Can an enum have abstract methods?

Question

Can an enum have abstract methods? If so, what is the use, and give a scenario which will illustrate this usage.

Answer 1

Yes enum can contain abstract method - you can use this approach if you have own implementation per enum constants. (or you can omit using abstract method with using customized value per enum constant Enum with customized value in Java)

for example (constant-specific method implementations)

enum TrafficSignal {

    RED{
        @Override
        public void action(){
            System.out.println("STOP");
        }
    }, 
    GREEN{
        @Override
        public void action(){
            System.out.println("GO");
        }
    }, 
    ORANGE{
        @Override
        public void action(){
            System.out.println("SLOW DOWN");
        }
    };


    public abstract void action();
}

If this method is common for all enum constants then consider using interface. (Java enums extend the java.lang.Enum generic class implicitly, so your enum types cannot extend another class(becouse java does not support multiple inheritance) but can implement interface)

interface ColorInterface{
    void inLowerCase();
}

enum Color implements ColorInterface{

    RED, GREEN, ORANGE;

    @Override
    public void inLowerCase(){
        System.out.println(this.toString().toLowerCase());
    }
}

Usage:

public class TestEnums{

     public static void main(String []args){
        TrafficSignal signal = TrafficSignal.RED;
        signal.action();

        ColorInterface color = Color.RED;
        color.inLowerCase();
     }
}

Out:

STOP
red

Consider the strategy enum pattern if some, but not all, enum constants share common behaviors. [Effective Java - Joshua Bloch - third edition. page 166]