and hashCode    Add factory method for a number represented by a String  Add some convenience factory methods  Add a toString    and Make it Serializable    In fact  try this on for size   It runs but may have some issues   public class BigRational extends Number implements Comparable lt BigRational gt   Serializable       public final static BigRational ZERO   new BigRational BigInteger ZERO  BigInteger ONE       private final static long serialVersionUID   1099377265582986378L       private final BigInteger numerator  denominator       private BigRational BigInteger numerator  BigInteger denominator            this numerator   numerator          this denominator   denominator             private static BigRational canonical BigInteger numerator  BigInteger denominator  boolean checkGcd            if  denominator signum      0                throw new IllegalArgumentException  denominator is zero                      if  numerator signum      0                return ZERO                    if  denominator signum    lt  0                numerator   numerator negate                denominator   denominator negate                      if  checkGcd                BigInteger gcd   numerator gcd denominator               if   gcd equals BigInteger ONE                     numerator   numerator divide gcd                   denominator   denominator divide gcd                                   return new BigRational numerator  denominator              public static BigRational getInstance BigInteger numerator  BigInteger denominator            return canonical numerator  denominator  true              public static BigRational getInstance long numerator  long denominator            return canonical new BigInteger      numerator   new BigInteger      denominator   true              public static BigRational getInstance String numerator  String denominator            return canonical new BigInteger numerator   new BigInteger denominator   true              public static BigRational valueOf String s            Pattern p   Pattern compile   -   d         d     0    e -   d                Matcher m   p matcher s           if   m matches                  throw new IllegalArgumentException  Unknown format      s                               this translates 23 123e5 to 25 123   1000   10 5   2 512 300   1  GCD          String whole   m group 1           String decimal   m group 2           String exponent   m group 3           String n   whole              23 123   gt  23123         if  decimal    null                n    decimal                    BigInteger numerator   new BigInteger n               exponent is an int because BigInteger pow   takes an int argument            it gets more difficult if exponent needs to be outside  -2 billion 2 billion          int exp   exponent    null   0   Integer valueOf exponent           int decimalPlaces   decimal    null   0   decimal length            exp -  decimalPlaces          BigInteger denominator          if  exp  lt  0                denominator   BigInteger TEN pow -exp             else               numerator   numerator multiply BigInteger TEN pow exp                denominator   BigInteger ONE                        done         return canonical numerator  denominator  true                 Comparable     public int compareTo BigRational o               note  this is a bit of cheat  relying on BigInteger compareTo   returning            -1  0 or 1   For the more general contract of compareTo    you d need to do            more checking         if  numerator signum      o numerator signum                  return numerator signum   - o numerator signum              else                  oddly BigInteger has gcd   but no lcm               BigInteger i1   numerator multiply o denominator               BigInteger i2   o numerator multiply denominator               return i1 compareTo i2      expensive                       public BigRational add BigRational o            if  o numerator signum      0                return this            else if  numerator signum      0                return o            else if  denominator equals o denominator                 return new BigRational numerator add o numerator   denominator             else               return canonical numerator multiply o denominator  add o numerator multiply denominator    denominator multiply o denominator   true                         public BigRational multiply BigRational o            if  numerator signum      0    o numerator signum      0                return ZERO            else if  numerator equals o denominator                 return canonical o numerator  denominator  true             else if  o numerator equals denominator                 return canonical numerator  o denominator  true             else if  numerator negate   equals o denominator                 return canonical o numerator negate    denominator  true             else if  o numerator negate   equals denominator                 return canonical numerator negate    o denominator  true             else               return canonical numerator multiply o numerator   denominator multiply o denominator   true                        public BigInteger getNumerator     return numerator        public BigInteger getDenominator     return denominator        public boolean isInteger     return numerator signum      0    denominator equals BigInteger ONE         public BigRational negate     return new BigRational numerator negate    denominator         public BigRational invert     return canonical denominator  numerator  false         public BigRational abs     return numerator signum    lt  0   negate     this        public BigRational pow int exp    return canonical numerator pow exp   denominator pow exp   true         public BigRational subtract BigRational o    return add o negate           public BigRational divide BigRational o    return multiply o invert           public BigRational min BigRational o    return compareTo o   lt   0   this   o        public BigRational max BigRational o    return compareTo o   gt   0   this   o         public BigDecimal toBigDecimal int scale  RoundingMode roundingMode            return isInteger     new BigDecimal numerator    new BigDecimal numerator  divide new BigDecimal denominator   scale  roundingMode                 Number     public int intValue     return isInteger     numerator intValue     numerator divide denominator  intValue          public long longValue     return isInteger     numerator longValue     numerator divide denominator  longValue          public float floatValue     return  float doubleValue          public double doubleValue     return isInteger     numerator doubleValue     numerator doubleValue     denominator doubleValue            Override     public String toString     return isInteger     String format    d   numerator    String format    d     d   numerator  denominator           Override     public boolean equals Object o            if  this    o  return true          if  o    null    getClass      o getClass    return false           BigRational that    BigRational  o           if  denominator    null    denominator equals that denominator    that denominator    null  return false          if  numerator    null    numerator equals that numerator    that numerator    null  return false           return true              Override     public int hashCode             int result   numerator    null   numerator hashCode     0          result   31   result    denominator    null   denominator hashCode     0           return result             public static void main String args              BigRational r1   BigRational valueOf  3 14e4            BigRational r2   BigRational getInstance 111  7           dump  r1   r1           dump  r2   r2           dump  r1   r2   r1 add r2            dump  r1 - r2   r1 subtract r2            dump  r1   r2   r1 multiply r2            dump  r1   r2   r1 divide r2            dump  r2   2   r2 pow 2               public static void dump String name  BigRational r            System out printf   s    s n   name  r           System out printf   s negate      s n   name  r negate             System out printf   s invert      s n   name  r invert             System out printf   s intValue       d n   name  r intValue             System out printf   s longValue       d n   name  r longValue             System out printf   s floatValue       f n   name  r floatValue             System out printf   s doubleValue       f n   name  r doubleValue             System out println              Output is   r1   31 400 r1 negate     -31 400 r1 invert     1   31 400 r1 intValue     31 400 r1 longValue     31 400 r1 floatValue     31 400 000000 r1 doubleValue     31 400 000000  r2   111   7 r2 negate     -111   7 r2 invert     7   111 r2 intValue     15 r2 longValue     15 r2 floatValue     15 857142 r2 doubleValue     15 857143  r1   r2   219 911   7 r1   r2 negate     -219 911   7 r1   r2 invert     7   219 911 r1   r2 intValue     31 415 r1   r2 longValue     31 415 r1   r2 floatValue     31 415 857422 r1   r2 doubleValue     31 415 857143  r1 - r2   219 689   7 r1 - r2 negate     -219 689   7 r1 - r2 invert     7   219 689 r1 - r2 intValue     31 384 r1 - r2 longValue     31 384 r1 - r2 floatValue     31 384 142578 r1 - r2 doubleValue     31 384 142857  r1   r2   3 485 400   7 r1   r2 negate     -3 485 400   7 r1   r2 invert     7   3 485 400 r1   r2 intValue     497 914 r1   r2 longValue     497 914 r1   r2 floatValue     497 914 281250 r1   r2 doubleValue     497 914 285714  r1   r2   219 800   111 r1   r2 negate     -219 800   111 r1   r2 invert     111   219 800 r1   r2 intValue     1 980 r1   r2 longValue     1 980 r1   r2 floatValue     1 980 180176 r1   r2 doubleValue     1 980 180180  r2   2   12 321   49 r2   2 negate     -12 321   49 r2   2 invert     49   12 321 r2   2 intValue     251 r2   2 longValue     251 r2   2 floatValue     251 448975 r2   2 doubleValue     251 448980

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Specifically  Is there a better way to handle being passed a zero denominator  Setting the denominator to 1 is feels mighty arbitrary  How can I do this right    I would say throw a ArithmeticException for divide by zero  since that s really what s happening   public Fraction int numerator  int denominator        if denominator    0          throw new ArithmeticException  Divide by zero         this numerator   numerator      this denominator   denominator      Instead of  Divide by zero    you might want to make the message say  Divide by zero  Denominator for Fraction is zero

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If you re feeling adventurous  take a look at JScience  It has a Rational class that represents fractions

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Once you ve created a fraction object why would you want to allow other objects to set the numerator or the denominator   I would think these should be read only   It makes the object immutable     Also   setting the denominator to zero should throw an invalid argument exception  I don t know what it is in Java

