overriding hashcode and equals.

[Aneek Banerjee]

Is it always necessary to override hashcode if I try to override equals?

[Matthew Brown]

You should do, yes. The contract says that if two objects are equal then they should have the same hash code (but not necessarily the other way round). So if you redefine equals() you should redefine hashCode() to match.

[Jeanne Boyarsky]

Not "you should.". You "must."

The JavaDoc says

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.

If you don't override hashCode when you've overwritten equals, you will get unpredictable behavior at times. T he only exception is when you "overwrite" equals with:

public boolean equals(Object o) {
  super.equals();
}

Do you know why this is a special case?