public interface MediaPlayerInvoker {
    public void run(MediaPlayer player);
}

public class Invoker implements Runnable {
    public static final BlockingQueue<MediaPlayerInvoker> queue = new LinkedBlockingQueue<MediaPlayerInvoker>();

    private MediaPlayer mediaPlayer;

    public Invoker() {
        mediaPlayer = new MediaPlayer(); //Или как он там создается
    }

    public void run() throws Exception {
        while(true) {
            queue.take().run(mediaPlayer);
        }
    }
}

//Хотим вызвать метод stop()
Invoker.queue.put(new MediaPlayerInvoker {
    @Override
    public void run(MediaPlayer player) {
        player.stop();
    }
});