Why would an Enum implement an Interface?

java enums
unj2 picture unj2 · Apr 25, 2010 · Viewed 163.2k times · Source

I just found out that Java allows enums to implement an interface. What would be a good use case for that?

Answer

helpermethod picture helpermethod · Apr 25, 2010

Here's one example (a similar/better one is found in Effective Java 2nd Edition):

public interface Operator {
    int apply (int a, int b);
}

public enum SimpleOperators implements Operator {
    PLUS { 
        int apply(int a, int b) { return a + b; }
    },
    MINUS { 
        int apply(int a, int b) { return a - b; }
    };
}

public enum ComplexOperators implements Operator {
    // can't think of an example right now :-/
}

Now to get a list of both the Simple + Complex Operators:

List<Operator> operators = new ArrayList<Operator>();

operators.addAll(Arrays.asList(SimpleOperators.values()));
operators.addAll(Arrays.asList(ComplexOperators.values()));

So here you use an interface to simulate extensible enums (which wouldn't be possible without using an interface).

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