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It just so happens that I wrote a BigFraction class not too long ago  for Project Euler problems   It keeps a BigInteger numerator and denominator  so it ll never overflow   But it ll be a tad slow for a lot of operations that you know will never overflow   anyway  use it if you want it   I ve been dying to show this off somehow      Edit  Latest and greatest version of this code  including unit tests is now hosted on GitHub and also available via Maven Central  I m leaving my original code here so that this answer isn t just a link       import java math            Arbitrary-precision fractions  utilizing BigIntegers for numerator and    denominator   Fraction is always kept in lowest terms   Fraction is    immutable  and guaranteed not to have a null numerator or denominator     Denominator will always be positive  so sign is carried by numerator     and a zero-denominator is impossible       public final class BigFraction extends Number implements Comparable lt BigFraction gt      private static final long serialVersionUID   1L    because Number is Serializable   private final BigInteger numerator    private final BigInteger denominator     public final static BigFraction ZERO   new BigFraction BigInteger ZERO  BigInteger ONE  true     public final static BigFraction ONE   new BigFraction BigInteger ONE  BigInteger ONE  true               Constructs a BigFraction with given numerator and denominator   Fraction      will be reduced to lowest terms   If fraction is negative  negative sign will      be carried on numerator  regardless of how the values were passed in          public BigFraction BigInteger numerator  BigInteger denominator          if numerator    null        throw new IllegalArgumentException  Numerator is null        if denominator    null        throw new IllegalArgumentException  Denominator is null        if denominator equals BigInteger ZERO         throw new ArithmeticException  Divide by zero            only numerator should be negative      if denominator signum    lt  0              numerator   numerator negate          denominator   denominator negate                 create a reduced fraction     BigInteger gcd   numerator gcd denominator       this numerator   numerator divide gcd       this denominator   denominator divide gcd                   Constructs a BigFraction from a whole number          public BigFraction BigInteger numerator          this numerator  BigInteger ONE  true          public BigFraction long numerator  long denominator          this BigInteger valueOf numerator   BigInteger valueOf denominator           public BigFraction long numerator          this BigInteger valueOf numerator   BigInteger ONE  true                   Constructs a BigFraction from a floating-point number             Warning  round-off error in IEEE floating point numbers can result      in answers that are unexpected   For example            System out println new BigFraction 1 1        will print           2476979795053773 2251799813685248            This is because 1 1 cannot be expressed exactly in binary form   The      given fraction is exactly equal to the internal representation of      the double-precision floating-point number    Which  for 1 1  is        -1  0   2 0    1   0x199999999999aL   0x10000000000000L               NOTE  In many cases  BigFraction Double toString d   may give a result      closer to what the user expects          public BigFraction double d          if Double isInfinite d         throw new IllegalArgumentException  double val is infinite        if Double isNaN d         throw new IllegalArgumentException  double val is NaN           special case - math below won t work right for 0 0 or -0 0     if d    0              numerator   BigInteger ZERO        denominator   BigInteger ONE        return             final long bits   Double doubleToLongBits d       final int sign    int  bits  gt  gt  63   amp  0x1      final int exponent     int  bits  gt  gt  52   amp  0x7ff  - 0x3ff      final long mantissa   bits  amp  0xfffffffffffffL         number is  -1  sign   2  exponent    1 mantissa     BigInteger tmpNumerator   BigInteger valueOf sign  0   1   -1       BigInteger tmpDenominator   BigInteger ONE         use shortcut  2 x    1  lt  lt  x   if x is negative  shift the denominator     if exponent  gt   0        tmpNumerator   tmpNumerator multiply BigInteger ONE shiftLeft exponent        else       tmpDenominator   tmpDenominator multiply BigInteger ONE shiftLeft -exponent           1 mantissa    1   mantissa 2 52     2 52   mantissa  2 52     tmpDenominator   tmpDenominator multiply BigInteger valueOf 0x10000000000000L        tmpNumerator   tmpNumerator multiply BigInteger valueOf 0x10000000000000L   mantissa         BigInteger gcd   tmpNumerator gcd tmpDenominator       numerator   tmpNumerator divide gcd       denominator   tmpDenominator divide gcd                   Constructs a BigFraction from two floating-point numbers             Warning  round-off error in IEEE floating point numbers can result      in answers that are unexpected   See BigFraction double  for more      information             NOTE  In many cases  BigFraction Double toString numerator          Double toString denominator        may give a result closer to what the user expects          public BigFraction double numerator  double denominator          if denominator    0        throw new ArithmeticException  Divide by zero          BigFraction tmp   new BigFraction numerator  divide new BigFraction denominator        this numerator   tmp numerator      this denominator   tmp denominator                  Constructs a new BigFraction from the given BigDecimal object          public BigFraction BigDecimal d          this d scale    lt  0   d unscaledValue   multiply BigInteger TEN pow -d scale       d unscaledValue             d scale    lt  0   BigInteger ONE                                               BigInteger TEN pow d scale             public BigFraction BigDecimal numerator  BigDecimal denominator          if denominator equals BigDecimal ZERO         throw new ArithmeticException  Divide by zero          BigFraction tmp   new BigFraction numerator  divide new BigFraction denominator        this numerator   tmp numerator      this denominator   tmp denominator                  Constructs a BigFraction from a String   Expected format is numerator denominator       but  denominator part is optional   Either numerator or denominator may be a floating-      point decimal number  which in the same format as a parameter to the       lt code gt BigDecimal String  lt  code gt  constructor              throws NumberFormatException  if the string cannot be properly parsed          public BigFraction String s          int slashPos   s indexOf           if slashPos  lt  0              BigFraction res   new BigFraction new BigDecimal s          this numerator   res numerator        this denominator   res denominator            else             BigDecimal num   new BigDecimal s substring 0  slashPos          BigDecimal den   new BigDecimal s substring slashPos 1  s length            BigFraction res   new BigFraction num  den         this numerator   res numerator        this denominator   res denominator                        Returns this   f          public BigFraction add BigFraction f          if f    null        throw new IllegalArgumentException  Null argument           n1 d1   n2 d2    n1 d2   d1 n2   d1 d2       return new BigFraction numerator multiply f denominator  add denominator multiply f numerator                               denominator multiply f denominator                    Returns this   b          public BigFraction add BigInteger b          if b    null        throw new IllegalArgumentException  Null argument           n1 d1   n2    n1   d1 n2  d1     return new BigFraction numerator add denominator multiply b                               denominator  true                   Returns this   n          public BigFraction add long n          return add BigInteger valueOf n                    Returns this - f          public BigFraction subtract BigFraction f          if f    null        throw new IllegalArgumentException  Null argument         return new BigFraction numerator multiply f denominator  subtract denominator multiply f numerator                               denominator multiply f denominator                    Returns this - b          public BigFraction subtract BigInteger b          if b    null        throw new IllegalArgumentException  Null argument         return new BigFraction numerator subtract denominator multiply b                               denominator  true                   Returns this - n          public BigFraction subtract long n          return subtract BigInteger valueOf n                    Returns this   f          public BigFraction multiply BigFraction f          if f    null        throw new IllegalArgumentException  Null argument         return new BigFraction numerator multiply f numerator   denominator multiply f denominator                    Returns this   b          public BigFraction multiply BigInteger b          if b    null        throw new IllegalArgumentException  Null argument         return new BigFraction numerator multiply b   denominator                   Returns this   n          public BigFraction multiply long n          return multiply BigInteger valueOf n                    Returns this   f          public BigFraction divide BigFraction f          if f    null        throw new IllegalArgumentException  Null argument         if f numerator equals BigInteger ZERO         throw new ArithmeticException  Divide by zero         return new BigFraction numerator multiply f denominator   denominator multiply f numerator                    Returns this   b          public BigFraction divide BigInteger b          if b    null        throw new IllegalArgumentException  Null argument         if b equals BigInteger ZERO         throw new ArithmeticException  Divide by zero         return new BigFraction numerator  denominator multiply b                    Returns this   n          public BigFraction divide long n          return divide BigInteger valueOf n                    Returns this exponent          public BigFraction pow int exponent          if exponent    0        return BigFraction ONE      else if  exponent    1        return this      else if  exponent  lt  0        return new BigFraction denominator pow -exponent   numerator pow -exponent   true       else       return new BigFraction numerator pow exponent   denominator pow exponent   true                   Returns 1 this          public BigFraction reciprocal           if this numerator equals BigInteger ZERO         throw new ArithmeticException  Divide by zero         return new BigFraction denominator  numerator  true                   Returns the complement of this fraction  which is equal to 1 - this       Useful for probabilities statistics           public BigFraction complement           return new BigFraction denominator subtract numerator   denominator  true                   Returns -this          public BigFraction negate           return new BigFraction numerator negate    denominator  true                   Returns -1  0  or 1  representing the sign of this fraction          public int signum           return numerator signum                    Returns the absolute value of this          public BigFraction abs           return  signum    lt  0   negate     this                   Returns a string representation of this  in the form      numerator denominator          public String toString           return numerator toString           denominator toString                    Returns if this object is equal to another object          public boolean equals Object o          if   o instanceof BigFraction         return false       BigFraction f    BigFraction o      return numerator equals f numerator   amp  amp  denominator equals f denominator                   Returns a hash code for this object          public int hashCode             using the method generated by Eclipse  but streamlined a bit       return  31   numerator hashCode    31   denominator hashCode                    Returns a negative  zero  or positive number  indicating if this object      is less than  equal to  or greater than f  respectively          public int compareTo BigFraction f          if f    null        throw new IllegalArgumentException  Null argument           easy case  this and f have different signs     if signum      f signum          return signum   - f signum           next easy case  this and f have the same denominator     if denominator equals f denominator         return numerator compareTo f numerator          not an easy case  so first make the denominators equal then compare the numerators      return numerator multiply f denominator  compareTo denominator multiply f numerator                    Returns the smaller of this and f          public BigFraction min BigFraction f          if f    null        throw new IllegalArgumentException  Null argument         return  this compareTo f   lt   0   this   f                   Returns the maximum of this and f          public BigFraction max BigFraction f          if f    null        throw new IllegalArgumentException  Null argument         return  this compareTo f   gt   0   this   f                   Returns a positive BigFraction  greater than or equal to zero  and less than one          public static BigFraction random           return new BigFraction Math random            public final BigInteger getNumerator     return numerator      public final BigInteger getDenominator     return denominator         implementation of Number class   may cause overflow    public byte   byteValue       return  byte  Math max Byte MIN VALUE     Math min Byte MAX VALUE     longValue          public short  shortValue      return  short Math max Short MIN VALUE    Math min Short MAX VALUE    longValue          public int    intValue        return  int   Math max Integer MIN VALUE  Math min Integer MAX VALUE  longValue          public long   longValue       return Math round doubleValue         public float  floatValue      return  float doubleValue        public double doubleValue     return toBigDecimal 18  doubleValue                  Returns a BigDecimal representation of this fraction   If possible  the      returned value will be exactly equal to the fraction   If not  the BigDecimal      will have a scale large enough to hold the same number of significant figures      as both numerator and denominator  or the equivalent of a double-precision      number  whichever is more          public BigDecimal toBigDecimal             Implementation note   A fraction can be represented exactly in base-10 iff its       denominator is of the form 2 a   5 b  where a and b are nonnegative integers         In other words  if there are no prime factors of the denominator except for       2 and 5  or if the denominator is 1    So to determine if this denominator is       of this form  continually divide by 2 to get the number of 2 s  and then       continually divide by 5 to get the number of 5 s   Afterward  if the denominator       is 1 then there are no other prime factors         Note  number of 2 s is given by the number of trailing 0 bits in the number     int twos   denominator getLowestSetBit        BigInteger tmpDen   denominator shiftRight twos      x   2 n     x  gt  gt  n      final BigInteger FIVE   BigInteger valueOf 5       int fives   0      BigInteger   divMod   null         while tmpDen   5    0    fives    tmpDen    5        while BigInteger ZERO equals  divMod   tmpDen divideAndRemainder FIVE   1                fives          tmpDen   divMod 0              if BigInteger ONE equals tmpDen                 This fraction will terminate in base 10  so it can be represented exactly as         a BigDecimal   We would now like to make the fraction of the form         unscaled   10 scale   We know that 2 x   5 x   10 x  and our denominator is         in the form 2 twos   5 fives   So use max twos  fives  as the scale  and         multiply the numerator and deminator by the appropriate number of 2 s or 5 s         such that the denominator is of the form 2 scale   5 scale    Of course  we         only have to actually multiply the numerator  since all we need for the         BigDecimal constructor is the scale        BigInteger unscaled   numerator        int scale   Math max twos  fives          if twos  lt  fives          unscaled   unscaled shiftLeft fives - twos     x   2 n     x  lt  lt  n       else if  fives  lt  twos          unscaled   unscaled multiply FIVE pow twos - fives           return new BigDecimal unscaled  scale                else  this number will repeat infinitely in base-10   So try to figure out       a good number of significant digits   Start with the number of digits required       to represent the numerator and denominator in base-10  which is given by       bitLength   log 2  10     bitLenth is the number of digits in base-2       final double LG10   3 321928094887362    Precomputed ln 10  ln 2   a k a  log 2  10      int precision   Math max numerator bitLength    denominator bitLength         precision    int Math ceil precision   LG10          If the precision is less than 18 digits  use 18 digits so that the number       will be at least as accurate as a cast to a double   For example  with       the fraction 1 3  precision will be 1  giving a result of 0 3   This is       quite a bit different from what a user would expect      if precision  lt  18        precision   18       return toBigDecimal precision                   Returns a BigDecimal representation of this fraction  with a given precision        param precision  the number of significant figures to be used in the result          public BigDecimal toBigDecimal int precision          return new BigDecimal numerator  divide new BigDecimal denominator   new MathContext precision  RoundingMode HALF EVEN             --------------------------------------------------------------------------       PRIVATE FUNCTIONS     --------------------------------------------------------------------------             Private constructor  used when you can be certain that the fraction is already in      lowest terms   No check is done to reduce numerator denominator   A check is still      done to maintain a positive denominator              param throwaway  unused variable  only here to signal to the compiler that this                        constructor should be used          private BigFraction BigInteger numerator  BigInteger denominator  boolean throwaway          if denominator signum    lt  0              this numerator   numerator negate          this denominator   denominator negate              else             this numerator   numerator        this denominator   denominator

