double-dispatch.java

import com.librato.alerts.IEmitter;

public class DoubleDispatch {
    static IEmitter emitter;

    public static void main(String[] args) {
        IMessage messageA = new MessageA();
        IMessage messageB = new MessageB();

        // without double dispatch
        emitter.emit(new Payload(messageA));
        emitter.emit(new Payload(messageB));

        // with double dispatch
        messageA.emit(emitter);
        messageB.emit(emitter);
    }

    static class Payload {
        public Payload(IMessage message) {
            // has to figure out what data to pull out of message
        }

        public Payload(MessageA message) {
            // initializes using specific MessageA data
        }

        public Payload(MessageB message) {
            // initializes using specific MessageB data
        }
    }

    interface IEmitter {
        void emit(Payload payload);
    }

    /*
     * Two implementations of IMessage, both know how to emit themselves
     * to an IEmitter
     */

    interface IMessage {
        void emit(IEmitter emitter);
    }

    static class MessageA implements IMessage {
        @Override
        public void emit(IEmitter emitter) {
            // emit, but build the payload based on the concrete type
            emitter.emit(new Payload(this));
        }


    }

    static class MessageB implements IMessage {
        @Override
        public void emit(IEmitter emitter) {
            // emit, but build the payload based on the concrete type
            emitter.emit(new Payload(this));
        }
    }

}