What s the best way to check if a String represents an integer in Java

Question

I normally use the following idiom to check if a String can be converted to an integer   public boolean isInteger  String input         try           Integer parseInt  input            return true            catch  Exception e             return false            Is it just me  or does this seem a bit hackish   What s a better way     See my answer  with benchmarks  based on the earlier answer by CodingWithSpike  to see why I ve reversed my position and accepted Jonas Klemming s answer to this problem   I think this original code will be used by most people because it s quicker to implement  and more maintainable  but it s orders of magnitude slower when non-integer data is provided

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is number   true  try     Integer parseInt mystr    catch  NumberFormatException  e      is number   false

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You can use the matches method of the string class  The  0-9  represents all the values it    can be  the   means it must be at least one character long  and the   means it can be zero or more characters long    boolean isNumeric   yourString matches   0-9         1 or more characters long  numbers only boolean isNumeric   yourString matches   0-9         0 or more characters long  numbers only

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This is shorter  but shorter isn t necessarily better  and it won t catch integer values which are out of range  as pointed out in danatel s comment    input matches   -   d        Personally  since the implementation is squirrelled away in a helper method and correctness trumps length  I would just go with something like what you have  minus catching the base Exception class rather than NumberFormatException

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is number   true  try     Integer parseInt mystr    catch  NumberFormatException  e      is number   false

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You can also use the Scanner class  and use hasNextInt   - and this allows you to test for other types  too  like floats  etc

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This is a Java 8 variation of Jonas Klemming answer   public static boolean isInteger String str        return str    null  amp  amp  str length    gt  0  amp  amp           IntStream range 0  str length    allMatch i - gt  i    0  amp  amp   str charAt i      -     str charAt i                               Character isDigit str charAt i         Test code   public static void main String   args  throws NoSuchAlgorithmException  UnsupportedEncodingException       Arrays asList  1231231    -1232312312     12313123131    qwqe123123211    2    0000000001111        123-      123                123-23   null    -123   forEach s - gt            System out printf   15s  s n   s  isInteger s                Results of the test code           1231231 true     -1232312312 true     12313123131 true   qwqe123123211 false               2 true   0000000001111 true                 false            123- false             123 false          123-23 false            null false            -123 false

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This is a modification of Jonas  code that checks if the string is within range to be cast into an integer     public static boolean isInteger String str        if  str    null            return false            int length   str length        int i   0          set the length and value for highest positive int or lowest negative int     int maxlength   10      String maxnum   String valueOf Integer MAX VALUE       if  str charAt 0      -              maxlength   11          i   1          maxnum   String valueOf Integer MIN VALUE                   verify digit length does not exceed int range     if  length  gt  maxlength             return false                 verify that all characters are numbers     if  maxlength    11  amp  amp  length    1            return false            for  int num   i  num  lt  length  num              char c   str charAt num           if  c  lt   0     c  gt   9                 return false                          verify that number value is within int range     if  length    maxlength            for    i  lt  length  i                  if  str charAt i   lt  maxnum charAt i                     return true                            else if  str charAt i   gt  maxnum charAt i                     return false                                    return true

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Number number  try       number   NumberFormat getInstance   parse  123      catch  ParseException e          not a number - do recovery      e printStackTrace        use number

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Did a quick benchmark  Exceptions aren t actually that expensivve  unless you start popping back multiple methods and the JVM has to do a lot of work to get the execution stack in place   When staying in the same method  they aren t bad performers    public void RunTests           String str    1234567890         long startTime   System currentTimeMillis         for int i   0  i  lt  100000  i             IsInt ByException str        long endTime   System currentTimeMillis         System out print  ByException           System out println endTime - startTime         startTime   System currentTimeMillis         for int i   0  i  lt  100000  i             IsInt ByRegex str        endTime   System currentTimeMillis         System out print  ByRegex           System out println endTime - startTime         startTime   System currentTimeMillis         for int i   0  i  lt  100000  i             IsInt ByJonas str        endTime   System currentTimeMillis         System out print  ByJonas           System out println endTime - startTime        private boolean IsInt ByException String str          try                 Integer parseInt str            return true              catch NumberFormatException nfe                  return false              private boolean IsInt ByRegex String str          return str matches   -   d           public boolean IsInt ByJonas String str          if  str    null                 return false              int length   str length         if  length    0                 return false              int i   0       if  str charAt 0      -                  if  length    1                         return false                              i   1              for    i  lt  length  i                   char c   str charAt i                if  c  lt          c  gt                               return false                             return true       Output      ByException  31      ByRegex  453  note  re-compiling the pattern every time       ByJonas  16   I do agree that Jonas K s solution is the most robust too  Looks like he wins

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You have it  but you should only catch NumberFormatException

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Did a quick benchmark  Exceptions aren t actually that expensivve  unless you start popping back multiple methods and the JVM has to do a lot of work to get the execution stack in place   When staying in the same method  they aren t bad performers    public void RunTests           String str    1234567890         long startTime   System currentTimeMillis         for int i   0  i  lt  100000  i             IsInt ByException str        long endTime   System currentTimeMillis         System out print  ByException           System out println endTime - startTime         startTime   System currentTimeMillis         for int i   0  i  lt  100000  i             IsInt ByRegex str        endTime   System currentTimeMillis         System out print  ByRegex           System out println endTime - startTime         startTime   System currentTimeMillis         for int i   0  i  lt  100000  i             IsInt ByJonas str        endTime   System currentTimeMillis         System out print  ByJonas           System out println endTime - startTime        private boolean IsInt ByException String str          try                 Integer parseInt str            return true              catch NumberFormatException nfe                  return false              private boolean IsInt ByRegex String str          return str matches   -   d           public boolean IsInt ByJonas String str          if  str    null                 return false              int length   str length         if  length    0                 return false              int i   0       if  str charAt 0      -                  if  length    1                         return false                              i   1              for    i  lt  length  i                   char c   str charAt i                if  c  lt          c  gt                               return false                             return true       Output      ByException  31      ByRegex  453  note  re-compiling the pattern every time       ByJonas  16   I do agree that Jonas K s solution is the most robust too  Looks like he wins

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It partly depend on what you mean by  can be converted to an integer    If you mean  can be converted into an int in Java  then the answer from Jonas is a good start  but doesn t quite finish the job  It would pass 999999999999999999999999999999 for example  I would add the normal try catch call from your own question at the end of the method   The character-by-character checks will efficiently reject  not an integer at all  cases  leaving  it s an integer but Java can t handle it  cases to be caught by the slower exception route  You could do this bit by hand too  but it would be a lot more complicated

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org apache commons lang StringUtils isNumeric    though Java s standard lib really misses such utility functions  I think that Apache Commons is a  must have  for every Java programmer  too bad it isn t ported to Java5 yet

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Just one comment about regexp  Every example provided here is wrong   If you want to use regexp don t forget that compiling the pattern take a lot of time  This   str matches   -   d       and also this   Pattern matches  -   d    input     causes compile of pattern in every method call  To used it correctly follow   import java util regex Pattern           author Rastislav Komara     public class NaturalNumberChecker       public static final Pattern PATTERN   Pattern compile     d           boolean isNaturalNumber CharSequence input            return input    null  amp  amp  PATTERN matcher input  matches

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There is guava version   import com google common primitives Ints   Integer intValue   Ints tryParse stringValue     It will return null instead of throwing an exception if it fails to parse string

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To check for all int chars  you can simply use a double negative    if   searchString matches    0-9              0-9    checks to see if there are any characters that are not integer  so the test fails if it s true  Just NOT that and you get true on success

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This is shorter  but shorter isn t necessarily better  and it won t catch integer values which are out of range  as pointed out in danatel s comment    input matches   -   d        Personally  since the implementation is squirrelled away in a helper method and correctness trumps length  I would just go with something like what you have  minus catching the base Exception class rather than NumberFormatException