User · Answer

There are several ways to improve this or any value type:

Make your class immutable, including making numerator and denominator final
Automatically convert fractions to a canonical form, e.g. 2/4 -> 1/2
Implement toString()
Implement "public static Fraction valueOf(String s)" to convert from strings to fractions. Implement similar factory methods for converting from int, double, etc.
Implement addition, multiplication, etc
Add constructor from whole numbers
Override equals/hashCode
Consider making Fraction an interface with an implementation that switches to BigInteger as necessary
Consider sub-classing Number
Consider including named constants for common values like 0 and 1
Consider making it serializable
Test for division by zero
Document your API

Basically, take a look at the API for other value classes like Double, Integer and do what they do :)

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You have a compareTo function already     I would implement the Comparable interface   May not really matter for whatever you re going to do with it though

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Use Rational class from JScience library  It s the best thing for fractional arithmetic I seen in Java

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You have a compareTo function already     I would implement the Comparable interface   May not really matter for whatever you re going to do with it though

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how I would improve that code    a constructor based on String Fraction String s    expect  number number  a copy constructor Fraction Fraction copy  override the clone method implements the equals  toString and hashcode methods implements the interface java io Serializable  Comparable a method  double getDoubleValue    a method add divide etc    I would make that class as immutable  no setters

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If you multiply the numerator and denominator of one Fraction with the denominator of the other and vice versa  you end up with two fractions  that are still the same values  with the same denominator and you can compare the numerators directly  Therefore you wouldn t need to calculate the double value   public int compareTo Fraction frac        int t   this numerator   frac getDenominator        int f   frac getNumerator     this denominator      if t gt f  return 1      if f gt t  return -1      return 0

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It s kinda pointless without arithmetic methods like add   and multiply    etc  You should definitely override equals   and hashCode    You should either add a method to normalize the fraction  or do it automatically  Think about whether you want 1 2 and 2 4 to be considered the same or not - this has implications for the equals    hashCode   and compareTo   methods

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It s kinda pointless without arithmetic methods like add   and multiply    etc  You should definitely override equals   and hashCode    You should either add a method to normalize the fraction  or do it automatically  Think about whether you want 1 2 and 2 4 to be considered the same or not - this has implications for the equals    hashCode   and compareTo   methods

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Might be useful to add simple things like reciprocate  get remainder and get whole

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Specifically  Is there a better way to handle being passed a zero denominator  Setting the denominator to 1 is feels mighty arbitrary  How can I do this right    I would say throw a ArithmeticException for divide by zero  since that s really what s happening   public Fraction int numerator  int denominator        if denominator    0          throw new ArithmeticException  Divide by zero         this numerator   numerator      this denominator   denominator      Instead of  Divide by zero    you might want to make the message say  Divide by zero  Denominator for Fraction is zero

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Once you ve created a fraction object why would you want to allow other objects to set the numerator or the denominator   I would think these should be read only   It makes the object immutable     Also   setting the denominator to zero should throw an invalid argument exception  I don t know what it is in Java

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I m trying to work with proper fractions in Java    Apache Commons Math has had a Fraction class for quite some time  Most times the answer to   Boy I wish Java had something like X in the core library   can be found under the umbrella of the Apache Commons library

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It s kinda pointless without arithmetic methods like add   and multiply    etc  You should definitely override equals   and hashCode    You should either add a method to normalize the fraction  or do it automatically  Think about whether you want 1 2 and 2 4 to be considered the same or not - this has implications for the equals    hashCode   and compareTo   methods

User · Answer

Please make it an immutable type  The value of a fraction doesn t change - a half doesn t become a third  for example  Instead of setDenominator  you could have withDenominator which returns a new fraction which has the same numerator but the specified denominator   Life is much easier with immutable types   Overriding equals and hashcode would be sensible too  so it can be used in maps and sets  Outlaw Programmer s points about arithmetic operators and string formatting are good too   As a general guide  have a look at BigInteger and BigDecimal  They re not doing the same thing  but they re similar enough to give you good ideas

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Well  for one  I d get rid of the setters and make Fractions immutable   You ll probably also want methods to add  subtract  etc   and maybe some way to get the representation in various String formats   EDIT  I d probably mark the fields as  final  to signal my intent but I guess it s not a big deal

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Even though you have the methods compareTo(), if you want to make use of utilities like Collections.sort(), then you should also implement Comparable.

public class Fraction extends Number implements Comparable<Fraction> {
 ...
}

Also, for pretty display I recommend overriding toString()

public String toString() {
    return this.getNumerator() + "/" + this.getDenominator();
}

And finally, I'd make the class public so that you can use it from different packages.

