Question

我需要找出surfaceview上drawCircle方法缺少的内容。在getholder()中调用onCreate后调用lockcanvas，并且从方法中尝试了同样的操作，但是我收到了错误。

这里是主要活动：

    public class MainActivity extends AppCompatActivity {

    private Main2Activity contentView;
    Tester tester;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        Log.d("App", "App launched");

        tester = new Tester(this);
//        GameView gameView = new GameView(this);
        Log.d("App", "set calling.");

        setContentView(tester);


    }

    @Override
    protected void onResume() {
        super.onResume();
        Log.d("App", "onResume");

        tester.resume();
    }
}

这里绘制圆圈的方法：

public class Tester extends SurfaceView {

Canvas canvas;
Paint paint;
boolean b;
SurfaceHolder surfaceHolder;

    public Tester(Context context) {
        super(context);
        Log.d("App", "in other context.");

//        canvas = new Canvas();
//        paint = new Paint(Paint.ANTI_ALIAS_FLAG);
//        surfaceHolder = getHolder();
//        canvas = surfaceHolder.lockCanvas();
//        setWillNotDraw(false); //this line is very important!

    }

/*
    public void run() {
        Log.d("App", "in running.");

        synchronized (canvas = surfaceHolder.lockCanvas()) {
            while (b) {

                drawing();
                Log.d("App", "drawing passed");
            }
        }
    }
*/
    private void drawing() {
        Log.d("App", "posting.");
        surfaceHolder = getHolder();
        canvas = surfaceHolder.lockCanvas();
        paint = new Paint(Paint.ANTI_ALIAS_FLAG);

        synchronized (canvas = surfaceHolder.lockCanvas()) {

                paint.setColor(Color.MAGENTA);
                canvas.drawCircle(100, 200, 20, paint);
                surfaceHolder.unlockCanvasAndPost(canvas);

        }
    void resume() {
        b = true;
        drawing();
    }
}

logcat的：

引起：java.lang.NullPointerException：尝试在空对象引用上调用虚方法'void android.graphics.Canvas.drawCircle（float，float，float，android.graphics.Paint）'

我找不到可能出错的内容，这个简单的代码片段没有实例化的内容？谢谢你的时间。

Answer 1

正如您在评论中提到的那样：

请提及nullPointer异常的原因。我想知道它。 :)

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.graphics.Canvas.drawCircle(float, float, float, android.graphics.Paint)' on a null object reference

您的LogError明确表示Canvas的对象为空，这意味着您在代码中使用的Canvas实例不存在或为空。

看这里：

here --> canvas.drawCircle(100, 200, 20, paint);

此处Canvas canvas的引用为null，请尝试初始化。

需要帮助，请说出nullpointer异常的原因，即使在calss和error中实例化的所有内容都在drawCircle方法中

1 个答案: