Can I make an abstract enum in Java?

Question

Below is a valid enum declaration.

public enum SomeEnumClass {

    ONE(1), TWO(2), THREE(3);

    private int someInt;

    public SomeEnumClass(int someInt         


        

Answer 1

If you really need to "extend an enum", you could use the the pre-Java 1.5 Typesafe Enum Pattern (see the bottom of http://www.javacamp.org/designPattern/enum.html ) which actually uses a class, not an enum. You lose the ability to use the EnumSet with your "enum"s and you lose some auto-generated methods such as items(), but you get the ability to override methods.

An example:

// Typesafe enum pattern
public static abstract class Operator {
    public static final Operator ADD = new Operator("+") {
        @Override
        public Double apply(Double firstParam, Double secondParam) {
            return firstParam + secondParam;
        }
    };
    public static final Operator SUB = new Operator("-") {
        @Override
        public Double apply(Double firstParam, Double secondParam) {
            return firstParam - secondParam;
        }
    };
    public static final Operator MUL = new Operator("*") {
        @Override
        public Double apply(Double firstParam, Double secondParam) {
            return firstParam * secondParam;
        }
    };
    public static final Operator DIV = new Operator("/") {
        @Override
        public Double apply(Double firstParam, Double secondParam) {
            return firstParam / secondParam;
        }
    };

    private static final Operator[] _ALL_VALUES = {ADD, SUB, MUL, DIV};
    private static final List ALL_VALUES = Collections.unmodifiableList(Arrays.asList(_ALL_VALUES));

    private final String operation;

    private Operator(String c) {
        operation = c;
    }

    // Factory method pattern
    public static Operator fromToken(String operation) {
        for (Operator o : Operator.items())
            if (o.operation.equals(operation))
                return o;
        return null;
    }

    public Iterable items() {
        return ALL_VALUES;
    }

    public abstract Double apply(Double firstParam, Double secondParam); 

}