User · Answer

Make it immutable  Make it canonical  meaning 6 4 becomes 3 2  greatest common divisor algorithm is useful for this   Call it Rational  since what you re representing is a rational number  You could use BigInteger to store arbitrarilyy-precise values   If not that then long  which has an easier implementation  Make the denominator always positive   Sign should be carried by the numerator  Extend Number  Implement Comparable lt T gt   Implement equals   and hashCode    Add factory method for a number represented by a String  Add some convenience factory methods  Add a toString    and Make it Serializable    In fact  try this on for size   It runs but may have some issues   public class BigRational extends Number implements Comparable lt BigRational gt   Serializable       public final static BigRational ZERO   new BigRational BigInteger ZERO  BigInteger ONE       private final static long serialVersionUID   1099377265582986378L       private final BigInteger numerator  denominator       private BigRational BigInteger numerator  BigInteger denominator            this numerator   numerator          this denominator   denominator             private static BigRational canonical BigInteger numerator  BigInteger denominator  boolean checkGcd            if  denominator signum      0                throw new IllegalArgumentException  denominator is zero                      if  numerator signum      0                return ZERO                    if  denominator signum    lt  0                numerator   numerator negate                denominator   denominator negate                      if  checkGcd                BigInteger gcd   numerator gcd denominator               if   gcd equals BigInteger ONE                     numerator   numerator divide gcd                   denominator   denominator divide gcd                                   return new BigRational numerator  denominator              public static BigRational getInstance BigInteger numerator  BigInteger denominator            return canonical numerator  denominator  true              public static BigRational getInstance long numerator  long denominator            return canonical new BigInteger      numerator   new BigInteger      denominator   true              public static BigRational getInstance String numerator  String denominator            return canonical new BigInteger numerator   new BigInteger denominator   true              public static BigRational valueOf String s            Pattern p   Pattern compile   -   d         d     0    e -   d                Matcher m   p matcher s           if   m matches                  throw new IllegalArgumentException  Unknown format      s                               this translates 23 123e5 to 25 123   1000   10 5   2 512 300   1  GCD          String whole   m group 1           String decimal   m group 2           String exponent   m group 3           String n   whole              23 123   gt  23123         if  decimal    null                n    decimal                    BigInteger numerator   new BigInteger n               exponent is an int because BigInteger pow   takes an int argument            it gets more difficult if exponent needs to be outside  -2 billion 2 billion          int exp   exponent    null   0   Integer valueOf exponent           int decimalPlaces   decimal    null   0   decimal length            exp -  decimalPlaces          BigInteger denominator          if  exp  lt  0                denominator   BigInteger TEN pow -exp             else               numerator   numerator multiply BigInteger TEN pow exp                denominator   BigInteger ONE                        done         return canonical numerator  denominator  true                 Comparable     public int compareTo BigRational o               note  this is a bit of cheat  relying on BigInteger compareTo   returning            -1  0 or 1   For the more general contract of compareTo    you d need to do            more checking         if  numerator signum      o numerator signum                  return numerator signum   - o numerator signum              else                  oddly BigInteger has gcd   but no lcm               BigInteger i1   numerator multiply o denominator               BigInteger i2   o numerator multiply denominator               return i1 compareTo i2      expensive                       public BigRational add BigRational o            if  o numerator signum      0                return this            else if  numerator signum      0                return o            else if  denominator equals o denominator                 return new BigRational numerator add o numerator   denominator             else               return canonical numerator multiply o denominator  add o numerator multiply denominator    denominator multiply o denominator   true                         public BigRational multiply BigRational o            if  numerator signum      0    o numerator signum      0                return ZERO            else if  numerator equals o denominator                 return canonical o numerator  denominator  true             else if  o numerator equals denominator                 return canonical numerator  o denominator  true             else if  numerator negate   equals o denominator                 return canonical o numerator negate    denominator  true             else if  o numerator negate   equals denominator                 return canonical numerator negate    o denominator  true             else               return canonical numerator multiply o numerator   denominator multiply o denominator   true                        public BigInteger getNumerator     return numerator        public BigInteger getDenominator     return denominator        public boolean isInteger     return numerator signum      0    denominator equals BigInteger ONE         public BigRational negate     return new BigRational numerator negate    denominator         public BigRational invert     return canonical denominator  numerator  false         public BigRational abs     return numerator signum    lt  0   negate     this        public BigRational pow int exp    return canonical numerator pow exp   denominator pow exp   true         public BigRational subtract BigRational o    return add o negate           public BigRational divide BigRational o    return multiply o invert           public BigRational min BigRational o    return compareTo o   lt   0   this   o        public BigRational max BigRational o    return compareTo o   gt   0   this   o         public BigDecimal toBigDecimal int scale  RoundingMode roundingMode            return isInteger     new BigDecimal numerator    new BigDecimal numerator  divide new BigDecimal denominator   scale  roundingMode                 Number     public int intValue     return isInteger     numerator intValue     numerator divide denominator  intValue          public long longValue     return isInteger     numerator longValue     numerator divide denominator  longValue          public float floatValue     return  float doubleValue          public double doubleValue     return isInteger     numerator doubleValue     numerator doubleValue     denominator doubleValue            Override     public String toString     return isInteger     String format    d   numerator    String format    d     d   numerator  denominator           Override     public boolean equals Object o            if  this    o  return true          if  o    null    getClass      o getClass    return false           BigRational that    BigRational  o           if  denominator    null    denominator equals that denominator    that denominator    null  return false          if  numerator    null    numerator equals that numerator    that numerator    null  return false           return true              Override     public int hashCode             int result   numerator    null   numerator hashCode     0          result   31   result    denominator    null   denominator hashCode     0           return result             public static void main String args              BigRational r1   BigRational valueOf  3 14e4            BigRational r2   BigRational getInstance 111  7           dump  r1   r1           dump  r2   r2           dump  r1   r2   r1 add r2            dump  r1 - r2   r1 subtract r2            dump  r1   r2   r1 multiply r2            dump  r1   r2   r1 divide r2            dump  r2   2   r2 pow 2               public static void dump String name  BigRational r            System out printf   s    s n   name  r           System out printf   s negate      s n   name  r negate             System out printf   s invert      s n   name  r invert             System out printf   s intValue       d n   name  r intValue             System out printf   s longValue       d n   name  r longValue             System out printf   s floatValue       f n   name  r floatValue             System out printf   s doubleValue       f n   name  r doubleValue             System out println              Output is   r1   31 400 r1 negate     -31 400 r1 invert     1   31 400 r1 intValue     31 400 r1 longValue     31 400 r1 floatValue     31 400 000000 r1 doubleValue     31 400 000000  r2   111   7 r2 negate     -111   7 r2 invert     7   111 r2 intValue     15 r2 longValue     15 r2 floatValue     15 857142 r2 doubleValue     15 857143  r1   r2   219 911   7 r1   r2 negate     -219 911   7 r1   r2 invert     7   219 911 r1   r2 intValue     31 415 r1   r2 longValue     31 415 r1   r2 floatValue     31 415 857422 r1   r2 doubleValue     31 415 857143  r1 - r2   219 689   7 r1 - r2 negate     -219 689   7 r1 - r2 invert     7   219 689 r1 - r2 intValue     31 384 r1 - r2 longValue     31 384 r1 - r2 floatValue     31 384 142578 r1 - r2 doubleValue     31 384 142857  r1   r2   3 485 400   7 r1   r2 negate     -3 485 400   7 r1   r2 invert     7   3 485 400 r1   r2 intValue     497 914 r1   r2 longValue     497 914 r1   r2 floatValue     497 914 281250 r1   r2 doubleValue     497 914 285714  r1   r2   219 800   111 r1   r2 negate     -219 800   111 r1   r2 invert     111   219 800 r1   r2 intValue     1 980 r1   r2 longValue     1 980 r1   r2 floatValue     1 980 180176 r1   r2 doubleValue     1 980 180180  r2   2   12 321   49 r2   2 negate     -12 321   49 r2   2 invert     49   12 321 r2   2 intValue     251 r2   2 longValue     251 r2   2 floatValue     251 448975 r2   2 doubleValue     251 448980

User · Answer

Even though you have the methods compareTo(), if you want to make use of utilities like Collections.sort(), then you should also implement Comparable.

public class Fraction extends Number implements Comparable<Fraction> {
 ...
}

Also, for pretty display I recommend overriding toString()

public String toString() {
    return this.getNumerator() + "/" + this.getDenominator();
}

And finally, I'd make the class public so that you can use it from different packages.