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Since there s possibility that people still visit here and will be biased against Regex after the benchmarks    So i m gonna give an updated version of the benchmark  with a compiled version of the Regex  Which opposed to the previous benchmarks  this one shows Regex solution actually has consistently good performance   Copied from Bill the Lizard and updated with compiled version   private final Pattern pattern   Pattern compile   -   d       public void runTests         String big int    1234567890       String non int    1234XY7890        long startTime   System currentTimeMillis        for int i   0  i  lt  100000  i                IsInt ByException big int       long endTime   System currentTimeMillis        System out print  ByException - integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i                IsInt ByException non int       endTime   System currentTimeMillis        System out print  ByException - non-integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i                IsInt ByRegex big int       endTime   System currentTimeMillis        System out print   nByRegex - integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i                IsInt ByRegex non int       endTime   System currentTimeMillis        System out print  ByRegex - non-integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for  int i   0  i  lt  100000  i                IsInt ByCompiledRegex big int       endTime   System currentTimeMillis        System out print   nByCompiledRegex - integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for  int i   0  i  lt  100000  i                IsInt ByCompiledRegex non int       endTime   System currentTimeMillis        System out print  ByCompiledRegex - non-integer data          System out println endTime - startTime         startTime   System currentTimeMillis        for int i   0  i  lt  100000  i                IsInt ByJonas big int       endTime   System currentTimeMillis        System out print   nByJonas - integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i                IsInt ByJonas non int       endTime   System currentTimeMillis        System out print  ByJonas - non-integer data          System out println endTime - startTime      private boolean IsInt ByException String str        try               Integer parseInt str           return true            catch NumberFormatException nfe                return false           private boolean IsInt ByRegex String str        return str matches   -   d         private boolean IsInt ByCompiledRegex String str        return pattern matcher str  find       public boolean IsInt ByJonas String str        if  str    null                return false            int length   str length        if  length    0                return false            int i   0      if  str charAt 0      -                 if  length    1                        return false                            i   1            for    i  lt  length  i                  char c   str charAt i               if  c  lt          c  gt                              return false                          return true      Results   ByException - integer data  45 ByException - non-integer data  465  ByRegex - integer data  272 ByRegex - non-integer data  131  ByCompiledRegex - integer data  45 ByCompiledRegex - non-integer data  26  ByJonas - integer data  8 ByJonas - non-integer data  2

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I have seen a lot of answers here  but most of them are able to determine whether the String is numeric  but they fail checking whether the number is in Integer range     Therefore I purpose something like this   public static boolean isInteger String str        if  str    null    str isEmpty              return false            try           long value   Long valueOf str           return value  gt   -2147483648  amp  amp  value  lt   2147483647        catch  Exception ex            return false

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Number number  try       number   NumberFormat getInstance   parse  123      catch  ParseException e          not a number - do recovery      e printStackTrace        use number

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When explanations are more important than performance  I noticed many discussions centering how efficient certain solutions are  but none on why an string is not an integer   Also  everyone seemed to assume that the number  2 00  is not equal to  2   Mathematically and humanly speaking  they are equal  even though computer science says that they are not  and for good reason   This is why the  Integer parseInt  solutions above are weak  depending on your requirements    At any rate  to make software smarter and more human-friendly  we need to create software that thinks like we do and explains why something failed  In this case   public static boolean isIntegerFromDecimalString String possibleInteger    possibleInteger   possibleInteger trim    try          Integer parsing works great for  regular  integers like 42 or 13      int num   Integer parseInt possibleInteger       System out println  The possibleInteger   possibleInteger   is a pure integer         return true    catch  NumberFormatException e        if  possibleInteger equals                 System out println  The possibleInteger     possibleInteger     is NOT an integer because it is only a decimal point             return false        else if  possibleInteger startsWith       amp  amp  possibleInteger matches      0-9                if  possibleInteger matches      0                    System out println  The possibleInteger     possibleInteger     is an integer because it starts with a decimal point and afterwards is all zeros                 return true            else               System out println  The possibleInteger     possibleInteger     is NOT an integer because it starts with a decimal point and afterwards is not all zeros                 return false                  else if  possibleInteger endsWith        amp  amp  possibleInteger matches   0-9                   System out println  The possibleInteger   possibleInteger   is an impure integer  ends with decimal point              return true        else if  possibleInteger contains                 String   partsOfPossibleInteger   possibleInteger split                 if  partsOfPossibleInteger length    2                  System out println  The possibleInteger     possibleInteger     is split into      partsOfPossibleInteger 0       and      partsOfPossibleInteger 1                       if  partsOfPossibleInteger 0  matches   0-9                        if  partsOfPossibleInteger 1  matches   0                            System out println  The possibleInteger   possibleInteger   is an impure integer  ends with all zeros after the decimal point                          return true                    else if  partsOfPossibleInteger 1  matches   0-9                            System out println  The possibleInteger     possibleInteger     is NOT an integer because it the numbers after the decimal point                                       partsOfPossibleInteger 1       are not all zeros                         return false                    else                       System out println  The possibleInteger     possibleInteger     is NOT an integer because it the  numbers  after the decimal point                                   partsOfPossibleInteger 1       are not all numeric digits                         return false                                  else                   System out println  The possibleInteger     possibleInteger     is NOT an integer because it the  number  before the decimal point                               partsOfPossibleInteger 0       is not a number                     return false                          else               System out println  The possibleInteger   possibleInteger   is NOT an integer because it has a strange number of decimal-period separated parts                          partsOfPossibleInteger length                      return false                     else     System out println  The possibleInteger    possibleInteger    is NOT an integer  even though it has no decimal point         return false        Test code   String   testData     0    0     0 0     000    2    2     2 0    2 0000    3 14159     0001          4 0    3E24    6 0221409e 23    int i   0  for  String possibleInteger   testData         System out println          System out println i      possibleInteger      possibleInteger     isIntegerFromDecimalString     isIntegerFromDecimalString possibleInteger        i

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If you want to check if the string represents an integer that fits in an int type  I did a little modification to the jonas  answer  so that strings that represent integers bigger than Integer MAX VALUE or smaller than Integer MIN VALUE  will now return false  For example   3147483647  will return false because 3147483647 is bigger than 2147483647  and likewise   -2147483649  will also return false because -2147483649 is smaller than -2147483648   public static boolean isInt String s      if s    null        return false        s   s trim      Don t get tricked by whitespaces    int len   s length      if len    0        return false          The bottom limit of an int is -2147483648 which is 11 chars long       note that the upper limit  2147483647  is only 10 chars long      Thus any string with more than 11 chars  even if represents a valid integer       it won t fit in an int    if len  gt  11        return false        char c   s charAt 0     int i   0      I don t mind the plus sign  so   13  will return true    if c     -     c                 A single     or  -  is not a valid integer      if len    1          return false            i   1          Check if all chars are digits   for   i  lt  len  i          c   s charAt i       if c  lt   0     c  gt   9           return false                If we reached this point then we know for sure that the string has at     most 11 chars and that they re all digits  the first one might be a          or  -  thought       Now we just need to check  for 10 and 11 chars long strings  if the numbers     represented by the them don t surpass the limits    c   s charAt 0     char l    String limit    if len    10  amp  amp  c     -   amp  amp  c               limit    2147483647         Now we are going to compare each char of the string with the char in       the limit string that has the same index  so if the string is  ABC  and       the limit string is  DEF  then we are gonna compare A to D  B to E and so on        c is the current string s char and l is the corresponding limit s char       Note that the loop only continues if c    l  Now imagine that our string       is  2150000000   2    2  next   1    1  next   5  gt  4 as you can see        because 5  gt  4 we can guarantee that the string will represent a bigger integer        Similarly  if our string was  2139999999   when we find out that 3  lt  4        we can also guarantee that the integer represented will fit in an int      for i   0  i  lt  len  i            c   s charAt i         l   limit charAt i         if c  gt  l            return false                if c  lt  l            return true                      c   s charAt 0     if len    11          If the first char is neither     nor  -  then 11 digits represent a        bigger integer than 2147483647  10 digits       if c         amp  amp  c     -           return false            limit    c     -      -2147483648      2147483647         Here we re applying the same logic that we applied in the previous case       ignoring the first char      for i   1  i  lt  len  i            c   s charAt i         l   limit charAt i         if c  gt  l            return false                if c  lt  l            return true                        The string passed all tests  so it must represent a number that fits     in an int      return true

