In Java, why must equals() and hashCode() be consistent?

Question

If I override either method on a class, it must make sure that if A.equals(B) == true then A.hashCode() == B.hashCode must also be true.

Can

Answer 1

It's exactly because of hash tables.

Because of the possibility of hash code collisions, hash tables need to check identity as well, otherwise the table can't determine if it found the object it was looking for, or one with the same hash code. So every get() in a hash table calls key.equals(potentialMatch) before returning a value.

If equals() and hashCode() are inconsistent you can get very inconsistent behavior. Say for two objects, a and b, a.equals(b) returns true, but a.hashCode() != b.hashCode(). Insert a and a HashSet will return false for .contains(b), but a List created from that set will return true (because the list doesn't use hash codes).

HashSet set = new HashSet();
set.add(a);
set.contains(b); // false
new ArrayList(set).contains(b); // true

Obviously, that could be bad.