java - android中出现NullPointerException的原因是什么？

Question

我是 Android 游戏开发的新手。我一直在尝试一个游戏开发的例子，但有些无法理解android中空点异常的原因。

    public void onLoadResources() {
         this.mBitmapTextureAtlas = new BitmapTextureAtlas(512, 512,
               TextureOptions.BILINEAR_PREMULTIPLYALPHA);
        BitmapTextureAtlasTextureRegionFactory.setAssetBasePath("gfx/");

        this.mPlayerTextureRegion = BitmapTextureAtlasTextureRegionFactory
                .createFromAsset(this.mBitmapTextureAtlas, this, "Player.png",
                0, 0);
        this.mEngine.getTextureManager().loadTexture(this.mBitmapTextureAtlas);
        final int PlayerX = (int)(mCamera.getWidth() / 2);
        final int PlayerY = (int) ((mCamera.getHeight() - mPlayerTextureRegion
                .getHeight()) / 2);
        this.player = new Sprite(PlayerX, PlayerY, this.mPlayerTextureRegion);
        this.player.setScale(2);
        this.mMainScene.attachChild(this.player);
    }

最后一行“this.mMainScene.attachChild(this.player);”导致空点异常，即使所有内容都已初始化。评论此行后，一切正常，但显示没有 Sprite 。

这是初始化mMainScene的代码

    public Scene onLoadScene() {
        this.mEngine.registerUpdateHandler(new FPSLogger());
        this.mMainScene = new Scene();
        this.mMainScene.setBackground(new ColorBackground(0.09804f, 0.6274f, 0.8784f));
        return mMainScene;
    }

Answer 1

您尚未初始化mMainScene。您无法引用(即在定义 mMainScene 之前附加 Children/调用方法。)

您能否检查一下 mMainScene 是否为 null，或者将 mMainScene 的分配移至 onLoadResources() 中？

能否将 mMainScene 定义从 onLoadScene 移至 onLoadResources 中？

public void onLoadResources() {
     this.mBitmapTextureAtlas = new BitmapTextureAtlas(512, 512,
           TextureOptions.BILINEAR_PREMULTIPLYALPHA);
    BitmapTextureAtlasTextureRegionFactory.setAssetBasePath("gfx/");

    this.mPlayerTextureRegion = BitmapTextureAtlasTextureRegionFactory
            .createFromAsset(this.mBitmapTextureAtlas, this, "Player.png",
            0, 0);
    this.mEngine.getTextureManager().loadTexture(this.mBitmapTextureAtlas);
    final int PlayerX = (int)(mCamera.getWidth() / 2);
    final int PlayerY = (int) ((mCamera.getHeight() - mPlayerTextureRegion
            .getHeight()) / 2);
    this.player = new Sprite(PlayerX, PlayerY, this.mPlayerTextureRegion);
    this.player.setScale(2);
    this.mMainScene = new Scene();
    this.mMainScene.setBackground(new ColorBackground(0.09804f, 0.6274f, 0.8784f));
    this.mMainScene.attachChild(this.player);
}

public Scene onLoadScene() {
    return mMainScene;
}

编辑:

基于 Snake 的示例看来您应该在 onLoadScene 方法中将子项添加到场景中:

    public Scene onLoadScene() {
        this.mEngine.registerUpdateHandler(new FPSLogger());

        this.mScene = new Scene();
        for(int i = 0; i < LAYER_COUNT; i++) {
            this.mScene.attachChild(new Entity());
    }