When do you override hashCode() and equals()?

15 Oct , 2016  

hashCode() and equals() methods have been defined in Object class which is parent class for java objects. For this reason, all java objects inherit a default implementation of these methods.

hashCode() method is used to get a unique integer for given object. This integer is used for determining the bucket location, when this object needs to be stored in some HashTable like data structure. By default, Object’s hashCode()method returns and integer representation of memory address where object is stored.
equals() method, as name suggest, is used to simply verify the equality of two objects. Default implementation simply check the object references of two objects to verify their equality.

Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode() method, which states that equal objects must have equal hash codes.

  • equals()must define an equality relation (it must be reflexive, symmetric and transitive). In addition, it must be consistent (if the objects are not modified, then it must keep returning the same value). Furthermore,equals(null) must always return false.
  • hashCode()must also be consistent (if the object is not modified in terms of equals(), it must keep returning the same value).

The relation between the two methods is:

Whenever a.equals(b) then a.hashCode() must be same as b.hashCode().

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