User · Answer

I cleaned up cletus  answer    Added Javadoc for all methods  Added checks for method preconditions  Replaced custom parsing in valueOf String  with the BigInteger String  which is both more flexible and faster      import com google common base Splitter  import java math BigDecimal  import java math BigInteger  import java math RoundingMode  import java util List  import java util Objects  import org bitbucket cowwoc preconditions Preconditions          A rational fraction  represented by   code numerator   denominator       lt p gt     This implementation is based on  lt a    href  https   stackoverflow com a 474577 14731  gt https   stackoverflow com a 474577 14731 lt  a gt      lt p gt      author Gili Tzabari     public final class BigRational extends Number implements Comparable lt BigRational gt        private static final long serialVersionUID   0L      public static final BigRational ZERO   new BigRational BigInteger ZERO  BigInteger ONE       public static final BigRational ONE   new BigRational BigInteger ONE  BigInteger ONE                   Ensures the fraction the denominator is positive and optionally divides the numerator and        denominator by the greatest common factor          lt p gt          param numerator   a numerator         param denominator a denominator         param checkGcd    true if the numerator and denominator should be divided by the greatest                           common factor         return the canonical representation of the rational fraction             private static BigRational canonical BigInteger numerator  BigInteger denominator          boolean checkGcd                assert  numerator    null           assert  denominator    null           if  denominator signum      0              throw new IllegalArgumentException  denominator is zero            if  numerator signum      0              return ZERO          BigInteger newNumerator   numerator          BigInteger newDenominator   denominator          if  newDenominator signum    lt  0                        newNumerator   newNumerator negate                newDenominator   newDenominator negate                      if  checkGcd                        BigInteger gcd   newNumerator gcd newDenominator               if   gcd equals BigInteger ONE                                 newNumerator   newNumerator divide gcd                   newDenominator   newDenominator divide gcd                                   return new BigRational newNumerator  newDenominator                          param numerator   a numerator         param denominator a denominator         return a BigRational having value   code numerator   denominator          throws NullPointerException if numerator or denominator are null             public static BigRational valueOf BigInteger numerator  BigInteger denominator                Preconditions requireThat numerator   numerator   isNotNull            Preconditions requireThat denominator   denominator   isNotNull            return canonical numerator  denominator  true                          param numerator   a numerator         param denominator a denominator         return a BigRational having value   code numerator   denominator              public static BigRational valueOf long numerator  long denominator                BigInteger bigNumerator   BigInteger valueOf numerator           BigInteger bigDenominator   BigInteger valueOf denominator           return canonical bigNumerator  bigDenominator  true                          param value the parameter value         param name  the parameter name         return the BigInteger representation of the parameter         throws NumberFormatException if value is not a valid representation of BigInteger             private static BigInteger requireBigInteger String value  String name          throws NumberFormatException               try                       return new BigInteger value                     catch  NumberFormatException e                        throw  NumberFormatException  new NumberFormatException  Invalid     name          value                   initCause e                                    param numerator   a numerator         param denominator a denominator         return a BigRational having value   code numerator   denominator          throws NullPointerException     if numerator or denominator are null         throws IllegalArgumentException if numerator or denominator are empty         throws NumberFormatException    if numerator or denominator are not a valid representation of                                         BigDecimal             public static BigRational valueOf String numerator  String denominator          throws NullPointerException  IllegalArgumentException  NumberFormatException               Preconditions requireThat numerator   numerator   isNotNull   isNotEmpty            Preconditions requireThat denominator   denominator   isNotNull   isNotEmpty            BigInteger bigNumerator   requireBigInteger numerator   numerator            BigInteger bigDenominator   requireBigInteger denominator   denominator            return canonical bigNumerator  bigDenominator  true                          param value a string representation of a rational fraction  e g   12 34e5  or  3 4           return a BigRational representation of the String         throws NullPointerException     if value is null         throws IllegalArgumentException if value is empty         throws NumberFormatException    if numerator or denominator are not a valid representation of                                         BigDecimal             public static BigRational valueOf String value          throws NullPointerException  IllegalArgumentException  NumberFormatException               Preconditions requireThat value   value   isNotNull   isNotEmpty            List lt String gt  fractionParts   Splitter on      splitToList value           if  fractionParts size      1              return valueOfRational value           if  fractionParts size      2              return BigRational valueOf fractionParts get 0   fractionParts get 1            throw new IllegalArgumentException  Too many slashes      value                          param value a string representation of a rational fraction  e g   12 34e5           return a BigRational representation of the String         throws NullPointerException     if value is null         throws IllegalArgumentException if value is empty         throws NumberFormatException    if numerator or denominator are not a valid representation of                                         BigDecimal             private static BigRational valueOfRational String value          throws NullPointerException  IllegalArgumentException  NumberFormatException               Preconditions requireThat value   value   isNotNull   isNotEmpty            BigDecimal bigDecimal   new BigDecimal value           int scale   bigDecimal scale            BigInteger numerator   bigDecimal unscaledValue            BigInteger denominator          if  scale  gt  0              denominator   BigInteger TEN pow scale           else                       numerator   numerator multiply BigInteger TEN pow -scale                denominator   BigInteger ONE                     return canonical numerator  denominator  true              private final BigInteger numerator      private final BigInteger denominator                   param numerator   the numerator         param denominator the denominator         throws NullPointerException if numerator or denominator are null             private BigRational BigInteger numerator  BigInteger denominator                Preconditions requireThat numerator   numerator   isNotNull            Preconditions requireThat denominator   denominator   isNotNull            this numerator   numerator          this denominator   denominator                         return the numerator             public BigInteger getNumerator                 return numerator                         return the denominator             public BigInteger getDenominator                 return denominator              Override      SuppressWarnings  AccessingNonPublicFieldOfAnotherObject       public int compareTo BigRational other                Preconditions requireThat other   other   isNotNull                canonical   ensures denominator is positive         if  numerator signum      other numerator signum                return numerator signum   - other numerator signum                Set the denominator to a common multiple before comparing the numerators         BigInteger first   numerator multiply other denominator           BigInteger second   other numerator multiply denominator           return first compareTo second                          param other another rational fraction         return the result of adding this object to   code other          throws NullPointerException if other is null              SuppressWarnings  AccessingNonPublicFieldOfAnotherObject       public BigRational add BigRational other                Preconditions requireThat other   other   isNotNull            if  other numerator signum      0              return this          if  numerator signum      0              return other          if  denominator equals other denominator               return new BigRational numerator add other numerator   denominator           return canonical numerator multiply other denominator               add other numerator multiply denominator                denominator multiply other denominator   true                          param other another rational fraction         return the result of subtracting   code other  from this object         throws NullPointerException if other is null              SuppressWarnings  AccessingNonPublicFieldOfAnotherObject       public BigRational subtract BigRational other                return add other negate                            param other another rational fraction         return the result of multiplying this object by   code other          throws NullPointerException if other is null              SuppressWarnings  AccessingNonPublicFieldOfAnotherObject       public BigRational multiply BigRational other                Preconditions requireThat other   other   isNotNull            if  numerator signum      0    other numerator signum      0              return ZERO          if  numerator equals other denominator               return canonical other numerator  denominator  true           if  other numerator equals denominator               return canonical numerator  other denominator  true           if  numerator negate   equals other denominator               return canonical other numerator negate    denominator  true           if  other numerator negate   equals denominator               return canonical numerator negate    other denominator  true           return canonical numerator multiply other numerator   denominator multiply other denominator               true                          param other another rational fraction         return the result of dividing this object by   code other          throws NullPointerException if other is null             public BigRational divide BigRational other                return multiply other invert                            return true if the object is a whole number             public boolean isInteger                 return numerator signum      0    denominator equals BigInteger ONE                         Returns a BigRational whose value is  -this           lt p gt          return -this             public BigRational negate                 return new BigRational numerator negate    denominator                          return a rational fraction with the numerator and denominator swapped             public BigRational invert                 return canonical denominator  numerator  false                          return the absolute value of this   code BigRational              public BigRational abs                 if  numerator signum    lt  0              return negate            return this                         param exponent exponent to which both numerator and denominator is to be raised          return a BigRational whose value is  this lt sup gt exponent lt  sup gt                public BigRational pow int exponent                return canonical numerator pow exponent   denominator pow exponent   true                          param other another rational fraction         return the minimum of this object and the other fraction             public BigRational min BigRational other                if  compareTo other   lt   0              return this          return other                         param other another rational fraction         return the maximum of this object and the other fraction             public BigRational max BigRational other                if  compareTo other   gt   0              return this          return other                         param scale        scale of the BigDecimal quotient to be returned         param roundingMode the rounding mode to apply         return a BigDecimal representation of this object         throws NullPointerException if roundingMode is null             public BigDecimal toBigDecimal int scale  RoundingMode roundingMode                Preconditions requireThat roundingMode   roundingMode   isNotNull            if  isInteger                return new BigDecimal numerator           return new BigDecimal numerator  divide new BigDecimal denominator   scale  roundingMode               Override     public int intValue                 return  int  longValue                Override     public long longValue                 if  isInteger                return numerator longValue            return numerator divide denominator  longValue                Override     public float floatValue                 return  float  doubleValue                Override     public double doubleValue                 if  isInteger                return numerator doubleValue            return numerator doubleValue     denominator doubleValue                Override      SuppressWarnings  AccessingNonPublicFieldOfAnotherObject       public boolean equals Object o                if  this    o              return true          if    o instanceof BigRational               return false          BigRational other    BigRational  o           return numerator equals other denominator   amp  amp  Objects equals denominator  other denominator               Override     public int hashCode                 return Objects hash numerator  denominator                         Returns the String representation    code numerator   denominator                Override     public String toString                 if  isInteger                return String format    d   numerator           return String format    d     d   numerator  denominator

