How to implement enum with generics?

Question

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?

Answer 1

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.