Android Studio-尝试调用接口方法“Android”。图样画布android。看法表面层。空对象引用上的lockCanvas()'

我是Android Studio和StackOverflow的新手,请提前原谅我。

我试着制作一个简单的android程序,输出自己的帧速率。

当我运行它时,会出现以下错误:

Attempt to invoke interface method 'android.graphics.Canvas android.view.SurfaceHolder.lockCanvas()' on a null object reference

我相当肯定这是该计划的问题领域:

while(running){
        startTime=System.nanoTime();
        canvas=null;
        try{
            canvas=this.surfaceHolder.lockCanvas();
            synchronized(surfaceHolder){
                this.gamePanel.update();
                this.gamePanel.draw(canvas);
            }
        }catch(Exception e) {
            e.printStackTrace();
        }finally{
            if(canvas!=null){
                try{
                    surfaceHolder.unlockCanvasAndPost(canvas);
                }catch(Exception e) {
                    e.printStackTrace();}
            }
        }

我直接从本教程中复制了它: https://www.youtube.com/watch?v=OojQitoAEXs 据我所知,我逐行复制了它。你知道我如何修复这个错误吗?

以下是整个程序,如果有帮助的话(很抱歉转储):

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.app.Activity;
import android.view.Window;
import android.view.WindowManager;
public class MainActivity extends Activity {

    @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, 
    WindowManager.LayoutParams.FLAG_FULLSCREEN);
        this.requestWindowFeature(Window.FEATURE_NO_TITLE);
        setContentView(new GamePanel(this));
    }
}

import android.view.MotionEvent;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.MotionEvent;
import android.graphics.Canvas;
import android.content.Context;
import java.lang.Thread;

public class GamePanel extends SurfaceView implements SurfaceHolder.Callback 
{
    private MainThread thread;

    public GamePanel(Context context){
        super(context);

        getHolder().addCallback(this);

        thread=new MainThread(getHolder(), this);
        setFocusable(true);
    }

    public void surfaceChanged(SurfaceHolder holder, int format, int width, int height){

    }

    public void surfaceCreated(SurfaceHolder holder){
        thread=new MainThread(getHolder(), this);
        thread.setRunning(true);
        thread.start();
    }

    public void surfaceDestroyed(SurfaceHolder holder){
        boolean retry=true;
        while(true){
            try{
                thread.setRunning(false);
                thread.join();
            }catch(Exception e){
                e.printStackTrace();}

            retry=false;
        }
    }

    public boolean onTouchEvent(MotionEvent event) {
        return super.onTouchEvent(event);
    }

    public void update(){

    }

    public void draw(Canvas canvas) {
        super.draw(canvas);

    }

}

import android.view.SurfaceHolder;
import android.graphics.Canvas;

public class MainThread extends Thread {
    public static final int MAX_FPS = 30;
    private double averageFPS;
    private SurfaceHolder surfaceHolder;
    private GamePanel gamePanel;
    private boolean running;
    public static Canvas canvas;

    public void setRunning(boolean running){
        this.running=running;
    }

    public MainThread(SurfaceHolder surfaceholder, GamePanel gamepanel){
        super();
        this.surfaceHolder=surfaceHolder;
        this.gamePanel=gamePanel;
    }

    public void run(){
        long startTime;
        long timeMillis=1000/MAX_FPS;
        long waitTime;
        int frameCount=0;
        long totalTime=0;
        long targetTime=1000/MAX_FPS;

        while(running){
            startTime=System.nanoTime();
            canvas=null;
            try{
                canvas=this.surfaceHolder.lockCanvas();
                synchronized(surfaceHolder){
                    this.gamePanel.update();
                    this.gamePanel.draw(canvas);
                }
            }catch(Exception e) {
                e.printStackTrace();
            }finally{
                if(canvas!=null){
                    try{
                        surfaceHolder.unlockCanvasAndPost(canvas);
                    }catch(Exception e) {
                        e.printStackTrace();}
                }
            }
            timeMillis=(System.nanoTime()-startTime)/1000000;
            waitTime=targetTime-timeMillis;
            try{
                if(waitTime>0){
                    this.sleep(waitTime);
                }
            }catch(Exception e) {
                e.printStackTrace();
            }
            totalTime+=System.nanoTime()-startTime;
            frameCount++;
            if(frameCount==MAX_FPS){
                averageFPS=1000/(totalTime/frameCount)/1000000;
                frameCount=0;
                totalTime=0;
                System.out.println(averageFPS);
            }
        }
    }
}