Here is the contract that hashCode needs to satisfy:
‘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(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.’