为什么我在 Libgdx 中更改屏幕并绘制 Sprite 后得到 NPE

Question

这是我在这里问的第一个问题，所以我很抱歉我在这里做错了。

我目前正在尝试在 libgdx 开发我的第一款自己的游戏。所以一开始我有 4 classes.

FlipX 一个 class 扩展自 Game.
StartMenu class 从屏幕实现。
GameScreen class 从屏幕实现。
从 Sprite 扩展的按钮 class。

我希望我能在这里向您展示一些代码，但让我先解释一下。
当启动 Class FlipX 代码时，我要在其中绘制 SpriteBatch，将屏幕设置为 StartMenu Class。在那里我可以单击从 FlipX 绘制到 SpriteBatch 的 2 个按钮。
这些按钮来自我创建的用于将多个纹理存储到一个所谓的按钮的按钮 Class。此按钮 Class 从 Sprite 扩展而来。
当我点击它时，PlayBtn 会将 Screen 设置为 GameScreen class。
当 GameScreen Class 在 Screen 设置为 GameScreen 后在同一批次上绘制另一个 Button 时，就会出现问题。
它是 drawBatch() 函数中来自 GameScreen 的线，我在其中绘制 jumpBtn.draw(game.batch)。当我这样做时，出现以下错误。

Exception in thread "LWJGL Application" java.lang.NullPointerException
    at com.badlogic.gdx.graphics.g2d.SpriteBatch.switchTexture(SpriteBatch.java:1067)
    at com.badlogic.gdx.graphics.g2d.SpriteBatch.draw(SpriteBatch.java:558)
    at com.badlogic.gdx.graphics.g2d.Sprite.draw(Sprite.java:580)
    at de.geecogames.flipx.Screens.GameScreen.drawBatch(GameScreen.java:88)
    at de.geecogames.flipx.Screens.GameScreen.render(GameScreen.java:63)
    at com.badlogic.gdx.Game.render(Game.java:46)
    at de.geecogames.flipx.FlipX.render(FlipX.java:26)
    at com.badlogic.gdx.backends.lwjgl.LwjglApplication.mainLoop(LwjglApplication.java:232)
    at com.badlogic.gdx.backends.lwjgl.LwjglApplication.run(LwjglApplication.java:127)

当我用简单的 Sprite 替换 Button 时，不会发生此错误，这很奇怪，因为 Button Class 只是扩展 Sprite 的 class。 private Sprite jumpBtn = new Sprite(new Texture("game/UpDef.png"));

public class FlipX extends Game {
    public SpriteBatch batch;

//Filemanager to load Assets from an Asset Manager
    public FileManager manager;

    StartMenu startMenu;
    
    @Override
    public void create () {
        manager= new FileManager();
        startMenu=new StartMenu(this);
        batch = new SpriteBatch();
        setScreen(startMenu);
    }

    @Override
    public void render () {
        super.render();
    }
    
    @Override
    public void dispose () {
        batch.dispose();
    }

    public FileManager getFileManager(){
        return manager;
    }
}

public class StartMenu implements Screen {
private final FlipX game;
    private final FileManager fileM;

    private final OrthographicCamera camera;

    private final FitViewport viewport;

    private Sprite img = new Sprite(new Texture("badlogic.jpg"));

    private final Button playBtn, exitBtn;

    private final Sprite backGround;

    private boolean playPressed, exitPressed;
    public StartMenu(FlipX game) {
        this.game = game;

        //Get Manager and load Assets
        this.fileM = game.getFileManager();
        fileM.loadAssetsMenu();

        //Testlabel
        Label testlabel = new Label(String.format("test"), new Label.LabelStyle(new BitmapFont(), Color.BLACK));

