Poor Man's Sealed Classes (visitor pattern)

java-sealed-class

/**
 * For Java developers all excited by and jealous of Kotlin's sealed classes.
 * Do this when you wish you could put parameters on an enum.
 */
public class PoorMan {
  interface Event {
    <T> T dispatch(EventHandler<T> handler);
  }

  interface EventHandler<T> {
    T onEvent(ThingHappened event);

    T onEvent(AnotherThingHappened event);

    T onEvent(AThingWithParametersHappened event);
  }

  static final class ThingHappened implements Event {
    static final ThingHappened INSTANCE = new ThingHappened();

    private ThingHappened() {
    }

    @Override public <T> T dispatch(EventHandler<T> handler) {
      return handler.onEvent(this);
    }
  }

  static final class AnotherThingHappened implements Event {
    static final AnotherThingHappened INSTANCE = new AnotherThingHappened();

    private AnotherThingHappened() {
    }

    @Override public <T> T dispatch(EventHandler<T> handler) {
      return handler.onEvent(this);
    }
  }

  static final class AThingWithParametersHappened implements Event {
    final String foo;
    final String bar;

    AThingWithParametersHappened(String foo, String bar) {
      this.foo = foo;
      this.bar = bar;
    }

    @Override public <T> T dispatch(EventHandler<T> handler) {
      return handler.onEvent(this);
    }
  }

  static void omgAnEvent(Event event) {
    String whatHappened = event.dispatch(new EventHandler<String>() {
      @Override public String onEvent(ThingHappened event) {
        return "A Thing";
      }

      @Override public String onEvent(AnotherThingHappened event) {
        return "A Nother Thing";
      }

      @Override public String onEvent(AThingWithParametersHappened event) {
        return "Foo: " + event.foo + ", Bar: " + event.bar;
      }
    });

    System.out.println(whatHappened);
  }

  public static void main(String... args) {
    omgAnEvent(ThingHappened.INSTANCE);
    omgAnEvent(AnotherThingHappened.INSTANCE);
    omgAnEvent(new AThingWithParametersHappened("FOOOOO!", "bar"));
  }
}

The omgAnEvent method illustrates what you do instead of a when / switch statement.

Updated to a) compile and b) add return values, to make this useful for pipelines.

@rjrj Why not make the interface event private? It can't be instantiated from outside the file then.

Please rename the file to JavaSealedClass.java instead for code highlighting and better readability.