compareTo vs equals

Question

When testing for equality of String s in Java I have always used equals   because to me this seems to be the most natural method for it  After all  its name already says what it is intended to do  However  a colleague of mine recently told me had been taught to use compareTo      0 instead of equals    This feels unnatural  as compareTo   is meant to provide an ordering and not compare for equality  and even somewhat dangerous  because compareTo      0 does not necessarily imply equality in all cases  even though I know it does for String s  to me   He did not know why he was taught to use compareTo   instead of equals   for String s  and I could also not find any reason why  Is this really a matter of personal taste  or is there any real reason for either method

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equals   should be the method of choice in the case of the OP   Looking at the implementation of equals   and compareTo   in java lang String on grepcode  we can easily see that equals is better if we are just concerned with the equality of two Strings   equals     1012  public boolean equals Object anObject   1013      if  this    anObject   1014          return true 1015       1016      if  anObject instanceof String   1017          String anotherString    String anObject 1018          int n   count 1019          if  n    anotherString count   1020              char v1     value 1021              char v2     anotherString value 1022              int i   offset 1023              int j   anotherString offset 1024              while  n--    0   1025                  if  v1 i       v2 j    1026                      return false 1027               1028              return true 1029           1030       1031      return false 1032     and compareTo     1174  public int compareTo String anotherString   1175      int len1   count 1176      int len2   anotherString count 1177      int n   Math min len1  len2  1178      char v1     value 1179      char v2     anotherString value 1180      int i   offset 1181      int j   anotherString offset 1183      if  i    j   1184          int k   i 1185          int lim   n   i 1186          while  k  lt  lim   1187              char c1   v1 k  1188              char c2   v2 k  1189              if  c1    c2   1190                  return c1 - c2 1191               1192              k   1193           1194        else  1195          while  n--    0   1196              char c1   v1 i    1197              char c2   v2 j    1198              if  c1    c2   1199                  return c1 - c2 1200               1201           1202       1203      return len1 - len2 1204     When one of the strings is a prefix of another  the performance of compareTo   is worse as it still needs to determine the lexicographical ordering while equals   won t worry any more and return false immediately   In my opinion  we should use these two as they were intended    equals   to check for equality  and compareTo   to find the lexical ordering

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I believe equals and equalsIgnoreCase methods of String return true and false which is useful if you wanted to compare the values of the string object  But in case of implementing compareTo and compareToIgnoreCase methods returns positive  negative and zero value which will be useful in case of sorting

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equals      required for checking equality and restricting duplicates  Many classes of Java Library use this in case they wanted to find duplicates  e g  HashSet add ob1  will only add if that doesn t exist  So if you are extending some classes like this then override equals    compareTo        required for ordering of element  Again for stable sorting you require equality  so there is a return 0

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The 2 main differences are that    equals will take any Object as a parameter  but compareTo will only take Strings  equals only tells you whether they re equal or not  but compareTo gives information on how the Strings compare lexicographically    I took a look at the String class code  and the algorithm within compareTo and equals looks basically the same   I believe his opinion was just a matter of taste  and I agree with you -- if all you need to know is the equality of the Strings and not which one comes first lexicographically  then I would use equals

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equals   can be more efficient then compareTo      A very important difference between compareTo and equals    myString  compareTo null      Throws java lang NullPointerException  myString  equals null         Returns false   equals   checks if two objects are the same or not and returns a boolean    compareTo    from interface Comparable  returns an integer  It checks which of the two objects is  less than    equal to  or  greater than  the other  Not all objects can be logically ordered  so a compareTo   method doesn t always make sense    Note that equals   doesn t define the ordering between objects  which compareTo   does    Now I advise you to review the source code of both methods to conclude that equals is preferable over compareTo that involves some Math calculations

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This is an experiment in necromancy  -   Most answers compare performance and API differences  They miss the fundamental point that the two operations simply have different semantics   Your intuition is correct  x equals y  is not interchangeable with x compareTo y     0  The first compares identity  while the other compares the notion of  size   It is true that in many cases  especially with primitive types  these two co-align   The general case is this   If x and y are identical  they share the same  size   if x equals y  is true    x compareTo y  is 0    However  if x and y share the same size  it does not mean they are identical   if x compareTo y  is 0 does not necessarily mean x equals y  is true   A compelling example where identity differs from size would be complex numbers  Assume that the comparison is done by their absolute value  So given two complex numbers  Z1   a1   b1 i and Z2   a2   b2 i   Z1 equals z2  returns true if and only if a1   a2 and b1   b2   However Z1 compareTo Z2  returns 0 for and infinite number of  a1 b1  and  a2 b2  pairs as long as they satisfy the condition a1 2   b1 2    a2 2   b2 2

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equals  compare objects and return true or false and   compare to  return 0 if is true or  an number      0  or   lt  0  if is false here an example    lt  -- language  lang-java -- gt    Objects Integer Integer num1   1  Integer num2   1    equal System out println num1 equals num2    System out println num1 compareTo num2      New Value num2   3   set value   diferent System out println num1 equals num2    System out println num1 compareTo num2      Results    num1 equals num2   true num1 compareTo num2   0 num1 equals num2   false num1 compareTo num2   -1   Documentation Compare to  https   docs oracle com javase 7 docs api java lang Comparable html  Documentation Equals   https   docs oracle com javase 7 docs api java lang Object html equals java lang Object

