Question

我正在建立自己的活动制度,并试图使它像现在这样成为现实。

EventSystem.get(EventClientStart.class)
    .setCallback((event) -> {
        System.out.println("Client Started");
    });

EventSystem.get(EventClientStart.class)
    .invoke(new EventClientStart());

But got a problem with checks of type, what I missed? T is extends by Event, I think it should work and it actually is (only if I will cast it to (EventController), but then I will get warning "Unchecked Cast")

活动.java

package im.silkproject.event;

import im.silkproject.event.internal.EventController;

import java.util.HashMap;
import java.util.Map;

public final class EventSystem
{
    private static final Map<Class<? extends Event>, EventController<? extends Event>> map = new HashMap<>();

    private EventSystem() { }

    public static <T extends Event> EventController<T> get(Class<T> event)
    {
        return map.computeIfAbsent(event, k -> new EventController<>());
    }
}

活动.java

package im.silkproject.event;

public class Event
{
    private boolean cancelled;

    public void cancel()
    {
        cancelled = true;
    }

    public boolean isCancelled()
    {
        return cancelled;
    }
}

活动Callback.java

package im.silkproject.event.internal;

@FunctionalInterface
public interface EventCallback<T>
{
    void __call(T event);
}

活动主计长

package im.silkproject.event.internal;

import java.util.concurrent.CopyOnWriteArrayList;

public class EventController<T>
{
    private final CopyOnWriteArrayList<EventCallback<T>> callbacks = new CopyOnWriteArrayList<>();

    public void invoke(T event)
    {
        for (EventCallback<T> callback : callbacks)
        {
            callback.__call(event);
        }
    }

    public int length()
    {
        return callbacks.size();
    }

    public boolean setCallback(EventCallback<T> event)
    {
        return callbacks.addIfAbsent(event);
    }

    public boolean unsetCallback(EventCallback<T> event)
    {
        return callbacks.remove(event);
    }
}

Answer 1

I think it relates to Covariance in Java. You can only read from List or Map if you use Upper Bounded Wildcards. You can find more details in this answer here

友情链接