Question

I truly understand that it is not able to extend any class to implement an enum, since this would mean to have multiple inheritance. But what I don’t understand is, why the enum classes, created by the compiler when using “enum” instead of “class” are final.

Is there any good reason why enums cannot be extended?

Plus: Is there any way to implement common behavior to different enums without Copy&Paste?

Answer 1

A very important aspect of enums is that the number or values are known at compile time (at least at compile time of the enum).

So if you have an enum like this:

public enum Foo {
  BAR,
  BAZ;
}

then you know that it has exactly two values: BAR and BAZ.

If you could extend an enum then you could introduce an ExtendedFoo enum that adds QUUX. Now since ExtendedFoo would be a Foo, QUUX would suddenly be a valid Foo value. And nothing would stop you from adding any number of additional Foo values.

This means that the compiler can no longer check if your switch covers all Foo cases. Similar static analysis steps are also no longer possible. Effectively your enum would cease to be a special type and there would be very little difference to a normal class with some pre-defined instances.

Side note: the generated enum class is not always final: If (at least) one of your enum values has a value-specific body, then the “base” class will not be final, but the compiler will still prevent you from extending it.