equals vs Arrays equals in Java

Question

When comparing arrays in Java  are there any differences between the following 2 statements   Object   array1  array2  array1 equals array2   Arrays equals array1  array2     And if so  what are they

User · Answer

Sigh  Back in the 70s I was the  system programmer   sysadmin  for an IBM 370 system  and my employer was a member of the IBM users group SHARE  It would sometimes happen thatsomebody submitted an APAR  bug report  on some unexpected behavior of some CMS command  and IBM would respond NOTABUG  the command does what it was designed to do  and what the documentation says    SHARE came up with a counter to this  BAD -- Broken As Designed  I think this might apply to this implementation of equals for arrays   There s nothing wrong with the implementation of Object equals  Object has no data members  so there is nothing to compare  Two  Object s are equal if and only if they are  in fact  the same Object  internally  the same address and length    But that logic doesn t apply to arrays  Arrays have data  and you expect comparison  via equals  to compare the data  Ideally  the way Arrays deepEquals does  but at least the way Arrays equals does  shallow comparison of the elements    So the problem is that array  as a built-in object  does not override Object equals  String  as a named class  does override Object equals and give the result you expect   Other answers given are correct        equals         simply compares the pointers and not the contents  Maybe someday somebody will correct this  Or maybe not  how many existing programs would break if       equals actually compared the elements  Not many  I suspect  but more than zero

User · Answer

It s an infamous problem   equals   for arrays is badly broken  just don t use it  ever   That said  it s not  broken  as in  someone has done it in a really wrong way      it s just doing what s defined and not what s usually expected  So for purists  it s perfectly fine  and that also means  don t use it  ever   Now the expected behaviour for equals is to compare data  The default behaviour is to compare the identity  as Object does not have any data  for purists  yes it has  but it s not the point   assumption is  if you need equals in subclasses  you ll implement it  In arrays  there s no implementation for you  so you re not supposed to use it   So the difference is  Arrays equals array1  array2  works as you would expect  i e  compares content   array1 equals array2  falls back to Object equals implementation  which in turn compares identity  and thus better replaced by     for purists  yes I know about null    Problem is  even Arrays equals array1  array2  will bite you hard if elements of array do not implement equals properly  It s a very naive statement  I know  but there s a very important less-than-obvious case  consider a 2D array    2D array in Java is an array of arrays  and arrays  equals is broken  or useless if you prefer   so Arrays equals array1  array2  will not work as you expect on 2D arrays   Hope that helps

User · Answer

The equals   of arrays is inherited from Object  so it does not look at the contents of the arrrays  it only considers each array equal to itself   The Arrays equals   methods do compare the arrays  contents  There s overloads for all primitive types  and the one for objects uses the objects  own equals   methods

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The Arrays equals array1  array2      check if both arrays contain the same number of elements  and all corresponding pairs of elements in the two arrays are equal   The array1 equals array2     compare the object to another object and return true only if the reference of the two object are equal as in the Object equals

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import java util Arrays  public class ArrayDemo      public static void main String   args          initializing three object arrays    Object   array1   new Object     1  123       Object   array2   new Object     1  123  22  4       Object   array3   new Object     1  123           comparing array1 and array2    boolean retval Arrays equals array1  array2      System out println  array1 and array2 equal      retval      System out println  array1 and array2 equal      array1 equals array2           comparing array1 and array3    boolean retval2 Arrays equals array1  array3      System out println  array1 and array3 equal      retval2      System out println  array1 and array3 equal      array1 equals array3              Here is the output       array1 and array2 equal  false     array1 and array2 equal  false      array1 and array3 equal  true     array1 and array3 equal  false   Seeing this kind of problem I would personally go for Arrays equals array1  array2  as per your question to avoid confusion

User · Answer

Arrays inherit equals   from Object and hence compare only returns true if comparing an array against itself   On the other hand  Arrays equals compares the elements of the arrays   This snippet elucidates the difference   Object o1   new Object    Object o2   new Object    Object   a1     o1  o2    Object   a2     o1  o2    System out println a1 equals a2       prints false System out println Arrays equals a1  a2       prints true   See also Arrays equals    Another static method there may also be of interest  Arrays deepEquals

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Look inside the implementation of the two methods to understand them deeply   array1 equals array2              Indicates whether some other object is  equal to  this one      lt p gt     The   code equals  method implements an equivalence relation    on non-null object references      lt ul gt      lt li gt It is  lt i gt reflexive lt  i gt   for any non-null reference value          code x     code x equals x   should return          code true       lt li gt It is  lt i gt symmetric lt  i gt   for any non-null reference values          code x  and   code y     code x equals y          should return   code true  if and only if          code y equals x   returns   code true       lt li gt It is  lt i gt transitive lt  i gt   for any non-null reference values          code x     code y   and   code z   if          code x equals y   returns   code true  and          code y equals z   returns   code true   then          code x equals z   should return   code true       lt li gt It is  lt i gt consistent lt  i gt   for any non-null reference values          code x  and   code y   multiple invocations of          code x equals y   consistently return   code true         or consistently return   code false   provided no        information used in   code equals  comparisons on the        objects is modified      lt li gt For any non-null reference value   code x            code x equals null   should return   code false       lt  ul gt      lt p gt     The   code equals  method for class   code Object  implements    the most discriminating possible equivalence relation on objects     that is  for any non-null reference values   code x  and      code y   this method returns   code true  if and only    if   code x  and   code y  refer to the same object       code x    y  has the value   code true        lt p gt     Note that it is generally necessary to override the   code hashCode     method whenever this method is overridden  so as to maintain the    general contract for the   code hashCode  method  which states    that equal objects must have equal hash codes         param   obj   the reference object with which to compare      return    code true  if this object is the same as the obj             argument    code false  otherwise      see      hashCode       see     java util HashMap     public boolean equals Object obj        return  this    obj       while   Arrays equals array1  array2              Returns  lt tt gt true lt  tt gt  if the two specified arrays of Objects are     lt i gt equal lt  i gt  to one another   The two arrays are considered equal if    both arrays contain the same number of elements  and all corresponding    pairs of elements in the two arrays are equal   Two objects  lt tt gt e1 lt  tt gt     and  lt tt gt e2 lt  tt gt  are considered  lt i gt equal lt  i gt  if  lt tt gt  e1  null   e2  null      e1 equals e2   lt  tt gt    In other words  the two arrays are equal if    they contain the same elements in the same order   Also  two array    references are considered equal if both are  lt tt gt null lt  tt gt   lt p gt         param a one array to be tested for equality     param a2 the other array to be tested for equality     return  lt tt gt true lt  tt gt  if the two arrays are equal     public static boolean equals Object   a  Object   a2        if  a  a2          return true      if  a  null    a2  null          return false       int length   a length      if  a2 length    length          return false       for  int i 0  i lt length  i              Object o1   a i           Object o2   a2 i           if    o1  null   o2  null   o1 equals o2                return false             return true

User · Answer

array1 equals array2  is the same as array1    array2  i e  is it the same array  As  alf points out it s not what most people expect   Arrays equals array1  array2  compares the contents of the arrays     Similarly array toString   may not be very useful and you need to use Arrays toString array

[java] equals vs Arrays.equals in Java

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