程序报出NullPointerException

Question

在第39行代码：modelBatch.begin(cam);报出 NullPointerException 异常。请帮忙看看怎么回事，谢谢。public class Loading implements Screen { private boolean AP; private Chemistry chemistry; public PerspectiveCamera cam; public ModelBatch modelBatch; public Model model; public ModelInstance instance; public Lights lights; public Loading(boolean AP, Chemistry chemistry) {     this.AP = AP;     this.chemistry = chemistry; } @Override public void render(float delta) {     Gdx.gl.glViewport(0, 0, Gdx.graphics.getWidth(),             Gdx.graphics.getHeight());     Gdx.gl.glClear(GL10.GL_COLOR_BUFFER_BIT | GL10.GL_DEPTH_BUFFER_BIT);     modelBatch.begin(cam);     modelBatch.render(instance, lights);     modelBatch.end(); } @Override public void resize(int width, int height) { } @Override public void show() {             modelBatch = new ModelBatch();     lights = new Lights();     lights.ambientLight.set(0.4f, 0.4f, 0.4f, 1f);     lights.add(new DirectionalLight().set(0.8f, 0.8f, 0.8f, -1f, -0.8f,             -0.2f));     cam = new PerspectiveCamera(70, Gdx.graphics.getWidth(),             Gdx.graphics.getHeight());     cam.position.set(10f, 10f, 10f);     cam.lookAt(0, 0, 0);     cam.near = 0.1f;     cam.far = 300f;     cam.update();     ModelBuilder modelBuilder = new ModelBuilder();     model = modelBuilder.createBox(5f, 5f, 5f,             new Material(ColorAttribute.createDiffuse(Color.GREEN)),             Usage.Position | Usage.Normal);     instance = new ModelInstance(model); }        ... }

Answer 1

肯定报空指针，因为首先你的cam没有初始化，也没有作为左值进行赋值。