Why Java does not allow overriding equals(Object) in an Enum?

java enums api-design
Ashwin Prabhu picture Ashwin Prabhu · Jun 3, 2010 · Viewed 18.1k times · Source

I've noticed that the following snippet...

@Override
public boolean equals(Object otherObject) {
    ...
}

...is not allowed for an Enum, since the method equals(Object x) is defined as final in Enum. Why is this so?

I cannot think of any use case which would require overriding equals(Object) for Enum. I'm just curious to know the reasoning behind this behavior.

Answer

aioobe picture aioobe · Jun 3, 2010

Anything but return this == other would be counter intuitive and violate the principle of least astonishment. Two enum constants are expected to be equal if and only if they are the same object and the ability to override this behavior would be error prone.

Same reasoning applies to hashCode(), clone(), compareTo(Object), name(), ordinal(), and getDeclaringClass().

The JLS does not motivate the choice of making it final, but mentions equals in the context of enums here. Snippet:

The equals method in Enum is a final method that merely invokes super.equals on its argument and returns the result, thus performing an identity comparison.

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