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If you want to check if the string represents an integer that fits in an int type  I did a little modification to the jonas  answer  so that strings that represent integers bigger than Integer MAX VALUE or smaller than Integer MIN VALUE  will now return false  For example   3147483647  will return false because 3147483647 is bigger than 2147483647  and likewise   -2147483649  will also return false because -2147483649 is smaller than -2147483648   public static boolean isInt String s      if s    null        return false        s   s trim      Don t get tricked by whitespaces    int len   s length      if len    0        return false          The bottom limit of an int is -2147483648 which is 11 chars long       note that the upper limit  2147483647  is only 10 chars long      Thus any string with more than 11 chars  even if represents a valid integer       it won t fit in an int    if len  gt  11        return false        char c   s charAt 0     int i   0      I don t mind the plus sign  so   13  will return true    if c     -     c                 A single     or  -  is not a valid integer      if len    1          return false            i   1          Check if all chars are digits   for   i  lt  len  i          c   s charAt i       if c  lt   0     c  gt   9           return false                If we reached this point then we know for sure that the string has at     most 11 chars and that they re all digits  the first one might be a          or  -  thought       Now we just need to check  for 10 and 11 chars long strings  if the numbers     represented by the them don t surpass the limits    c   s charAt 0     char l    String limit    if len    10  amp  amp  c     -   amp  amp  c               limit    2147483647         Now we are going to compare each char of the string with the char in       the limit string that has the same index  so if the string is  ABC  and       the limit string is  DEF  then we are gonna compare A to D  B to E and so on        c is the current string s char and l is the corresponding limit s char       Note that the loop only continues if c    l  Now imagine that our string       is  2150000000   2    2  next   1    1  next   5  gt  4 as you can see        because 5  gt  4 we can guarantee that the string will represent a bigger integer        Similarly  if our string was  2139999999   when we find out that 3  lt  4        we can also guarantee that the integer represented will fit in an int      for i   0  i  lt  len  i            c   s charAt i         l   limit charAt i         if c  gt  l            return false                if c  lt  l            return true                      c   s charAt 0     if len    11          If the first char is neither     nor  -  then 11 digits represent a        bigger integer than 2147483647  10 digits       if c         amp  amp  c     -           return false            limit    c     -      -2147483648      2147483647         Here we re applying the same logic that we applied in the previous case       ignoring the first char      for i   1  i  lt  len  i            c   s charAt i         l   limit charAt i         if c  gt  l            return false                if c  lt  l            return true                        The string passed all tests  so it must represent a number that fits     in an int      return true

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This works for me  Simply to identify whether a String is a primitive or a number   private boolean isPrimitive String value           boolean status true          if value length   lt 1              return false          for int i   0 i lt value length   i                 char c value charAt i               if Character isDigit c     c                      else                  status false                  break                                  return status

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How about   return Pattern matches  -   d    input

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You just check NumberFormatException -   String value  123    try            int s Integer parseInt any int val          do something when integer values comes        catch NumberFormatException nfe                      do something when string values comes

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public class HelloWorld       static boolean validateIP String s           String   value   s split                 if value length  4  return false          int   v   new int 4           for int i 0 i lt 4 i                 for int j 0 j lt value i  length   j                     if  Character isDigit value i  charAt j                      return false                            v i  Integer parseInt value i                if   v i  gt  0  amp  amp  v i  lt  255   return false                    return true             public static void main String   argv           String test    12 23 8 9j           if validateIP test                System out println    test

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You can also use the Scanner class  and use hasNextInt   - and this allows you to test for other types  too  like floats  etc

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Since there s possibility that people still visit here and will be biased against Regex after the benchmarks    So i m gonna give an updated version of the benchmark  with a compiled version of the Regex  Which opposed to the previous benchmarks  this one shows Regex solution actually has consistently good performance   Copied from Bill the Lizard and updated with compiled version   private final Pattern pattern   Pattern compile   -   d       public void runTests         String big int    1234567890       String non int    1234XY7890        long startTime   System currentTimeMillis        for int i   0  i  lt  100000  i                IsInt ByException big int       long endTime   System currentTimeMillis        System out print  ByException - integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i                IsInt ByException non int       endTime   System currentTimeMillis        System out print  ByException - non-integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i                IsInt ByRegex big int       endTime   System currentTimeMillis        System out print   nByRegex - integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i                IsInt ByRegex non int       endTime   System currentTimeMillis        System out print  ByRegex - non-integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for  int i   0  i  lt  100000  i                IsInt ByCompiledRegex big int       endTime   System currentTimeMillis        System out print   nByCompiledRegex - integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for  int i   0  i  lt  100000  i                IsInt ByCompiledRegex non int       endTime   System currentTimeMillis        System out print  ByCompiledRegex - non-integer data          System out println endTime - startTime         startTime   System currentTimeMillis        for int i   0  i  lt  100000  i                IsInt ByJonas big int       endTime   System currentTimeMillis        System out print   nByJonas - integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i                IsInt ByJonas non int       endTime   System currentTimeMillis        System out print  ByJonas - non-integer data          System out println endTime - startTime      private boolean IsInt ByException String str        try               Integer parseInt str           return true            catch NumberFormatException nfe                return false           private boolean IsInt ByRegex String str        return str matches   -   d         private boolean IsInt ByCompiledRegex String str        return pattern matcher str  find       public boolean IsInt ByJonas String str        if  str    null                return false            int length   str length        if  length    0                return false            int i   0      if  str charAt 0      -                 if  length    1                        return false                            i   1            for    i  lt  length  i                  char c   str charAt i               if  c  lt          c  gt                              return false                          return true      Results   ByException - integer data  45 ByException - non-integer data  465  ByRegex - integer data  272 ByRegex - non-integer data  131  ByCompiledRegex - integer data  45 ByCompiledRegex - non-integer data  26  ByJonas - integer data  8 ByJonas - non-integer data  2

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What you did works  but you probably shouldn t always check that way   Throwing exceptions should be reserved for  exceptional  situations  maybe that fits in your case  though   and are very costly in terms of performance

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If you are not concerned with potential overflow problems this function will perform about 20-30 times faster than using Integer parseInt     public static boolean isInteger String str        if  str    null            return false            int length   str length        if  length    0            return false            int i   0      if  str charAt 0      -             if  length    1                return false                    i   1            for    i  lt  length  i              char c   str charAt i           if  c  lt   0     c  gt   9                 return false                      return true

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What you did works  but you probably shouldn t always check that way   Throwing exceptions should be reserved for  exceptional  situations  maybe that fits in your case  though   and are very costly in terms of performance

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Just one comment about regexp  Every example provided here is wrong   If you want to use regexp don t forget that compiling the pattern take a lot of time  This   str matches   -   d       and also this   Pattern matches  -   d    input     causes compile of pattern in every method call  To used it correctly follow   import java util regex Pattern           author Rastislav Komara     public class NaturalNumberChecker       public static final Pattern PATTERN   Pattern compile     d           boolean isNaturalNumber CharSequence input            return input    null  amp  amp  PATTERN matcher input  matches

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It partly depend on what you mean by  can be converted to an integer    If you mean  can be converted into an int in Java  then the answer from Jonas is a good start  but doesn t quite finish the job  It would pass 999999999999999999999999999999 for example  I would add the normal try catch call from your own question at the end of the method   The character-by-character checks will efficiently reject  not an integer at all  cases  leaving  it s an integer but Java can t handle it  cases to be caught by the slower exception route  You could do this bit by hand too  but it would be a lot more complicated

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Integer valueOf string      works for me most of the time

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You probably need to take the use case in account too   If most of the time you expect numbers to be valid  then catching the exception is only causing a performance overhead when attempting to convert invalid numbers  Whereas calling some isInteger   method and then convert using Integer parseInt   will always cause a performance overhead for valid numbers - the strings are parsed twice  once by the check and once by the conversion

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To check for all int chars  you can simply use a double negative    if   searchString matches    0-9              0-9    checks to see if there are any characters that are not integer  so the test fails if it s true  Just NOT that and you get true on success

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Integer valueOf string      works for me most of the time

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I believe there s zero risk running into an exception  because as you can see below you always safely parse int to String and not the other way around  So   You check if every slot of character in your string matches at least one of the characters   quot 0 quot   quot 1 quot   quot 2 quot   quot 3 quot   quot 4 quot   quot 5 quot   quot 6 quot   quot 7 quot   quot 8 quot   quot 9 quot    if aString substring j  j 1  equals String valueOf i      You sum all the times that you encountered in the slots the above characters  digits      And finally you check if the times that you encountered integers as characters equals with the length of the given string  if digits    aString length       And in practice we have      String aString    quot 1234224245 quot       int digits   0   count how many digits you encountered     for int j 0 j lt aString length   j             for int i 0 i lt  9 i                 if aString substring j  j 1  equals String valueOf i                        digits                        if digits    aString length             System out println  quot It s an integer   quot                  else          System out println  quot It s not an integer   quot                   String anotherString    quot 1234f22a4245 quot       int anotherDigits   0   count how many digits you encountered     for int j 0 j lt anotherString length   j             for int i 0 i lt  9 i                 if anotherString substring j  j 1  equals String valueOf i                        anotherDigits                        if anotherDigits    anotherString length             System out println  quot It s an integer   quot                  else          System out println  quot It s not an integer   quot           And the results are   It s an integer   It s not an integer    Similarly  you can validate if a String is a float or a double but in those cases you have to encounter only one    dot  in the String and of course check if  digits     aString length  -1   Again  there s zero risk running into a parsing exception here  but if you plan on parsing a string that it is known that contains a number  let s say int data type  you must first check if it fits in the data type  Otherwise you must cast it   I hope I helped

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This is shorter  but shorter isn t necessarily better  and it won t catch integer values which are out of range  as pointed out in danatel s comment    input matches   -   d        Personally  since the implementation is squirrelled away in a helper method and correctness trumps length  I would just go with something like what you have  minus catching the base Exception class rather than NumberFormatException

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You can use the matches method of the string class  The  0-9  represents all the values it    can be  the   means it must be at least one character long  and the   means it can be zero or more characters long    boolean isNumeric   yourString matches   0-9         1 or more characters long  numbers only boolean isNumeric   yourString matches   0-9         0 or more characters long  numbers only

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Find this may helpful   public static boolean isInteger String self        try           Integer valueOf self trim             return true        catch  NumberFormatException nfe            return false

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For those readers who arrive here like me years after the question was asked  I have a more general solution for this question          Checks  if the string represents a number         param string the string     return true  if the string is a number     public static boolean isANumber final String string        if  string    null            final int length   string length            if  length    0                int i   0              if  string charAt 0      -                     if  length    1                        return false                                    i                              for    i  lt  length  i                      final char c   string charAt i                   if   c  lt   PERIOD       c  gt   COLON                          final String strC   Character toString c  toUpperCase                        final boolean isExponent   strC equals  E                        final boolean isPeriod    c    PERIOD                       final boolean isPlus    c    PLUS                        if   isExponent  amp  amp   isPeriod  amp  amp   isPlus                            return false                                                                    return true                      return false

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What you did works  but you probably shouldn t always check that way   Throwing exceptions should be reserved for  exceptional  situations  maybe that fits in your case  though   and are very costly in terms of performance

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How about   return Pattern matches  -   d    input

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You have it  but you should only catch NumberFormatException

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If you are using the Android API  you can use   TextUtils isDigitsOnly str

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For kotlin  isDigitsOnly    Also for Java s TextUtils isDigitsOnly    of String always returns false it has a negative sign in front though the rest of the character is digit only  For example -     For kotlin   var str    quot -123 quot   str isDigitsOnly      Result will be false       For Java    String str    quot -123 quot  TextUtils isDigitsOnly str    Result will be also false   So I made a quick fix by this -  var isDigit str matches  quot -   d       d    quot  toRegex         Result will be true for now

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If you are not concerned with potential overflow problems this function will perform about 20-30 times faster than using Integer parseInt     public static boolean isInteger String str        if  str    null            return false            int length   str length        if  length    0            return false            int i   0      if  str charAt 0      -             if  length    1                return false                    i   1            for    i  lt  length  i              char c   str charAt i           if  c  lt   0     c  gt   9                 return false                      return true

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I do not like method with regex  because regex can not check ranges  Integer MIN VALUE  Integer MAX VALUE    If you expect int value in most cases  and not int is something uncommon  then I suggest version with Integer valueOf or Integer parseInt with NumberFormatException catching  Advantage of this approach - your code has good readability   public static boolean isInt String s      try       Integer parseInt s       return true      catch  NumberFormatException nfe        return false          If you need to check if String is integer  and care about perfomance then the best way will be to use java jdk implementation of Integer parseInt  but little modified  replacing throw with return false    This function has good perfomence and garantee right result      public static boolean isInt String s        int radix   10       if  s    null            return false             if  radix  lt  Character MIN RADIX            return false             if  radix  gt  Character MAX RADIX            return false             int result   0      boolean negative   false      int i   0  len   s length        int limit   -Integer MAX VALUE      int multmin      int digit       if  len  gt  0            char firstChar   s charAt 0           if  firstChar  lt   0        Possible leading     or  -              if  firstChar     -                     negative   true                  limit   Integer MIN VALUE                else if  firstChar                         return false               if  len    1     Cannot have lone     or  -                  return false              i                      multmin   limit   radix          while  i  lt  len                   Accumulating negatively avoids surprises near MAX VALUE             digit   Character digit s charAt i     radix               if  digit  lt  0                    return false                            if  result  lt  multmin                    return false                            result    radix              if  result  lt  limit   digit                    return false                            result -  digit                  else           return false            return true

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is number   true  try     Integer parseInt mystr    catch  NumberFormatException  e      is number   false

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Another option   private boolean isNumber String s        boolean isNumber   true      for  char c   s toCharArray              isNumber   isNumber  amp  amp  Character isDigit c             return isNumber

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For kotlin  isDigitsOnly    Also for Java s TextUtils isDigitsOnly    of String always returns false it has a negative sign in front though the rest of the character is digit only  For example -     For kotlin   var str    quot -123 quot   str isDigitsOnly      Result will be false       For Java    String str    quot -123 quot  TextUtils isDigitsOnly str    Result will be also false   So I made a quick fix by this -  var isDigit str matches  quot -   d       d    quot  toRegex         Result will be true for now

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org apache commons lang StringUtils isNumeric    though Java s standard lib really misses such utility functions  I think that Apache Commons is a  must have  for every Java programmer  too bad it isn t ported to Java5 yet

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a little improvement to  Jonas K anwser  this function will rule out one single operator like  quot   quot   public boolean isInteger String str           null pointer     if  str    null            return false            int len   str length           empty string     if  len    0            return false               one digit  cannot begin with 0     if  len    1            char c   str charAt 0           if   c  lt   1       c  gt   9                  return false                       for  int i   0  i  lt  len  i              char c   str charAt i              check positive  negative sign         if  i    0                if  c     -     c                           continue                                     check each character matches  0-9          if   c  lt   0       c  gt   9                  return false                      return true

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org apache commons lang StringUtils isNumeric    though Java s standard lib really misses such utility functions  I think that Apache Commons is a  must have  for every Java programmer  too bad it isn t ported to Java5 yet

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This is a Java 8 variation of Jonas Klemming answer   public static boolean isInteger String str        return str    null  amp  amp  str length    gt  0  amp  amp           IntStream range 0  str length    allMatch i - gt  i    0  amp  amp   str charAt i      -     str charAt i                               Character isDigit str charAt i         Test code   public static void main String   args  throws NoSuchAlgorithmException  UnsupportedEncodingException       Arrays asList  1231231    -1232312312     12313123131    qwqe123123211    2    0000000001111        123-      123                123-23   null    -123   forEach s - gt            System out printf   15s  s n   s  isInteger s                Results of the test code           1231231 true     -1232312312 true     12313123131 true   qwqe123123211 false               2 true   0000000001111 true                 false            123- false             123 false          123-23 false            null false            -123 false