User · Answer

I will need to order them from smallest to largest, so eventually I will need to represent them as a double also

Not strictly necessary. (In fact if you want to handle equality correctly, don't rely on double to work properly.) If b*d is positive, a/b < c/d if ad < bc. If there are negative integers involved, that can be handled appropriately...

I might rewrite as:

public int compareTo(Fraction frac)
{
    // we are comparing this=a/b with frac=c/d 
    // by multiplying both sides by bd.
    // If bd is positive, then a/b < c/d <=> ad < bc.
    // If bd is negative, then a/b < c/d <=> ad > bc.
    // If bd is 0, then you've got other problems (either b=0 or d=0)
    int d = frac.getDenominator();
    long ad = (long)this.numerator * d;
    long bc = (long)this.denominator * frac.getNumerator();
    long diff = ((long)d*this.denominator > 0) ? (ad-bc) : (bc-ad);
    return (diff > 0 ? 1 : (diff < 0 ? -1 : 0));
}

The use of long here is to ensure there's not an overflow if you multiply two large ints. handle If you can guarantee that the denominator is always nonnegative (if it's negative, just negate both numerator and denominator), then you can get rid of having to check whether b*d is positive and save a few steps. I'm not sure what behavior you're looking for with zero denominator.

Not sure how performance compares to using doubles to compare. (that is, if you care about performance that much) Here's a test method I used to check. (Appears to work properly.)

public static void main(String[] args)
{
    int a = Integer.parseInt(args[0]);
    int b = Integer.parseInt(args[1]);
    int c = Integer.parseInt(args[2]);
    int d = Integer.parseInt(args[3]);
    Fraction f1 = new Fraction(a,b); 
    Fraction f2 = new Fraction(c,d);
    int rel = f1.compareTo(f2);
    String relstr = "<=>";
    System.out.println(a+"/"+b+" "+relstr.charAt(rel+1)+" "+c+"/"+d);
}

(p.s. you might consider restructuring to implement Comparable or Comparator for your class.)

User · Answer

Well  for one  I d get rid of the setters and make Fractions immutable   You ll probably also want methods to add  subtract  etc   and maybe some way to get the representation in various String formats   EDIT  I d probably mark the fields as  final  to signal my intent but I guess it s not a big deal

User · Answer

how I would improve that code    a constructor based on String Fraction String s    expect  number number  a copy constructor Fraction Fraction copy  override the clone method implements the equals  toString and hashcode methods implements the interface java io Serializable  Comparable a method  double getDoubleValue    a method add divide etc    I would make that class as immutable  no setters

