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@rjrjr
Last active March 4, 2023 22:10
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Poor Man's Sealed Classes (visitor pattern)
/**
* 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"));
}
}
@rjrjr
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rjrjr commented Sep 28, 2017

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

@rjrjr
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rjrjr commented Sep 29, 2017

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

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

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

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