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Did a quick benchmark  Exceptions aren t actually that expensivve  unless you start popping back multiple methods and the JVM has to do a lot of work to get the execution stack in place   When staying in the same method  they aren t bad performers    public void RunTests           String str    1234567890         long startTime   System currentTimeMillis         for int i   0  i  lt  100000  i             IsInt ByException str        long endTime   System currentTimeMillis         System out print  ByException           System out println endTime - startTime         startTime   System currentTimeMillis         for int i   0  i  lt  100000  i             IsInt ByRegex str        endTime   System currentTimeMillis         System out print  ByRegex           System out println endTime - startTime         startTime   System currentTimeMillis         for int i   0  i  lt  100000  i             IsInt ByJonas str        endTime   System currentTimeMillis         System out print  ByJonas           System out println endTime - startTime        private boolean IsInt ByException String str          try                 Integer parseInt str            return true              catch NumberFormatException nfe                  return false              private boolean IsInt ByRegex String str          return str matches   -   d           public boolean IsInt ByJonas String str          if  str    null                 return false              int length   str length         if  length    0                 return false              int i   0       if  str charAt 0      -                  if  length    1                         return false                              i   1              for    i  lt  length  i                   char c   str charAt i                if  c  lt          c  gt                               return false                             return true       Output      ByException  31      ByRegex  453  note  re-compiling the pattern every time       ByJonas  16   I do agree that Jonas K s solution is the most robust too  Looks like he wins

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I copied the code from rally25rs answer and added some tests for non-integer data   The results are undeniably in favor of the method posted by Jonas Klemming   The results for the Exception method that I originally posted are pretty good when you have integer data  but they re the worst when you don t  while the results for the RegEx solution  that I ll bet a lot of people use  were consistently bad  See Felipe s answer for a compiled regex example  which is much faster   public void runTests         String big int    1234567890       String non int    1234XY7890        long startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByException big int       long endTime   System currentTimeMillis        System out print  ByException - integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByException non int       endTime   System currentTimeMillis        System out print  ByException - non-integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByRegex big int       endTime   System currentTimeMillis        System out print   nByRegex - integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByRegex non int       endTime   System currentTimeMillis        System out print  ByRegex - non-integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByJonas big int       endTime   System currentTimeMillis        System out print   nByJonas - integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByJonas non int       endTime   System currentTimeMillis        System out print  ByJonas - non-integer data          System out println endTime - startTime      private boolean IsInt ByException String str        try               Integer parseInt str           return true            catch NumberFormatException nfe                return false           private boolean IsInt ByRegex String str        return str matches   -   d         public boolean IsInt ByJonas String str        if  str    null                return false            int length   str length        if  length    0                return false            int i   0      if  str charAt 0      -                 if  length    1                        return false                            i   1            for    i  lt  length  i                  char c   str charAt i               if  c  lt          c  gt                              return false                          return true      Results   ByException - integer data  47 ByException - non-integer data  547  ByRegex - integer data  390 ByRegex - non-integer data  313  ByJonas - integer data  0 ByJonas - non-integer data  16

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I recently  today  needed to figure out a quick way to do this and of course I was going to use the exception approach for ease when the monkey on the shoulder  conscience  woke up so it took me down this old familiar rabbit hole  no exceptions are not that much more expensive in fact sometimes exceptions are faster  old AIX multiprocessor systems  but regardless it   s to elegant so I did something that the younger me never did and to my amazement nobody here did either  apologize if someone did and I missed it I honestly did not find    so what did I think we all missed  taking a look at how the JRE implemented it  yes they threw an exception but we can always skip that part  The younger me from 10 years ago would have felt this to be beneath him  but then again he is a loud mouthed show off with a poor temperament and a god complex so there is that  I am putting this here for the benefit of anyone coming here in the future  Here is what I found  public static int parseInt String s  int radix  throws NumberFormatException                 WARNING  This method may be invoked early during VM initialization        before IntegerCache is initialized  Care must be taken to not use        the valueOf method               if  s    null            throw new NumberFormatException  quot null quot               if  radix  lt  Character MIN RADIX            throw new NumberFormatException  quot radix  quot    radix                                            quot  less than Character MIN RADIX quot               if  radix  gt  Character MAX RADIX            throw new NumberFormatException  quot radix  quot    radix                                            quot  greater than Character MAX RADIX quot               int result   0      boolean negative   false      int i   0  len   s length        int limit   -Integer MAX VALUE      int multmin      int digit       if  len  gt  0            char firstChar   s charAt 0           if  firstChar  lt   0        Possible leading  quot   quot  or  quot - quot              if  firstChar     -                     negative   true                  limit   Integer MIN VALUE                else if  firstChar                         throw NumberFormatException forInputString s                if  len    1     Cannot have lone  quot   quot  or  quot - quot                  throw NumberFormatException forInputString s               i                      multmin   limit   radix          while  i  lt  len                   Accumulating negatively avoids surprises near MAX VALUE             digit   Character digit s charAt i    radix               if  digit  lt  0                    throw NumberFormatException forInputString s                             if  result  lt  multmin                    throw NumberFormatException forInputString s                             result    radix              if  result  lt  limit   digit                    throw NumberFormatException forInputString s                             result -  digit                  else           throw NumberFormatException forInputString s             return negative   result   -result

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is number   true  try     Integer parseInt mystr    catch  NumberFormatException  e      is number   false

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I copied the code from rally25rs answer and added some tests for non-integer data   The results are undeniably in favor of the method posted by Jonas Klemming   The results for the Exception method that I originally posted are pretty good when you have integer data  but they re the worst when you don t  while the results for the RegEx solution  that I ll bet a lot of people use  were consistently bad  See Felipe s answer for a compiled regex example  which is much faster   public void runTests         String big int    1234567890       String non int    1234XY7890        long startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByException big int       long endTime   System currentTimeMillis        System out print  ByException - integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByException non int       endTime   System currentTimeMillis        System out print  ByException - non-integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByRegex big int       endTime   System currentTimeMillis        System out print   nByRegex - integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByRegex non int       endTime   System currentTimeMillis        System out print  ByRegex - non-integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByJonas big int       endTime   System currentTimeMillis        System out print   nByJonas - integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByJonas non int       endTime   System currentTimeMillis        System out print  ByJonas - non-integer data          System out println endTime - startTime      private boolean IsInt ByException String str        try               Integer parseInt str           return true            catch NumberFormatException nfe                return false           private boolean IsInt ByRegex String str        return str matches   -   d         public boolean IsInt ByJonas String str        if  str    null                return false            int length   str length        if  length    0                return false            int i   0      if  str charAt 0      -                 if  length    1                        return false                            i   1            for    i  lt  length  i                  char c   str charAt i               if  c  lt          c  gt                              return false                          return true      Results   ByException - integer data  47 ByException - non-integer data  547  ByRegex - integer data  390 ByRegex - non-integer data  313  ByJonas - integer data  0 ByJonas - non-integer data  16

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I recently  today  needed to figure out a quick way to do this and of course I was going to use the exception approach for ease when the monkey on the shoulder  conscience  woke up so it took me down this old familiar rabbit hole  no exceptions are not that much more expensive in fact sometimes exceptions are faster  old AIX multiprocessor systems  but regardless it   s to elegant so I did something that the younger me never did and to my amazement nobody here did either  apologize if someone did and I missed it I honestly did not find    so what did I think we all missed  taking a look at how the JRE implemented it  yes they threw an exception but we can always skip that part  The younger me from 10 years ago would have felt this to be beneath him  but then again he is a loud mouthed show off with a poor temperament and a god complex so there is that  I am putting this here for the benefit of anyone coming here in the future  Here is what I found  public static int parseInt String s  int radix  throws NumberFormatException                 WARNING  This method may be invoked early during VM initialization        before IntegerCache is initialized  Care must be taken to not use        the valueOf method               if  s    null            throw new NumberFormatException  quot null quot               if  radix  lt  Character MIN RADIX            throw new NumberFormatException  quot radix  quot    radix                                            quot  less than Character MIN RADIX quot               if  radix  gt  Character MAX RADIX            throw new NumberFormatException  quot radix  quot    radix                                            quot  greater than Character MAX RADIX quot               int result   0      boolean negative   false      int i   0  len   s length        int limit   -Integer MAX VALUE      int multmin      int digit       if  len  gt  0            char firstChar   s charAt 0           if  firstChar  lt   0        Possible leading  quot   quot  or  quot - quot              if  firstChar     -                     negative   true                  limit   Integer MIN VALUE                else if  firstChar                         throw NumberFormatException forInputString s                if  len    1     Cannot have lone  quot   quot  or  quot - quot                  throw NumberFormatException forInputString s               i                      multmin   limit   radix          while  i  lt  len                   Accumulating negatively avoids surprises near MAX VALUE             digit   Character digit s charAt i    radix               if  digit  lt  0                    throw NumberFormatException forInputString s                             if  result  lt  multmin                    throw NumberFormatException forInputString s                             result    radix              if  result  lt  limit   digit                    throw NumberFormatException forInputString s                             result -  digit                  else           throw NumberFormatException forInputString s             return negative   result   -result

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You could   Check that the string is a number Check that it is not too long to be parsed as a long Check if the resulting long value is small enough to be represented by an int   Assuming you for some reason would have to implement this yourself  you should probably first take a look at com google common primitives Ints tryParse String  and see if that is good enough for your purposes  as suggested in another answer       Credit to Rastislav Komara   s answer  https   stackoverflow com a 237895 1725151 private static final Pattern nonZero   Pattern compile  quot  -  1-9   d   quot        See if  str  can be parsed as an  int   does not trim     Strings like  0023  are rejected  leading zeros   public static boolean parsableAsInt  Nonnull String str        if  str isEmpty              return false            if  str equals  quot 0 quot              return true            if  canParseAsLong str             long value   Long valueOf str           if  value  gt   Integer MIN VALUE  amp  amp  value  lt   Integer MAX VALUE                return true                      return false     private static boolean canParseAsLong String str        final int intMaxLength   11      return str length    lt   intMaxLength  amp  amp  nonZero matcher str  matches       This method can also be converted to return Optional lt Integer gt  so that you don   t have to parse the string twice in the client code  once to check if it   s possible and the second time to do it    for real       For example  if  canParseAsLong str         long value   Long valueOf str       if  value  gt   Integer MIN VALUE  amp  amp  value  lt   Integer MAX VALUE            return Optional of  int  value

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Integer valueOf string      works for me most of the time

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This works for me  Simply to identify whether a String is a primitive or a number   private boolean isPrimitive String value           boolean status true          if value length   lt 1              return false          for int i   0 i lt value length   i                 char c value charAt i               if Character isDigit c     c                      else                  status false                  break                                  return status

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There is guava version   import com google common primitives Ints   Integer intValue   Ints tryParse stringValue     It will return null instead of throwing an exception if it fails to parse string

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What you did works  but you probably shouldn t always check that way   Throwing exceptions should be reserved for  exceptional  situations  maybe that fits in your case  though   and are very costly in terms of performance

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This is a modification of Jonas  code that checks if the string is within range to be cast into an integer     public static boolean isInteger String str        if  str    null            return false            int length   str length        int i   0          set the length and value for highest positive int or lowest negative int     int maxlength   10      String maxnum   String valueOf Integer MAX VALUE       if  str charAt 0      -              maxlength   11          i   1          maxnum   String valueOf Integer MIN VALUE                   verify digit length does not exceed int range     if  length  gt  maxlength             return false                 verify that all characters are numbers     if  maxlength    11  amp  amp  length    1            return false            for  int num   i  num  lt  length  num              char c   str charAt num           if  c  lt   0     c  gt   9                 return false                          verify that number value is within int range     if  length    maxlength            for    i  lt  length  i                  if  str charAt i   lt  maxnum charAt i                     return true                            else if  str charAt i   gt  maxnum charAt i                     return false                                    return true

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If you are not concerned with potential overflow problems this function will perform about 20-30 times faster than using Integer parseInt     public static boolean isInteger String str        if  str    null            return false            int length   str length        if  length    0            return false            int i   0      if  str charAt 0      -             if  length    1                return false                    i   1            for    i  lt  length  i              char c   str charAt i           if  c  lt   0     c  gt   9                 return false                      return true

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I copied the code from rally25rs answer and added some tests for non-integer data   The results are undeniably in favor of the method posted by Jonas Klemming   The results for the Exception method that I originally posted are pretty good when you have integer data  but they re the worst when you don t  while the results for the RegEx solution  that I ll bet a lot of people use  were consistently bad  See Felipe s answer for a compiled regex example  which is much faster   public void runTests         String big int    1234567890       String non int    1234XY7890        long startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByException big int       long endTime   System currentTimeMillis        System out print  ByException - integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByException non int       endTime   System currentTimeMillis        System out print  ByException - non-integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByRegex big int       endTime   System currentTimeMillis        System out print   nByRegex - integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByRegex non int       endTime   System currentTimeMillis        System out print  ByRegex - non-integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByJonas big int       endTime   System currentTimeMillis        System out print   nByJonas - integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByJonas non int       endTime   System currentTimeMillis        System out print  ByJonas - non-integer data          System out println endTime - startTime      private boolean IsInt ByException String str        try               Integer parseInt str           return true            catch NumberFormatException nfe                return false           private boolean IsInt ByRegex String str        return str matches   -   d         public boolean IsInt ByJonas String str        if  str    null                return false            int length   str length        if  length    0                return false            int i   0      if  str charAt 0      -                 if  length    1                        return false                            i   1            for    i  lt  length  i                  char c   str charAt i               if  c  lt          c  gt                              return false                          return true      Results   ByException - integer data  47 ByException - non-integer data  547  ByRegex - integer data  390 ByRegex - non-integer data  313  ByJonas - integer data  0 ByJonas - non-integer data  16

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Just one comment about regexp  Every example provided here is wrong   If you want to use regexp don t forget that compiling the pattern take a lot of time  This   str matches   -   d       and also this   Pattern matches  -   d    input     causes compile of pattern in every method call  To used it correctly follow   import java util regex Pattern           author Rastislav Komara     public class NaturalNumberChecker       public static final Pattern PATTERN   Pattern compile     d           boolean isNaturalNumber CharSequence input            return input    null  amp  amp  PATTERN matcher input  matches

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This would work only for positive integers   public static boolean isInt String str        if  str    null  amp  amp  str length      0            for  int i   0  i  lt  str length    i                  if   Character isDigit str charAt i    return false                      return true

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I do not like method with regex  because regex can not check ranges  Integer MIN VALUE  Integer MAX VALUE    If you expect int value in most cases  and not int is something uncommon  then I suggest version with Integer valueOf or Integer parseInt with NumberFormatException catching  Advantage of this approach - your code has good readability   public static boolean isInt String s      try       Integer parseInt s       return true      catch  NumberFormatException nfe        return false          If you need to check if String is integer  and care about perfomance then the best way will be to use java jdk implementation of Integer parseInt  but little modified  replacing throw with return false    This function has good perfomence and garantee right result      public static boolean isInt String s        int radix   10       if  s    null            return false             if  radix  lt  Character MIN RADIX            return false             if  radix  gt  Character MAX RADIX            return false             int result   0      boolean negative   false      int i   0  len   s length        int limit   -Integer MAX VALUE      int multmin      int digit       if  len  gt  0            char firstChar   s charAt 0           if  firstChar  lt   0        Possible leading     or  -              if  firstChar     -                     negative   true                  limit   Integer MIN VALUE                else if  firstChar                         return false               if  len    1     Cannot have lone     or  -                  return false              i                      multmin   limit   radix          while  i  lt  len                   Accumulating negatively avoids surprises near MAX VALUE             digit   Character digit s charAt i     radix               if  digit  lt  0                    return false                            if  result  lt  multmin                    return false                            result    radix              if  result  lt  limit   digit                    return false                            result -  digit                  else           return false            return true