User · Answer

It just so happens that I wrote a BigFraction class not too long ago  for Project Euler problems   It keeps a BigInteger numerator and denominator  so it ll never overflow   But it ll be a tad slow for a lot of operations that you know will never overflow   anyway  use it if you want it   I ve been dying to show this off somehow      Edit  Latest and greatest version of this code  including unit tests is now hosted on GitHub and also available via Maven Central  I m leaving my original code here so that this answer isn t just a link       import java math            Arbitrary-precision fractions  utilizing BigIntegers for numerator and    denominator   Fraction is always kept in lowest terms   Fraction is    immutable  and guaranteed not to have a null numerator or denominator     Denominator will always be positive  so sign is carried by numerator     and a zero-denominator is impossible       public final class BigFraction extends Number implements Comparable lt BigFraction gt      private static final long serialVersionUID   1L    because Number is Serializable   private final BigInteger numerator    private final BigInteger denominator     public final static BigFraction ZERO   new BigFraction BigInteger ZERO  BigInteger ONE  true     public final static BigFraction ONE   new BigFraction BigInteger ONE  BigInteger ONE  true               Constructs a BigFraction with given numerator and denominator   Fraction      will be reduced to lowest terms   If fraction is negative  negative sign will      be carried on numerator  regardless of how the values were passed in          public BigFraction BigInteger numerator  BigInteger denominator          if numerator    null        throw new IllegalArgumentException  Numerator is null        if denominator    null        throw new IllegalArgumentException  Denominator is null        if denominator equals BigInteger ZERO         throw new ArithmeticException  Divide by zero            only numerator should be negative      if denominator signum    lt  0              numerator   numerator negate          denominator   denominator negate                 create a reduced fraction     BigInteger gcd   numerator gcd denominator       this numerator   numerator divide gcd       this denominator   denominator divide gcd                   Constructs a BigFraction from a whole number          public BigFraction BigInteger numerator          this numerator  BigInteger ONE  true          public BigFraction long numerator  long denominator          this BigInteger valueOf numerator   BigInteger valueOf denominator           public BigFraction long numerator          this BigInteger valueOf numerator   BigInteger ONE  true                   Constructs a BigFraction from a floating-point number             Warning  round-off error in IEEE floating point numbers can result      in answers that are unexpected   For example            System out println new BigFraction 1 1        will print           2476979795053773 2251799813685248            This is because 1 1 cannot be expressed exactly in binary form   The      given fraction is exactly equal to the internal representation of      the double-precision floating-point number    Which  for 1 1  is        -1  0   2 0    1   0x199999999999aL   0x10000000000000L               NOTE  In many cases  BigFraction Double toString d   may give a result      closer to what the user expects          public BigFraction double d          if Double isInfinite d         throw new IllegalArgumentException  double val is infinite        if Double isNaN d         throw new IllegalArgumentException  double val is NaN           special case - math below won t work right for 0 0 or -0 0     if d    0              numerator   BigInteger ZERO        denominator   BigInteger ONE        return             final long bits   Double doubleToLongBits d       final int sign    int  bits  gt  gt  63   amp  0x1      final int exponent     int  bits  gt  gt  52   amp  0x7ff  - 0x3ff      final long mantissa   bits  amp  0xfffffffffffffL         number is  -1  sign   2  exponent    1 mantissa     BigInteger tmpNumerator   BigInteger valueOf sign  0   1   -1       BigInteger tmpDenominator   BigInteger ONE         use shortcut  2 x    1  lt  lt  x   if x is negative  shift the denominator     if exponent  gt   0        tmpNumerator   tmpNumerator multiply BigInteger ONE shiftLeft exponent        else       tmpDenominator   tmpDenominator multiply BigInteger ONE shiftLeft -exponent           1 mantissa    1   mantissa 2 52     2 52   mantissa  2 52     tmpDenominator   tmpDenominator multiply BigInteger valueOf 0x10000000000000L        tmpNumerator   tmpNumerator multiply BigInteger valueOf 0x10000000000000L   mantissa         BigInteger gcd   tmpNumerator gcd tmpDenominator       numerator   tmpNumerator divide gcd       denominator   tmpDenominator divide gcd                   Constructs a BigFraction from two floating-point numbers             Warning  round-off error in IEEE floating point numbers can result      in answers that are unexpected   See BigFraction double  for more      information             NOTE  In many cases  BigFraction Double toString numerator          Double toString denominator        may give a result closer to what the user expects          public BigFraction double numerator  double denominator          if denominator    0        throw new ArithmeticException  Divide by zero          BigFraction tmp   new BigFraction numerator  divide new BigFraction denominator        this numerator   tmp numerator      this denominator   tmp denominator                  Constructs a new BigFraction from the given BigDecimal object          public BigFraction BigDecimal d          this d scale    lt  0   d unscaledValue   multiply BigInteger TEN pow -d scale       d unscaledValue             d scale    lt  0   BigInteger ONE                                               BigInteger TEN pow d scale             public BigFraction BigDecimal numerator  BigDecimal denominator          if denominator equals BigDecimal ZERO         throw new ArithmeticException  Divide by zero          BigFraction tmp   new BigFraction numerator  divide new BigFraction denominator        this numerator   tmp numerator      this denominator   tmp denominator                  Constructs a BigFraction from a String   Expected format is numerator denominator       but  denominator part is optional   Either numerator or denominator may be a floating-      point decimal number  which in the same format as a parameter to the       lt code gt BigDecimal String  lt  code gt  constructor              throws NumberFormatException  if the string cannot be properly parsed          public BigFraction String s          int slashPos   s indexOf           if slashPos  lt  0              BigFraction res   new BigFraction new BigDecimal s          this numerator   res numerator        this denominator   res denominator            else             BigDecimal num   new BigDecimal s substring 0  slashPos          BigDecimal den   new BigDecimal s substring slashPos 1  s length            BigFraction res   new BigFraction num  den         this numerator   res numerator        this denominator   res denominator                        Returns this   f          public BigFraction add BigFraction f          if f    null        throw new IllegalArgumentException  Null argument           n1 d1   n2 d2    n1 d2   d1 n2   d1 d2       return new BigFraction numerator multiply f denominator  add denominator multiply f numerator                               denominator multiply f denominator                    Returns this   b          public BigFraction add BigInteger b          if b    null        throw new IllegalArgumentException  Null argument           n1 d1   n2    n1   d1 n2  d1     return new BigFraction numerator add denominator multiply b                               denominator  true                   Returns this   n          public BigFraction add long n          return add BigInteger valueOf n                    Returns this - f          public BigFraction subtract BigFraction f          if f    null        throw new IllegalArgumentException  Null argument         return new BigFraction numerator multiply f denominator  subtract denominator multiply f numerator                               denominator multiply f denominator                    Returns this - b          public BigFraction subtract BigInteger b          if b    null        throw new IllegalArgumentException  Null argument         return new BigFraction numerator subtract denominator multiply b                               denominator  true                   Returns this - n          public BigFraction subtract long n          return subtract BigInteger valueOf n                    Returns this   f          public BigFraction multiply BigFraction f          if f    null        throw new IllegalArgumentException  Null argument         return new BigFraction numerator multiply f numerator   denominator multiply f denominator                    Returns this   b          public BigFraction multiply BigInteger b          if b    null        throw new IllegalArgumentException  Null argument         return new BigFraction numerator multiply b   denominator                   Returns this   n          public BigFraction multiply long n          return multiply BigInteger valueOf n                    Returns this   f          public BigFraction divide BigFraction f          if f    null        throw new IllegalArgumentException  Null argument         if f numerator equals BigInteger ZERO         throw new ArithmeticException  Divide by zero         return new BigFraction numerator multiply f denominator   denominator multiply f numerator                    Returns this   b          public BigFraction divide BigInteger b          if b    null        throw new IllegalArgumentException  Null argument         if b equals BigInteger ZERO         throw new ArithmeticException  Divide by zero         return new BigFraction numerator  denominator multiply b                    Returns this   n          public BigFraction divide long n          return divide BigInteger valueOf n                    Returns this exponent          public BigFraction pow int exponent          if exponent    0        return BigFraction ONE      else if  exponent    1        return this      else if  exponent  lt  0        return new BigFraction denominator pow -exponent   numerator pow -exponent   true       else       return new BigFraction numerator pow exponent   denominator pow exponent   true                   Returns 1 this          public BigFraction reciprocal           if this numerator equals BigInteger ZERO         throw new ArithmeticException  Divide by zero         return new BigFraction denominator  numerator  true                   Returns the complement of this fraction  which is equal to 1 - this       Useful for probabilities statistics           public BigFraction complement           return new BigFraction denominator subtract numerator   denominator  true                   Returns -this          public BigFraction negate           return new BigFraction numerator negate    denominator  true                   Returns -1  0  or 1  representing the sign of this fraction          public int signum           return numerator signum                    Returns the absolute value of this          public BigFraction abs           return  signum    lt  0   negate     this                   Returns a string representation of this  in the form      numerator denominator          public String toString           return numerator toString           denominator toString                    Returns if this object is equal to another object          public boolean equals Object o          if   o instanceof BigFraction         return false       BigFraction f    BigFraction o      return numerator equals f numerator   amp  amp  denominator equals f denominator                   Returns a hash code for this object          public int hashCode             using the method generated by Eclipse  but streamlined a bit       return  31   numerator hashCode    31   denominator hashCode                    Returns a negative  zero  or positive number  indicating if this object      is less than  equal to  or greater than f  respectively          public int compareTo BigFraction f          if f    null        throw new IllegalArgumentException  Null argument           easy case  this and f have different signs     if signum      f signum          return signum   - f signum           next easy case  this and f have the same denominator     if denominator equals f denominator         return numerator compareTo f numerator          not an easy case  so first make the denominators equal then compare the numerators      return numerator multiply f denominator  compareTo denominator multiply f numerator                    Returns the smaller of this and f          public BigFraction min BigFraction f          if f    null        throw new IllegalArgumentException  Null argument         return  this compareTo f   lt   0   this   f                   Returns the maximum of this and f          public BigFraction max BigFraction f          if f    null        throw new IllegalArgumentException  Null argument         return  this compareTo f   gt   0   this   f                   Returns a positive BigFraction  greater than or equal to zero  and less than one          public static BigFraction random           return new BigFraction Math random            public final BigInteger getNumerator     return numerator      public final BigInteger getDenominator     return denominator         implementation of Number class   may cause overflow    public byte   byteValue       return  byte  Math max Byte MIN VALUE     Math min Byte MAX VALUE     longValue          public short  shortValue      return  short Math max Short MIN VALUE    Math min Short MAX VALUE    longValue          public int    intValue        return  int   Math max Integer MIN VALUE  Math min Integer MAX VALUE  longValue          public long   longValue       return Math round doubleValue         public float  floatValue      return  float doubleValue        public double doubleValue     return toBigDecimal 18  doubleValue                  Returns a BigDecimal representation of this fraction   If possible  the      returned value will be exactly equal to the fraction   If not  the BigDecimal      will have a scale large enough to hold the same number of significant figures      as both numerator and denominator  or the equivalent of a double-precision      number  whichever is more          public BigDecimal toBigDecimal             Implementation note   A fraction can be represented exactly in base-10 iff its       denominator is of the form 2 a   5 b  where a and b are nonnegative integers         In other words  if there are no prime factors of the denominator except for       2 and 5  or if the denominator is 1    So to determine if this denominator is       of this form  continually divide by 2 to get the number of 2 s  and then       continually divide by 5 to get the number of 5 s   Afterward  if the denominator       is 1 then there are no other prime factors         Note  number of 2 s is given by the number of trailing 0 bits in the number     int twos   denominator getLowestSetBit        BigInteger tmpDen   denominator shiftRight twos      x   2 n     x  gt  gt  n      final BigInteger FIVE   BigInteger valueOf 5       int fives   0      BigInteger   divMod   null         while tmpDen   5    0    fives    tmpDen    5        while BigInteger ZERO equals  divMod   tmpDen divideAndRemainder FIVE   1                fives          tmpDen   divMod 0              if BigInteger ONE equals tmpDen                 This fraction will terminate in base 10  so it can be represented exactly as         a BigDecimal   We would now like to make the fraction of the form         unscaled   10 scale   We know that 2 x   5 x   10 x  and our denominator is         in the form 2 twos   5 fives   So use max twos  fives  as the scale  and         multiply the numerator and deminator by the appropriate number of 2 s or 5 s         such that the denominator is of the form 2 scale   5 scale    Of course  we         only have to actually multiply the numerator  since all we need for the         BigDecimal constructor is the scale        BigInteger unscaled   numerator        int scale   Math max twos  fives          if twos  lt  fives          unscaled   unscaled shiftLeft fives - twos     x   2 n     x  lt  lt  n       else if  fives  lt  twos          unscaled   unscaled multiply FIVE pow twos - fives           return new BigDecimal unscaled  scale                else  this number will repeat infinitely in base-10   So try to figure out       a good number of significant digits   Start with the number of digits required       to represent the numerator and denominator in base-10  which is given by       bitLength   log 2  10     bitLenth is the number of digits in base-2       final double LG10   3 321928094887362    Precomputed ln 10  ln 2   a k a  log 2  10      int precision   Math max numerator bitLength    denominator bitLength         precision    int Math ceil precision   LG10          If the precision is less than 18 digits  use 18 digits so that the number       will be at least as accurate as a cast to a double   For example  with       the fraction 1 3  precision will be 1  giving a result of 0 3   This is       quite a bit different from what a user would expect      if precision  lt  18        precision   18       return toBigDecimal precision                   Returns a BigDecimal representation of this fraction  with a given precision        param precision  the number of significant figures to be used in the result          public BigDecimal toBigDecimal int precision          return new BigDecimal numerator  divide new BigDecimal denominator   new MathContext precision  RoundingMode HALF EVEN             --------------------------------------------------------------------------       PRIVATE FUNCTIONS     --------------------------------------------------------------------------             Private constructor  used when you can be certain that the fraction is already in      lowest terms   No check is done to reduce numerator denominator   A check is still      done to maintain a positive denominator              param throwaway  unused variable  only here to signal to the compiler that this                        constructor should be used          private BigFraction BigInteger numerator  BigInteger denominator  boolean throwaway          if denominator signum    lt  0              this numerator   numerator negate          this denominator   denominator negate              else             this numerator   numerator        this denominator   denominator

