How to implement enum with generics?

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You can’t. Java doesn’t allow generic types on enum constants. They are allowed on enum types, though:

public enum B implements A<String> {
  A1, A2;
}

What you could do in this case is either have an enum type for each generic type, or ‘fake’ having an enum by just making it a class:

public class B<T> implements A<T> {
    public static final B<String> A1 = new B<String>();
    public static final B<Integer> A2 = new B<Integer>();
    private B() {};
}

Unfortunately, they both have drawbacks.

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