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This is shorter  but shorter isn t necessarily better  and it won t catch integer values which are out of range  as pointed out in danatel s comment    input matches   -   d        Personally  since the implementation is squirrelled away in a helper method and correctness trumps length  I would just go with something like what you have  minus catching the base Exception class rather than NumberFormatException

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several answers here saying to try parsing to an integer and catching the NumberFormatException but you should not do this  That way would create exception object and generates a stack trace each time you called it and it was not an integer  A better way with Java 8 would be to use a stream  boolean isInteger   returnValue chars   allMatch Character  isDigit

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Another option   private boolean isNumber String s        boolean isNumber   true      for  char c   s toCharArray              isNumber   isNumber  amp  amp  Character isDigit c             return isNumber

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If you are not concerned with potential overflow problems this function will perform about 20-30 times faster than using Integer parseInt     public static boolean isInteger String str        if  str    null            return false            int length   str length        if  length    0            return false            int i   0      if  str charAt 0      -             if  length    1                return false                    i   1            for    i  lt  length  i              char c   str charAt i           if  c  lt   0     c  gt   9                 return false                      return true

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You probably need to take the use case in account too   If most of the time you expect numbers to be valid  then catching the exception is only causing a performance overhead when attempting to convert invalid numbers  Whereas calling some isInteger   method and then convert using Integer parseInt   will always cause a performance overhead for valid numbers - the strings are parsed twice  once by the check and once by the conversion

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org apache commons lang StringUtils isNumeric    though Java s standard lib really misses such utility functions  I think that Apache Commons is a  must have  for every Java programmer  too bad it isn t ported to Java5 yet

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For those readers who arrive here like me years after the question was asked  I have a more general solution for this question          Checks  if the string represents a number         param string the string     return true  if the string is a number     public static boolean isANumber final String string        if  string    null            final int length   string length            if  length    0                int i   0              if  string charAt 0      -                     if  length    1                        return false                                    i                              for    i  lt  length  i                      final char c   string charAt i                   if   c  lt   PERIOD       c  gt   COLON                          final String strC   Character toString c  toUpperCase                        final boolean isExponent   strC equals  E                        final boolean isPeriod    c    PERIOD                       final boolean isPlus    c    PLUS                        if   isExponent  amp  amp   isPeriod  amp  amp   isPlus                            return false                                                                    return true                      return false

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You can also use the Scanner class  and use hasNextInt   - and this allows you to test for other types  too  like floats  etc

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It partly depend on what you mean by  can be converted to an integer    If you mean  can be converted into an int in Java  then the answer from Jonas is a good start  but doesn t quite finish the job  It would pass 999999999999999999999999999999 for example  I would add the normal try catch call from your own question at the end of the method   The character-by-character checks will efficiently reject  not an integer at all  cases  leaving  it s an integer but Java can t handle it  cases to be caught by the slower exception route  You could do this bit by hand too  but it would be a lot more complicated

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If your String array contains pure Integers and Strings  code below should work  You only have to look at first character  e g    4   44   abc   77   bond    if  Character isDigit string charAt 0            Do something with int

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Find this may helpful   public static boolean isInteger String self        try           Integer valueOf self trim             return true        catch  NumberFormatException nfe            return false

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How about   return Pattern matches  -   d    input

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Did a quick benchmark  Exceptions aren t actually that expensivve  unless you start popping back multiple methods and the JVM has to do a lot of work to get the execution stack in place   When staying in the same method  they aren t bad performers    public void RunTests           String str    1234567890         long startTime   System currentTimeMillis         for int i   0  i  lt  100000  i             IsInt ByException str        long endTime   System currentTimeMillis         System out print  ByException           System out println endTime - startTime         startTime   System currentTimeMillis         for int i   0  i  lt  100000  i             IsInt ByRegex str        endTime   System currentTimeMillis         System out print  ByRegex           System out println endTime - startTime         startTime   System currentTimeMillis         for int i   0  i  lt  100000  i             IsInt ByJonas str        endTime   System currentTimeMillis         System out print  ByJonas           System out println endTime - startTime        private boolean IsInt ByException String str          try                 Integer parseInt str            return true              catch NumberFormatException nfe                  return false              private boolean IsInt ByRegex String str          return str matches   -   d           public boolean IsInt ByJonas String str          if  str    null                 return false              int length   str length         if  length    0                 return false              int i   0       if  str charAt 0      -                  if  length    1                         return false                              i   1              for    i  lt  length  i                   char c   str charAt i                if  c  lt          c  gt                               return false                             return true       Output      ByException  31      ByRegex  453  note  re-compiling the pattern every time       ByJonas  16   I do agree that Jonas K s solution is the most robust too  Looks like he wins

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I have seen a lot of answers here  but most of them are able to determine whether the String is numeric  but they fail checking whether the number is in Integer range     Therefore I purpose something like this   public static boolean isInteger String str        if  str    null    str isEmpty              return false            try           long value   Long valueOf str           return value  gt   -2147483648  amp  amp  value  lt   2147483647        catch  Exception ex            return false

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You can also use the Scanner class  and use hasNextInt   - and this allows you to test for other types  too  like floats  etc

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I copied the code from rally25rs answer and added some tests for non-integer data   The results are undeniably in favor of the method posted by Jonas Klemming   The results for the Exception method that I originally posted are pretty good when you have integer data  but they re the worst when you don t  while the results for the RegEx solution  that I ll bet a lot of people use  were consistently bad  See Felipe s answer for a compiled regex example  which is much faster   public void runTests         String big int    1234567890       String non int    1234XY7890        long startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByException big int       long endTime   System currentTimeMillis        System out print  ByException - integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByException non int       endTime   System currentTimeMillis        System out print  ByException - non-integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByRegex big int       endTime   System currentTimeMillis        System out print   nByRegex - integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByRegex non int       endTime   System currentTimeMillis        System out print  ByRegex - non-integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByJonas big int       endTime   System currentTimeMillis        System out print   nByJonas - integer data          System out println endTime - startTime        startTime   System currentTimeMillis        for int i   0  i  lt  100000  i            IsInt ByJonas non int       endTime   System currentTimeMillis        System out print  ByJonas - non-integer data          System out println endTime - startTime      private boolean IsInt ByException String str        try               Integer parseInt str           return true            catch NumberFormatException nfe                return false           private boolean IsInt ByRegex String str        return str matches   -   d         public boolean IsInt ByJonas String str        if  str    null                return false            int length   str length        if  length    0                return false            int i   0      if  str charAt 0      -                 if  length    1                        return false                            i   1            for    i  lt  length  i                  char c   str charAt i               if  c  lt          c  gt                              return false                          return true      Results   ByException - integer data  47 ByException - non-integer data  547  ByRegex - integer data  390 ByRegex - non-integer data  313  ByJonas - integer data  0 ByJonas - non-integer data  16

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If you are using the Android API  you can use   TextUtils isDigitsOnly str

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You may try apache utils  NumberUtils isCreatable myText    See the javadoc here

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You have it  but you should only catch NumberFormatException

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several answers here saying to try parsing to an integer and catching the NumberFormatException but you should not do this  That way would create exception object and generates a stack trace each time you called it and it was not an integer  A better way with Java 8 would be to use a stream  boolean isInteger   returnValue chars   allMatch Character  isDigit

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Integer valueOf string      works for me most of the time

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public class HelloWorld       static boolean validateIP String s           String   value   s split                 if value length  4  return false          int   v   new int 4           for int i 0 i lt 4 i                 for int j 0 j lt value i  length   j                     if  Character isDigit value i  charAt j                      return false                            v i  Integer parseInt value i                if   v i  gt  0  amp  amp  v i  lt  255   return false                    return true             public static void main String   argv           String test    12 23 8 9j           if validateIP test                System out println    test

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This would work only for positive integers   public static boolean isInt String str        if  str    null  amp  amp  str length      0            for  int i   0  i  lt  str length    i                  if   Character isDigit str charAt i    return false                      return true