User · Answer

Once you ve created a fraction object why would you want to allow other objects to set the numerator or the denominator   I would think these should be read only   It makes the object immutable     Also   setting the denominator to zero should throw an invalid argument exception  I don t know what it is in Java

User · Answer

I ll third or fifth or whatever the recommendation for making your fraction immutable   I d also recommend that you have it extend the Number class   I d probably look at the Double class  since you re probably going to want to implement many of the same methods     You should probably also implement Comparable and Serializable since this behavior will probably be expected   Thus  you will need to implement compareTo     You will also need to override equals   and I cannot stress strongly enough that you also override hashCode     This might be one of the few cases though where you don t want compareTo   and equals   to be consistent since fractions reducable to each other are not necessarily equal

User · Answer

I will need to order them from smallest to largest, so eventually I will need to represent them as a double also

Not strictly necessary. (In fact if you want to handle equality correctly, don't rely on double to work properly.) If b*d is positive, a/b < c/d if ad < bc. If there are negative integers involved, that can be handled appropriately...

I might rewrite as:

public int compareTo(Fraction frac)
{
    // we are comparing this=a/b with frac=c/d 
    // by multiplying both sides by bd.
    // If bd is positive, then a/b < c/d <=> ad < bc.
    // If bd is negative, then a/b < c/d <=> ad > bc.
    // If bd is 0, then you've got other problems (either b=0 or d=0)
    int d = frac.getDenominator();
    long ad = (long)this.numerator * d;
    long bc = (long)this.denominator * frac.getNumerator();
    long diff = ((long)d*this.denominator > 0) ? (ad-bc) : (bc-ad);
    return (diff > 0 ? 1 : (diff < 0 ? -1 : 0));
}

The use of long here is to ensure there's not an overflow if you multiply two large ints. handle If you can guarantee that the denominator is always nonnegative (if it's negative, just negate both numerator and denominator), then you can get rid of having to check whether b*d is positive and save a few steps. I'm not sure what behavior you're looking for with zero denominator.

Not sure how performance compares to using doubles to compare. (that is, if you care about performance that much) Here's a test method I used to check. (Appears to work properly.)

public static void main(String[] args)
{
    int a = Integer.parseInt(args[0]);
    int b = Integer.parseInt(args[1]);
    int c = Integer.parseInt(args[2]);
    int d = Integer.parseInt(args[3]);
    Fraction f1 = new Fraction(a,b); 
    Fraction f2 = new Fraction(c,d);
    int rel = f1.compareTo(f2);
    String relstr = "<=>";
    System.out.println(a+"/"+b+" "+relstr.charAt(rel+1)+" "+c+"/"+d);
}

(p.s. you might consider restructuring to implement Comparable or Comparator for your class.)

User · Answer

Well  for one  I d get rid of the setters and make Fractions immutable   You ll probably also want methods to add  subtract  etc   and maybe some way to get the representation in various String formats   EDIT  I d probably mark the fields as  final  to signal my intent but I guess it s not a big deal

User · Answer

For industry-grade Fraction Rational implementation  I would implement it so it can represent NaN  positive infinity  negative infinity  and optionally negative zero with operational semantics exactly the same as the IEEE 754 standard states for floating point arithmetics  it also eases the conversion to from floating point values   Plus  since comparison to zero  one  and the special values above only needs simple  but combined comparison of the numerator and denominator against 0 and 1 - i would add several isXXX and compareToXXX methods for ease of use  eg  eq0   would use numerator    0  amp  amp  denominator    0 behind the scenes instead of letting the client to compare against a zero valued instance   Some statically predefined values  ZERO  ONE  TWO  TEN  ONE TENTH  NAN  etc   are also useful  since they appear at several places as constant values  This is the best way IMHO

User · Answer

A clean up practice that I like is to only have only one return      public int compareTo Fraction frac            int result   0         double t   this doubleValue            double f   frac doubleValue            if t gt f              result   1          else if f gt t              result -1          return result

User · Answer

If you re feeling adventurous  take a look at JScience  It has a Rational class that represents fractions

User · Answer

I m trying to work with proper fractions in Java    Apache Commons Math has had a Fraction class for quite some time  Most times the answer to   Boy I wish Java had something like X in the core library   can be found under the umbrella of the Apache Commons library

User · Answer

A clean up practice that I like is to only have only one return      public int compareTo Fraction frac            int result   0         double t   this doubleValue            double f   frac doubleValue            if t gt f              result   1          else if f gt t              result -1          return result

User · Answer

Initial remark   Never write this   if   condition   statement    This is much better   if   condition     statement      Just create to create a good habit   By making the class immutable as suggested  you can also take advantage of the double to perform the equals and hashCode and compareTo operations   Here s my quick dirty version   public final class Fraction implements Comparable        private final int numerator      private final int denominator      private final Double internal       public static Fraction createFraction  int numerator  int denominator              return new Fraction  numerator  denominator               private Fraction int numerator  int denominator            this numerator     numerator          this denominator   denominator          this internal     double  numerator    double  denominator               public int getNumerator             return this numerator             public int getDenominator             return this denominator              private double doubleValue             return internal             public int compareTo  Object o             if   o instanceof Fraction                  return internal compareTo    Fraction o  internal                      return 1             public boolean equals  Object o               if   o instanceof Fraction                    return this internal equals    Fraction o  internal                           return false             public int hashCode              return internal hashCode                 public String toString              return String format   d  d   numerator  denominator               public static void main  String    args              System out println  Fraction createFraction  1   2               System out println  Fraction createFraction  1   2   hashCode               System out println  Fraction createFraction  1   2   compareTo  Fraction createFraction 2 4                System out println  Fraction createFraction  1   2   equals  Fraction createFraction 4 8                System out println  Fraction createFraction  3   9   equals  Fraction createFraction 1 3                          About the static factory method  it may be useful later  if you subclass the Fraction to handle more complex things  or if you decide to use a pool for the most frequently used objects    It may not be the case  I just wanted to point it out       See Effective Java first item

User · Answer

Timothy Budd has a fine implementation of a Rational class in his "Data Structures in C++". Different language, of course, but it ports over to Java very nicely.

I'd recommend more constructors. A default constructor would have numerator 0, denominator 1. A single arg constructor would assume a denominator of 1. Think how your users might use this class.

No check for zero denominator? Programming by contract would have you add it.

User · Answer

Class Fraction        public class Fraction           private int num                numerator          private int denom              denominator             default constructor         public Fraction                 constructor         public Fraction  int a  int b                 num   a              if   b    0                   throw new ZeroDenomException                else                 denom   b                       return string representation of ComplexNumber          Override         public String toString                 return        num           denom                              the addition operation         public Fraction add Fraction x               return new Fraction                      x num   denom   x denom   num  x denom   denom                         the multiplication operation         public Fraction multiply Fraction x                return new Fraction x num   num  x denom   denom                  The main program       static void main String   args       Scanner input   new Scanner System in       System out println  Enter numerator and denominator of first fraction        int num1  input nextInt        int denom1  input nextInt        Fraction x   new Fraction num1  denom1       System out println  Enter numerator and denominator of second fraction        int num2  input nextInt        int denom2  input nextInt        Fraction y   new Fraction num2  denom2       Fraction result   new Fraction        System out println  Enter required operation  A  Add   M  Multiply         char op   input next   charAt 0       if op     A             result   x add y           System out println x           y           result

User · Answer

There are several ways to improve this or any value type:

Make your class immutable, including making numerator and denominator final
Automatically convert fractions to a canonical form, e.g. 2/4 -> 1/2
Implement toString()
Implement "public static Fraction valueOf(String s)" to convert from strings to fractions. Implement similar factory methods for converting from int, double, etc.
Implement addition, multiplication, etc
Add constructor from whole numbers
Override equals/hashCode
Consider making Fraction an interface with an implementation that switches to BigInteger as necessary
Consider sub-classing Number
Consider including named constants for common values like 0 and 1
Consider making it serializable
Test for division by zero
Document your API

Basically, take a look at the API for other value classes like Double, Integer and do what they do :)

User · Answer

Might be useful to add simple things like reciprocate  get remainder and get whole

[java] Best way to represent a fraction in Java?

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