Wednesday, May 14, 2008

While overriding equals() method which other method should also be considered?

It’s the hashCode() method, which returns a hash code value for every object and that value is used for the benefit of hashtables such as java.util.Hashtable. The general contract of the hashCode mentioned in the Java Language Specification is:-

  • Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
  • If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.
  • It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables.
The default implementation of the hashCode() method in the class ‘Object’ doesn’t return equals values for two distinct objects. This is typically implemented by converting the internal address of the object into an integer as what the object may contain as its value is not known at ‘Object’ level. The only thing known is that irrespective of what the object will contain, it’ll always have a memory address to reside at :-)


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