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When explanations are more important than performance  I noticed many discussions centering how efficient certain solutions are  but none on why an string is not an integer   Also  everyone seemed to assume that the number  2 00  is not equal to  2   Mathematically and humanly speaking  they are equal  even though computer science says that they are not  and for good reason   This is why the  Integer parseInt  solutions above are weak  depending on your requirements    At any rate  to make software smarter and more human-friendly  we need to create software that thinks like we do and explains why something failed  In this case   public static boolean isIntegerFromDecimalString String possibleInteger    possibleInteger   possibleInteger trim    try          Integer parsing works great for  regular  integers like 42 or 13      int num   Integer parseInt possibleInteger       System out println  The possibleInteger   possibleInteger   is a pure integer         return true    catch  NumberFormatException e        if  possibleInteger equals                 System out println  The possibleInteger     possibleInteger     is NOT an integer because it is only a decimal point             return false        else if  possibleInteger startsWith       amp  amp  possibleInteger matches      0-9                if  possibleInteger matches      0                    System out println  The possibleInteger     possibleInteger     is an integer because it starts with a decimal point and afterwards is all zeros                 return true            else               System out println  The possibleInteger     possibleInteger     is NOT an integer because it starts with a decimal point and afterwards is not all zeros                 return false                  else if  possibleInteger endsWith        amp  amp  possibleInteger matches   0-9                   System out println  The possibleInteger   possibleInteger   is an impure integer  ends with decimal point              return true        else if  possibleInteger contains                 String   partsOfPossibleInteger   possibleInteger split                 if  partsOfPossibleInteger length    2                  System out println  The possibleInteger     possibleInteger     is split into      partsOfPossibleInteger 0       and      partsOfPossibleInteger 1                       if  partsOfPossibleInteger 0  matches   0-9                        if  partsOfPossibleInteger 1  matches   0                            System out println  The possibleInteger   possibleInteger   is an impure integer  ends with all zeros after the decimal point                          return true                    else if  partsOfPossibleInteger 1  matches   0-9                            System out println  The possibleInteger     possibleInteger     is NOT an integer because it the numbers after the decimal point                                       partsOfPossibleInteger 1       are not all zeros                         return false                    else                       System out println  The possibleInteger     possibleInteger     is NOT an integer because it the  numbers  after the decimal point                                   partsOfPossibleInteger 1       are not all numeric digits                         return false                                  else                   System out println  The possibleInteger     possibleInteger     is NOT an integer because it the  number  before the decimal point                               partsOfPossibleInteger 0       is not a number                     return false                          else               System out println  The possibleInteger   possibleInteger   is NOT an integer because it has a strange number of decimal-period separated parts                          partsOfPossibleInteger length                      return false                     else     System out println  The possibleInteger    possibleInteger    is NOT an integer  even though it has no decimal point         return false        Test code   String   testData     0    0     0 0     000    2    2     2 0    2 0000    3 14159     0001          4 0    3E24    6 0221409e 23    int i   0  for  String possibleInteger   testData         System out println          System out println i      possibleInteger      possibleInteger     isIntegerFromDecimalString     isIntegerFromDecimalString possibleInteger        i

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You have it  but you should only catch NumberFormatException

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You may try apache utils  NumberUtils isCreatable myText    See the javadoc here

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If your String array contains pure Integers and Strings  code below should work  You only have to look at first character  e g    4   44   abc   77   bond    if  Character isDigit string charAt 0            Do something with int

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How about   return Pattern matches  -   d    input

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You could   Check that the string is a number Check that it is not too long to be parsed as a long Check if the resulting long value is small enough to be represented by an int   Assuming you for some reason would have to implement this yourself  you should probably first take a look at com google common primitives Ints tryParse String  and see if that is good enough for your purposes  as suggested in another answer       Credit to Rastislav Komara   s answer  https   stackoverflow com a 237895 1725151 private static final Pattern nonZero   Pattern compile  quot  -  1-9   d   quot        See if  str  can be parsed as an  int   does not trim     Strings like  0023  are rejected  leading zeros   public static boolean parsableAsInt  Nonnull String str        if  str isEmpty              return false            if  str equals  quot 0 quot              return true            if  canParseAsLong str             long value   Long valueOf str           if  value  gt   Integer MIN VALUE  amp  amp  value  lt   Integer MAX VALUE                return true                      return false     private static boolean canParseAsLong String str        final int intMaxLength   11      return str length    lt   intMaxLength  amp  amp  nonZero matcher str  matches       This method can also be converted to return Optional lt Integer gt  so that you don   t have to parse the string twice in the client code  once to check if it   s possible and the second time to do it    for real       For example  if  canParseAsLong str         long value   Long valueOf str       if  value  gt   Integer MIN VALUE  amp  amp  value  lt   Integer MAX VALUE            return Optional of  int  value

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I believe there s zero risk running into an exception  because as you can see below you always safely parse int to String and not the other way around  So   You check if every slot of character in your string matches at least one of the characters   quot 0 quot   quot 1 quot   quot 2 quot   quot 3 quot   quot 4 quot   quot 5 quot   quot 6 quot   quot 7 quot   quot 8 quot   quot 9 quot    if aString substring j  j 1  equals String valueOf i      You sum all the times that you encountered in the slots the above characters  digits      And finally you check if the times that you encountered integers as characters equals with the length of the given string  if digits    aString length       And in practice we have      String aString    quot 1234224245 quot       int digits   0   count how many digits you encountered     for int j 0 j lt aString length   j             for int i 0 i lt  9 i                 if aString substring j  j 1  equals String valueOf i                        digits                        if digits    aString length             System out println  quot It s an integer   quot                  else          System out println  quot It s not an integer   quot                   String anotherString    quot 1234f22a4245 quot       int anotherDigits   0   count how many digits you encountered     for int j 0 j lt anotherString length   j             for int i 0 i lt  9 i                 if anotherString substring j  j 1  equals String valueOf i                        anotherDigits                        if anotherDigits    anotherString length             System out println  quot It s an integer   quot                  else          System out println  quot It s not an integer   quot           And the results are   It s an integer   It s not an integer    Similarly  you can validate if a String is a float or a double but in those cases you have to encounter only one    dot  in the String and of course check if  digits     aString length  -1   Again  there s zero risk running into a parsing exception here  but if you plan on parsing a string that it is known that contains a number  let s say int data type  you must first check if it fits in the data type  Otherwise you must cast it   I hope I helped

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Number number  try       number   NumberFormat getInstance   parse  123      catch  ParseException e          not a number - do recovery      e printStackTrace        use number

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Number number  try       number   NumberFormat getInstance   parse  123      catch  ParseException e          not a number - do recovery      e printStackTrace        use number

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You just check NumberFormatException -   String value  123    try            int s Integer parseInt any int val          do something when integer values comes        catch NumberFormatException nfe                      do something when string values comes

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Just one comment about regexp  Every example provided here is wrong   If you want to use regexp don t forget that compiling the pattern take a lot of time  This   str matches   -   d       and also this   Pattern matches  -   d    input     causes compile of pattern in every method call  To used it correctly follow   import java util regex Pattern           author Rastislav Komara     public class NaturalNumberChecker       public static final Pattern PATTERN   Pattern compile     d           boolean isNaturalNumber CharSequence input            return input    null  amp  amp  PATTERN matcher input  matches

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a little improvement to  Jonas K anwser  this function will rule out one single operator like  quot   quot   public boolean isInteger String str           null pointer     if  str    null            return false            int len   str length           empty string     if  len    0            return false               one digit  cannot begin with 0     if  len    1            char c   str charAt 0           if   c  lt   1       c  gt   9                  return false                       for  int i   0  i  lt  len  i              char c   str charAt i              check positive  negative sign         if  i    0                if  c     -     c                           continue                                     check each character matches  0-9          if   c  lt   0       c  gt   9                  return false                      return true

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It partly depend on what you mean by  can be converted to an integer    If you mean  can be converted into an int in Java  then the answer from Jonas is a good start  but doesn t quite finish the job  It would pass 999999999999999999999999999999 for example  I would add the normal try catch call from your own question at the end of the method   The character-by-character checks will efficiently reject  not an integer at all  cases  leaving  it s an integer but Java can t handle it  cases to be caught by the slower exception route  You could do this bit by hand too  but it would be a lot more complicated

[java] What's the best way to check if a String represents an integer in Java?

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