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String s1    a   String s2    c    System out println s1 compareTo s2    System out println s1 equals s2      This prints -2 and false  String s1    c   String s2    a   System out println s1 compareTo s2    System out println s1 equals s2      This prints 2 and false  String s1    c   String s2    c   System out println s1 compareTo s2    System out println s1 equals s2      This prints 0 and true  equals returns boolean if and only if both strings match   compareTo is meant to not just tell if they match but also to tell which String is lesser than the other  and also by how much  lexicographically  This is mostly used while sorting in collection

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There are certain things which you need to keep in mind while overriding compareTo in Java e g  Compareto must be consistent with equals and subtraction should not be used for comparing integer fields as they can overflow  check Things to remember while overriding Comparator in Java for details

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Equals can be more efficient then compareTo   If the length of the character sequences in String doesn t match there is no way the Strings are equal so rejection can be much faster   Moreover if it is same object  identity equality rather then logical equality   it will also be more efficient   If they also implemented hashCode caching it could be even faster to reject non-equals in case their hashCode s doesn t match

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equals   checks whether two strings are equal or not It gives boolean value  compareTo   checks whether string object is equal to greater or smaller to the other string object It gives result as   1 if string object is greater 0 if both are equal -1 if string is smaller than other string  eq   String a    Amit   String b    Sumit   String c   new String  Amit    System out println a equals c     true System out println a compareTo c      0 System out println a compareTo b      1

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compareTo has do do more work if the strings have different lengths  equals can just return false  while compareTo must always examine enough characters to find the sorting order

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It appears that both methods pretty much do the same thing  but the compareTo   method takes in a String  not an Object  and adds some extra functionality on top of the normal equals   method  If all you care about is equality  then the equals   method is the best choice  simply because it makes more sense to the next programmer that takes a look at your code  The time difference between the two different functions shouldn t matter unless you re looping over some huge amount of items  The compareTo   is really useful when you need to know the order of Strings in a collection or when you need to know the difference in length between strings that start with the same sequence of characters   source  http   java sun com javase 6 docs api java lang String html

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compareTo   not only applies to Strings but also any other object because compareTo lt T gt  takes a generic argument T  String is one of the classes that has implemented the compareTo   method by implementing the Comparable interface  compareTo   is a method fo the comparable Interface   So any class is free to implement the Comparable interface    But compareTo   gives the ordering of objects  used typically in sorting objects in ascending or descending order while equals   will only talk about the equality and say whether they are equal or not

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When comparing for equality you should use equals    because it expresses your intent in a clear way   compareTo   has the additional drawback that it only works on objects that implement the Comparable interface   This applies in general  not only for Strings

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Equals -  1- Override the GetHashCode method to allow a type to work correctly in a hash table   2- Do not throw an exception in the implementation of an Equals method  Instead  return false for a null argument   3-    x Equals x  returns true     x Equals y  returns the same value as y Equals x       x Equals y   amp  amp  y Equals z   returns true if and only if x Equals z  returns true    Successive invocations of x Equals y  return the same value as long as the object referenced by x and y are not modified   x Equals null  returns false    4- For some kinds of objects  it is desirable to have Equals test for value equality instead of referential equality  Such implementations of Equals return true if the two objects have the same value  even if they are not the same instance   For Example -     Object obj1   new Object       Object obj2   new Object       Console WriteLine obj1 Equals obj2       obj1   obj2      Console WriteLine obj1 Equals obj2       Output  -  False True   while compareTo -  Compares the current instance with another object of the same type and returns an integer that indicates whether the current instance precedes  follows  or occurs in the same position in the sort order as the other object   It returns -  Less than zero - This instance precedes obj in the sort order  Zero - This instance occurs in the same position in the sort order as obj  Greater than zero - This instance follows obj in the sort order   It can throw ArgumentException if object is not the same type as instance   For example you can visit here   So I suggest better to use Equals in place of compareTo

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A difference is that  foo  equals  String null  returns false while  foo  compareTo  String null     0 throws a NullPointerException  So they are not always interchangeable even for Strings

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Here one thing is important while using compareTo   over equals   that compareTo works for the classes that implements  Comparable  interface otherwise it will throw a NullPointerException  String classes implements Comparable interface while StringBuffer does not hence you can use  foo  compareTo  doo   in String object but not in StringBuffer Object

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In String Context  compareTo  Compares two strings lexicographically  equals  Compares this string to the specified object   compareTo compares two strings by their characters  at same index  and returns an integer  positive or negative  accordingly   String s1    ab   String s2    ab   String s3    qb   s1 compareTo s2      is 0 s1 compareTo s3      is -16 s3 compareTo s1      is 16

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equals can take any Object as a parameter but compareTo can only take String  when cometo null compareTo will throw a exception when you want to know where the diff happen you can use compareTo

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String equals   requires invoking instanceof operator while compareTo   requires not  My colleague has noted large performance drop-down caused by excessive numbers of instanceof calls in equals   method  however my test has proved compareTo   to be only slightly faster    I was using  however  Java 1 6  On other versions  or other JDK vendors  the difference could be larger   The test compared each-to-each string in 1000 element arrays  repeated 10 times

[java] compareTo() vs. equals()

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