Is there any reason to override methods in enums in Java 8

Question

As pointed out here lambdas provide a quite elegant way of specifying behaviour for individual enum values.

Before Java 8 I would normally have implemented this as:

enum Operator {
    TIMES {
        public int operate(int n1, int n2) {
            return n1 * n2;
        }
    },
    PLUS {
        public int operate(int n1, int n2) {
            return n1 + n2;
        }
    };

    public int operate(int n1, int n2) {
        throw new AssertionError();
    }            
}

Now I tend to use:

enum Operator {
    TIMES((n1, n2) -> n1 * n2),
    PLUS((n1, n2) -> n1 + n2);

    private final BinaryOperator<Integer> operation;

    private Operator(BinaryOperator<Integer> operation) {
        this.operation = operation;
    }

    public int operate(int n1, int n2) {
        return operation.apply(n1, n2);
    }
}

This seems significantly more elegant.

I cannot think of a reason to override methods for a specific enum value now. So my question is, are there any good reasons to use method overrides in an enum now or should a functional interface always be preferred?

Answer 1

If you look at this answer which summarizes the advantages of using lambda expression in this enum scenario you might notice that these advantages all disappear in the pre-Java 8 variant. It's neither more readable than the old specialized enum variant nor does it improve the performance. Further, the interface BinaryOperator doesn't exist before Java 8 so it's another class you would need to add to your codebase to follow this approach.

The main reason to use this delegation approach in pre-Java 8 code would be to ease the migration if you plan to switch to Java 8 soon.

---

Update to your updated question:

If you mainly focus on the Java 8 use case, I would recommend to always use the delegation approach when all enum cases have a different behavior which still follows a similar pattern which can benefit from using lambda expressions, as it's the case when implementing operators like in your example.

A counter-example would be an enum where most of them share a common behavior that will be overridden for one or a few cases only. Eg:

enum Tokens {
    FOO, BAR, BAZ, AND, A, LOT, MORE // etc …

    /** Special Token End-Of-File */
    EOF {
        @Override
        public boolean matches(String input, int pos) {
            return input.length()==pos;
        }
    };

    // all ordinary tokens have the same behavior
    public boolean matches(String input, int pos) {
        return input.length()-pos >= name().length()
          && input.regionMatches(pos, name(), 0, name().length());
    }
}