From effective Java 2nd Edition
Item 9: Always override hashCode when you override equals
A common
source of bugs is the failure to override the hashCode method. You
must override hashCode in every class that overrides equals. Failure
to do so will result in a violation of the general contract for
Object.hashCode, which will prevent your class from functioning
properly in conjunction with all hash-based collections, including
HashMap, HashSet, and Hashtable.
I suggest you read that chapter. There are a lot of examples, that you can learn, what will happen if you don't do so.
