Relationship between hashCode and equals method in Java

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Answer 1

The problem you will have is with collections where unicity of elements is calculated according to both .equals() and .hashCode(), for instance keys in a HashMap.

As its name implies, it relies on hash tables, and hash buckets are a function of the object's .hashCode().

If you have two objects which are .equals(), but have different hash codes, you lose!

The part of the contract here which is important is: objects which are .equals() MUST have the same .hashCode().

This is all documented in the javadoc for Object. And Joshua Bloch says you must do it in Effective Java. Enough said.