出力: AAA

src_EventListener.java

public interface EventListener {

    void onMessage(String message);
}

src_EventManager.java

import java.lang.ref.WeakReference;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;

public class EventManager {

    private List<WeakReference<EventListener>> list = new LinkedList<>();


    public void addListener(EventListener e) {
        list.add(new WeakReference<>(e));
    }

    public void sendMessage(String message) {
        for (Iterator<WeakReference<EventListener>> i = list.iterator(); i.hasNext();) {
            WeakReference<EventListener> next = i.next();
            EventListener eventListener = next.get();
            if (eventListener == null) {
                i.remove();
                continue;
            }
            eventListener.onMessage(message);
        }
    }
}

src_Main.java

public class Main implements EventListener {

    public static void main(String[] args) throws InterruptedException {
        EventManager handler = new EventManager();
        Main main = new Main();
        handler.addListener(main);

        handler.sendMessage("AAA");
        main = null;
        System.gc();
        handler.sendMessage("BBB");

    }

    @Override
    public void onMessage(String message) {
        System.out.println(message);
    }
}