Java Community | Help. Code. Learn.•4w ago

Map with Generics

I got a class called EnchantmentStack<E extends Event>. And in that class there is a method called trigger(E event) And when you create a new EnchantmentStack, it gets stored in a map like this: Map<Event, List<EnchantmentStack<?>>> And when a certain event happens. I get all the events and loop in them calling the trigger method. But it won't work because it's not type safe and the generic is unknown <?>. So is there a way to solve it? Like I could call them and trigger them with the known event

39 Replies

dan1st•4w ago

basically you'd have to cast it

<E extends Event> void triggerListeners(E event) {
  List<EnchantmentStack<E>> listeners = (List<EnchantmentStack<E>>) listenerMap.get(event);
  for(EnchantmentStack<E> stack : listeners) {
    stack.trigger(event);
  }
}

<E extends Event> void triggerListeners(E event) {
  List<EnchantmentStack<E>> listeners = (List<EnchantmentStack<E>>) listenerMap.get(event);
  for(EnchantmentStack<E> stack : listeners) {
    stack.trigger(event);
  }
}

<E extends Event> void triggerListeners(E event) {
  List<EnchantmentStack<?>> listeners = listenerMap.get(event);
  for(EnchantmentStack<?> stack : listeners) {
    stack.trigger(event);
  }
}
private <E> void doTrigger(EnchantmentStack<E> stack, Event event) {
  stack.trigger((E)event);
}

<E extends Event> void triggerListeners(E event) {
  List<EnchantmentStack<?>> listeners = listenerMap.get(event);
  for(EnchantmentStack<?> stack : listeners) {
    stack.trigger(event);
  }
}
private <E> void doTrigger(EnchantmentStack<E> stack, Event event) {
  stack.trigger((E)event);
}

but do you really want events as the keys and not event types?

MDT 168OP•4w ago

The problem is that the map is static, and the method that triggers all of them is static public static void triggerEnchants(Event event) { for (EnchantmentStack<?> stack : triggers.get(event)) { stack.trigger(event); // This doesn't work } } So you can't really use generics for it

dan1st•4w ago

That's why I put casts in the code

MDT 168OP•4w ago

So casting is enough?

dan1st•4w ago

if you do it like that, it would work but it isn't type safe

MDT 168OP•4w ago

I didn't use generics much before. But I wanted to make a chance in my system So it may make errors if the Generic type and the event doesn't match?

dan1st•4w ago

you can't do it in a type-safe way just with a Map If the Map only contains entries where the key matches the value, no

MDT 168OP•4w ago

MDT 168OP•4w ago

It's like this. BlockDropItemEvent

dan1st•4w ago

If the register method ensures the event matches the listener, you wouldn't get the errors

MDT 168OP•4w ago

Okay good. But what is better. Do I store the event itselfEvent or do I store the Class<? extends Event>

dan1st•4w ago

that's what I mentioned before - if you use the Event as the key, the listeners would be for that specific event only so if you have data associated with the event, it would need to be the exact same data If you use the Class as the key, it would be the type of event

MDT 168OP•4w ago

Ooooo. I think I should use the Class for the Key,

Madjosz•4w ago

We are using the same construction for event listeners in our codebase so this is a common pattern. Unfortunately the Java type system is not powerful enough to have such associated generics to have the call typesafe without casting.

MDT 168OP•4w ago

The Event got data yes. So it will always be null when I try to get from the map right? I should use the Class It will be safe if I disturbute the enchantment stacks and the events correctly

Madjosz•4w ago

It is safe when you are caring for the correct type while putting/adding.

MDT 168OP•4w ago

This is the method

MDT 168OP•4w ago

This is the method

MDT 168OP•4w ago

Should this method work?

MDT 168OP•4w ago

dan1st•4w ago

I think it is possible with a small trick but not practical i.e. you'd need a different Map implementation that allows you to get the entry (which needs to be of a custom class) for any given key

Madjosz•4w ago

You are thinking of reified generics?

dan1st•4w ago

Madjosz•4w ago

This just sounds like hiding the unsafe cast behind another method.

MDT 168OP•4w ago

So I should just cast it? Should this one work? This is the old code

dan1st•4w ago

yes

MDT 168OP•4w ago

But how? I am not too bright with generics Never used them before in situations like these

dan1st•4w ago

not on its own

MDT 168OP•4w ago

Wdym?

dan1st•4w ago

something like that

Map<Class<? extends Event>, List<? extends EnchantmentStack<?>>> listenerMap = new HashMap<>();

<E extends Event> void triggerListeners(E event) {
  List<EnchantmentStack<E>> listeners = (List<EnchantmentStack<E>>) listenerMap.get(event.getClass()); // unsafe here
  for(EnchantmentStack<E> stack : listeners) {
    stack.trigger(event); // but only used here
  }
}

private <E extends Event> void addStack(Class<E> type, EnchantmentStack<E> stack) {
  listenerMap.put(type, stack);
}

Map<Class<? extends Event>, List<? extends EnchantmentStack<?>>> listenerMap = new HashMap<>();

<E extends Event> void triggerListeners(E event) {
  List<EnchantmentStack<E>> listeners = (List<EnchantmentStack<E>>) listenerMap.get(event.getClass()); // unsafe here
  for(EnchantmentStack<E> stack : listeners) {
    stack.trigger(event); // but only used here
  }
}

private <E extends Event> void addStack(Class<E> type, EnchantmentStack<E> stack) {
  listenerMap.put(type, stack);
}

While that solution includes an unsafe cast, you can be sure that it shouldn't throw an exception due to it if you only insert stuff via addStack it doesn't include the logic that ensures that you only have the right event

MDT 168OP•4w ago

I will try this. Thanks It says that I cannot cast Inconvertible types

dan1st•4w ago

where exactly?

MDT 168OP•4w ago

On the cast. List<...> listeners = (...) ...; The first line of the method Oh wait. You use EnchantmentStack The list is MysticEnchantmentComponent Ugh still inconvertible types

dan1st•4w ago

oh, my fault I edited it

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