How to implement enum with generics?

enums, generics, interface, java

Solution

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.

Problem

I have a generic interface like this: ``` interface A<T> { T getValue(); } ``` This interface has limited instances, hence it would be best to implement them as enum values. The problem is those instances have different type of values, so I tried the following approach but it does not compile: ``` public enum B implements A { A1<String> { @Override public String getValue() { return "value"; } }, A2<Integer> { @Override public Integer getValue() { return 0; } }; } ``` Any idea about this?

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