        //Get the Assets from Manager
        playBtn = new Button(fileM.getTexture(fileM.playBtnDefault), fileM.getTexture(fileM.playBtnHover), fileM.getTexture(fileM.playBtnClick));

        exitBtn = new Button(fileM.getTexture(fileM.exitBtnDefault), fileM.getTexture(fileM.exitBtnHover), fileM.getTexture(fileM.exitBtnClick));

        backGround = fileM.getSprite(fileM.backgroundMenu);

        camera = new OrthographicCamera(Gdx.graphics.getWidth(), Gdx.graphics.getHeight());
        viewport = new FitViewport(Z.V_WIDTH, Z.V_HEIGHT, camera);

        camera.position.set(viewport.getWorldWidth() / 2, viewport.getWorldHeight() / 2, 0);

        //Btn Position
        playBtn.setPosition(Z.V_WIDTH / 2 - playBtn.getWidth() / 2, Z.V_HEIGHT / 2 - playBtn.getHeight() / 2);
        exitBtn.setPosition(Z.V_WIDTH / 2 - exitBtn.getWidth() / 2, Z.V_HEIGHT / 2 - exitBtn.getHeight() / 2 - playBtn.getHeight() - playBtn.getHeight() / 2);

        img.setPosition(0, 0);
    }
    @Override
    public void render(float delta) {
        update(delta);

        clearColor();

        drawBatch();
    }

    private void update(float dt) {
        handleInput();

        cameraUpdates();
    }

    private void clearColor() {
        Gdx.gl.glClearColor(0, 0.5f, 0.9f, 1);
        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
    }

    private void drawBatch() {
        game.batch.setProjectionMatrix(camera.combined);
        game.batch.begin();
        backGround.draw(game.batch);
        img.draw(game.batch);

        //Buttons
        playBtn.draw(game.batch);
        exitBtn.draw(game.batch);

        game.batch.end();
    }

    private void handleInput() {
        Vector3 realCoords = viewport.unproject(new Vector3(Gdx.input.getX(), Gdx.input.getY(), 0));

        if (Gdx.input.isKeyPressed(Input.Keys.K)) {
            float addX = 2;
            float addY = 0;
            img.setPosition(img.getX() + addX, img.getY() + addY);
        }

        if (Gdx.input.isKeyPressed(Input.Keys.UP)) {
            camera.position.y = camera.position.y + 1;
        } else if (Gdx.input.isKeyPressed(Input.Keys.DOWN)) {
            camera.position.y = camera.position.y - 1;
        }

        if (Gdx.input.isKeyPressed(Input.Keys.LEFT)) {
            camera.position.x = camera.position.x - 1;
        } else if (Gdx.input.isKeyPressed(Input.Keys.RIGHT)) {
            camera.position.x = camera.position.x + 1;
        }

        if (Gdx.input.isKeyPressed(Input.Keys.PAGE_UP)) {
            camera.zoom = camera.zoom + 0.1f;
        } else if (Gdx.input.isKeyPressed(Input.Keys.PAGE_DOWN)) {
            camera.zoom = camera.zoom - 0.1f;
        }

        //btn test
        if (BasicFunctions.isInside(realCoords.x, realCoords.y, playBtn)) {
            if (Gdx.input.justTouched()) {
                playBtn.setClick();
                playPressed = true;
                fileM.playSound(fileM.playBtnSound);
            } else {
                if (playPressed) {
                    game.setScreen(new GameScreen(game));
                }
                playBtn.setHover();
            }
        } else
            playBtn.setNormal();


        if (BasicFunctions.isInside(realCoords.x, realCoords.y, exitBtn)) {
            if (Gdx.input.isTouched()) {
                exitBtn.setClick();
                exitPressed = true;
            } else {
                if (exitPressed)
                    Gdx.app.exit();
                exitBtn.setHover();
            }
        } else
            exitBtn.setNormal();
    }

    private void cameraUpdates() {
        camera.update();
    }
}

public class GameScreen implements Screen {

private final FlipX game;

    private final FileManager fileM;

    private final OrthographicCamera camera;

    private final FitViewport viewport;

    private final Button jumpBtn;

    private final Sprite background;

    public GameScreen(FlipX game){
        this.game=game;
        this.fileM=game.getFileManager();

