equals and hashCode

I see this mentioned on blogs fairly regularly, and it's always a surprise for people when they learn you have to override hashCode if you override equals.

In short: If you override equals, you must override hashCode.

(From the Javadoc comments from java.lang.Object)
The general contract of hashCode 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.

Here are a couple of references:

• java.lang.Object Javadoc
• JavaWorld article from 1999 on equals and hashCode
• Here is another link (from a Geocities site) explaining it.

Posted by rcleveng at March 3, 2005 03:09 PM

Comments