import java.lang.reflect.InvocationHandler;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.ArrayList;
import java.util.EventListener;
import java.util.List;

public class Announcer<T extends EventListener> {
    private final T proxy;
    private final List<T> listeners = new ArrayList<T>();


    public Announcer(Class<? extends T> listenerType) {
        proxy = listenerType.cast(Proxy.newProxyInstance(
            getClass().getClassLoader(),
            new Class<?>[]{listenerType},
            new InvocationHandler() {
                public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                    announce(method, args);
                    return null;
                }
            }));
    }

    public void addListener(T listener) {
        listeners.add(listener);
    }

    public void removeListener(T listener) {
        listeners.remove(listener);
    }

    public T announce() {
        return proxy;
    }

    private void announce(Method m, Object[] args) {
        try {
            for (T listener : listeners) {
                m.invoke(listener, args);
            }
        }
        catch (IllegalAccessException e) {
            throw new IllegalArgumentException("could not invoke listener", e);
        }
        catch (InvocationTargetException e) {
            Throwable cause = e.getCause();

            if (cause instanceof RuntimeException) {
                throw (RuntimeException)cause;
            }
            else if (cause instanceof Error) {
                throw (Error)cause;
            }
            else {
                throw new UnsupportedOperationException("listener threw exception", cause);
            }
        }
    }

    public static <T extends EventListener> Announcer<T> to(Class<? extends T> listenerType) {
        return new Announcer<T>(listenerType);
    }
}