        fileM.loadAssetsGame();

        camera=new OrthographicCamera(Gdx.graphics.getWidth(),Gdx.graphics.getHeight());
        viewport = new FitViewport(Z.V_WIDTH,Z.V_HEIGHT,camera);



        jumpBtn = new Button(new Texture("game/UpDef.png"),new Texture("game/UpHov.png"),new Texture("game/UpClick.png"));
        jumpBtn.setPosition(2,2);

        background = fileM.getSprite(fileM.backgroundMenu);
    }

    @Override
    public void show() {

    }

    @Override
    public void render(float delta) {
        update(delta);

        clearColor();

        drawBatch();
    }

    private void update(float dt){
        handleInput(dt);
    }

    private void clearColor() {
        Gdx.gl.glClearColor(1, 0.5f, 0.9f, 1);
        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
    }

    private void drawBatch(){
        game.batch.setProjectionMatrix(camera.combined);
        game.batch.begin();

        background.draw(game.batch);

        //Bug here?
        jumpBtn.draw(game.batch);

        game.batch.end();
    }

    private void handleInput(float dt){
        Vector3 realCoords = viewport.unproject(new Vector3(Gdx.input.getX(), Gdx.input.getY(), 0));

        if(Gdx.input.isTouched()){
            if(BasicFunctions.isInside(realCoords.x,realCoords.y,jumpBtn)){
                EzLog.logging("X " + jumpBtn.getX() + " Y " + jumpBtn.getY());
            }
        }
    }

public class Button extends Sprite {
    private final Texture normal, hover, click;

    private float stateTimer;

    public Button(Texture normal, Texture hover, Texture click){
        this.normal=normal;
        this.hover=hover;
        this.click=click;

        stateTimer=0;

        setSize();
    }

    private void setSize(){
        float height,width;

        width=normal.getWidth();
        height=normal.getHeight();

        setBounds(0,0,width*3,height*3);
    }

    public void setNormal(){
        setRegion(normal);
    }
    public void setHover(){
        setRegion(hover);
    }
    public void setClick(){
        setRegion(click);
    }
}

Answer 1

几小时后我找到了解决方案。

问题是我的 Class 按钮最多可以保存 3 个纹理。要绘制其中一个纹理，我们可以使用它的 draw 函数，因为 Button 是从 Sprite 扩展而来的。要在 SpriteBatch 上显示它，只需将纹理设置为该区域。

setRegion("TextureHere.png")

所以每次我用我的 playBtn.draw(game.batch) 绘制它时，它都会在我之前用 setPosition(x,y)

给他的位置显示这个纹理

但我实际上从未在构造函数中执行“setRegion()”，因此它会尝试绘制不存在的内容。这就是它说 NullPointerException 的原因。由于 SpriteBatch 的 switchTexture 函数，我只是对错误感到困惑。
在 Class StartMenu 中，它巧合地只是调用了“setRegion()”，因为它总是在“handleInput”中的条件的 else 部分中使用按钮 Class 中的函数“setNormal()”调用.

我知道的解决方法如下：

我在构造函数中添加了 setNormal() 函数，因此第一个纹理将始终设置为区域，如果我可以这样说的话，这很有效。

public class Button extends Sprite {
    private final Texture normal, hover, click;

    public Button(Texture normal, Texture hover, Texture click){
        this.normal=normal;
        this.hover=hover;
        this.click=click;

        setSize();
        setNormal();
    }

    private void setSize(){
        float height,width;

        width=normal.getWidth();
        height=normal.getHeight();

        setBounds(0,0,width*3,height*3);
    }

    public void setNormal(){
        setRegion(normal);
    }
    public void setHover(){
        setRegion(hover);
    }
    public void setClick(){
        setRegion(click);
    }

为什么我在 Libgdx 中更改屏幕并绘制 Sprite 后得到 NPE

Why i get an NPE after changing Screen and Drawing a Sprite in Libgdx

java

nullpointerexception

libgdx