Impl.java

ExampleEventListener.EVENT.register(outerEvent -> {
  if (event.parameter.isDoingIt) {
    outerEvent.setPayload(innerEvent -> {
      innerEvent.parameter.doTheThingWith(outerEvent.world);
    });
    return EventResult.EXECUTE;
  }
  return EventResult.CANCEL;
});

Listener.java

public interface ExampleEventListerner {
  Event<ExampleEventListerner> EVENT = PayloadedEventFactory.createArrayBacked(
    ExampleEventListerner.Determinator.class,
    ExampleEventListener.Payload.class,
    (determinators) -> (event) -> {},    
    (payloads) -> (event) -> {}
  );
  
  interface Determinator {
    EventResult onExampleEvent(Event event);
  }
  
  interface Payload {
    void onExampleEvent(Event event);
  }
}