java - 如何正确地将对象传递到 libgdx 中的 ApplicationListener

Question

我想将对象传递到我的 ApplicationListener 实现中，但我不断收到 NullPointerExceptions，所以我可能做错了什么。
我收到异常:callback.onReady();

这里是异常(exception)(行号是错误的，因为我已经删除了导入语句):

Exception in thread "LWJGL Application" java.lang.NullPointerException
    at com.dewi.jones.game_Implementation.create(game_Implementation.java:56)
    at com.badlogic.gdx.backends.lwjgl.LwjglApplication.mainLoop(LwjglApplication.java:144)
    at com.badlogic.gdx.backends.lwjgl.LwjglApplication$1.run(LwjglApplication.java:131)

基本上，我希望在 onCreate 方法结束时触发回调。
请帮忙
谢谢这是我的 ApplicationListener 实现代码:

 public class game_Implementation implements ApplicationListener {

    ICallbackTest callback;

    public game_Implementation(ICallbackTest callback) {
        this.callback = callback;
    }

    @Override
    public void create() {
        // initialize stuff
        // call the onReady method at the end of the create method, when
        // everything has been initialized and created
        callback.onReady();
    }

    public void sayHello() {
        System.out.println("Hello, this game is ready");
    }

    @Override
    public void resize(int width, int height) {
        // TODO Auto-generated method stub

    }

    @Override
    public void render() {
        // TODO Auto-generated method stub

    }

    @Override
    public void pause() {
        // TODO Auto-generated method stub

    }

    @Override
    public void resume() {
        // TODO Auto-generated method stub

    }

    @Override
    public void dispose() {
        // TODO Auto-generated method stub

    }

}

这是我的桌面游戏类:

public class desktopGame {
public static void main(String[] args) {
    LwjglApplicationConfiguration cfg = new LwjglApplicationConfiguration();
    cfg.title = "game";
    cfg.useGL20 = true;
    cfg.width = 480;
    cfg.height = 320;
    ICallbackTest callback = null;
    final game_Implementation gameInstance = new game_Implementation(
            callback);

    callback = new ICallbackTest() {
        @Override
        public void onReady() {
            gameInstance.sayHello();
        }
    };

    LwjglApplication game = new LwjglApplication(gameInstance, cfg);


}

}

来自@Mel Nicholso 给出的建议
这是我尝试的解决方案:

        ICallbackTest callback = null;
    final game_Implementation gameInstance = null;
    callback = new ICallbackTest() {
        @Override
        public void onReady() {
            gameInstance.sayHello();
        }
    };
    gameInstance = new game_Implementation(
            callback);

    LwjglApplication game = new LwjglApplication(gameInstance, cfg);

现在我收到错误消息:无法分配最终局部变量 gameInstance。它必须为空且不使用复合赋值
请帮忙，谢谢

Answer 1

您需要在实例化 game_Implementation 之前而不是之后设置变量 callback。交换两行的顺序。

[编辑添加]您需要打破循环引用

static class MyGameCallback {
    private MyGame game;
    public void onReady() {
        game.sayHello();
    }
    public void setGame(MyGame game) {
        this.game = game;
    }
}

public static void main(String arg[]) {
    MyGameCallback mgc = new MyGameCallback();
    MyGame game = new MyGame(mgc);
    mgc.setGame(game);

    game.whateverMakesMeReady();
}