带有Map< Class&lt ;?的EventSource extends Event&gt ;, List< EventListener&lt ;?扩展事件>>没有未经检查的呼叫

问题描述

我有：

我需要实现观察者模式和每个事件类的监听器列表。空界面事件

  p> 
 
 
必须知道该类型的意义。我会制作 EventSource 泛型，指定具有有界泛型类型参数的 Event 类型。
  class EventSource< E extends Event> {
  
许多？扩展事件声明将改为使用 E ，例如 
 
 
 私人最终地图< Class< extends Event>，List< EventListener< E>>> listeners = new HashMap<>（）; 
  
还有许多其他声明会改为使用 E  $ b 
现在， fire 将会带一个 E ，以便它可以传递给句柄：
  public void fire（E event）{
 for（EventListener< E> listener：listeners.get（event.getClass（）））{
 listener.handle（event）; //现在编译
} 
} 
  
 
I need to implement Observer pattern with List of listeners per Event class.
I have: 
empty interface Event
public interface Event {}
interface for Listeners:
public interface EventListener<T extends Event> {
void handle(T event);

Class<T> getEventClass();}
and event source:
public class EventSource {
private final Map<Class<? extends Event>, List<EventListener<? extends Event>>> listeners = new HashMap<>();

public <T> void subscribe(EventListener<? extends Event> listener) {
    Class<? extends Event> eventClass = listener.getEventClass();
    if (!listeners.containsKey(eventClass)) {
        listeners.put(eventClass, new ArrayList<>());
    }
    listeners.get(eventClass).add(listener);
}

public void unsubscribe(EventListener listener) {
    listeners.remove(listener.getEventClass());
}

public void fire(Event event) {
    for (EventListener listener : listeners.get(event.getClass())) {
        listener.handle(event); //<-- Unchecked call to 'handle(T)' as a member of raw type...
    }
}}
It works, but I have "Unchecked call" warning. How to avoid it?
I tried:
public void fire(Event event) {
    for (EventListener<? extends Event> listener : listeners.get(event.getClass())) {
        listener.handle(event);//<-- compilation error
    }
}
but in this case I have "handle cannot be applied to..." compilation error.
Thanks in advance!
 解决方案 
The "unchecked call" warning was reported for a good reason - you used the raw form of EventListener.  When you gave a type parameter ? extends Event, it gave the compilation error because the compiler doesn't which Event the EventListener handles - it only knows that it is a specific type of Event.  Likewise, it doesn't know the runtime type of event - it could be any specific type of Event.
The type must be known for this to make sense.  I would make EventSource generic, specifying the type of Event with a bounded generic type parameter.
class EventSource<E extends Event> {
Many ? extends Event declarations will change to use E, e.g.
private final Map<Class<? extends Event>, List<EventListener<E>>> listeners = new HashMap<>();
There are many other declarations that will change to use E.
Now, fire will take an E so that it can be passed to handle:
public void fire(E event) {
   for (EventListener<E> listener : listeners.get(event.getClass())) {
      listener.handle(event); // Now this compiles
   }
}
                        
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