java - 为什么我的 BufferStrategy 抛出 NullpointerException 以及如何修复它？

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 5 年前。

我一直在尝试制作 3d 游戏，但由于某种原因，我的代码抛出 NullPointerException。我收到的错误是:

Exception in thread "Thread-3" 
java.lang.NullPointerException
at Display_3d.render(Display_3d.java:73)
at Display_3d.run(Display_3d.java:55)
at java.lang.Thread.run(Unknown Source)

这个程序应该只是显示随机颜色的像素作为测试，以便稍后制作 3D 图形。它还使用其他两个类，尽管这两个类不会抛出错误，但它们仍然可能是问题所在，所以我也会发布它们。下面的类是抛出错误的类

import java.awt.Canvas;
import javax.swing.JFrame;
import java.awt.image.BufferedImage;
import java.awt.image.BufferStrategy;
import java.awt.image.DataBufferInt;
import java.awt.Graphics;

public class Display_3d extends Canvas implements Runnable {
private static final long serialVersionUID = 1L;

public static final int width = 800;
public static final int height = 600;
public static final String title = "Scott's Game Pre-Alpha 0.01";

private Thread thread;
private Screen screen;
private BufferedImage img;
private boolean running = false;
private Render render;
private int[] pixels;

public Display_3d(){
  Screen screen = new Screen(width,height);
  img = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
  pixels = ((DataBufferInt)img.getRaster().getDataBuffer()).getData();
}

private void start(){
  if (running){
     return;
}
running = true;
Thread thread = new Thread(this);
thread.start();

System.out.println("start() has been called sucessfully"); 
}

private void stop(){
if (true != running){
  return;
}
running = false;
try {
  thread.join();
}catch (Exception e){
  e.printStackTrace();
  System.exit(0);
}
}

public void run(){
  while (running){
    tick();
/*Line 55*/render();

}
}

private void tick(){

}

private void render(){
  BufferStrategy bs = this.getBufferStrategy();
  if (bs == null){
    this.createBufferStrategy(3);

    return;
  }


  /*Line 73*/ screen.render();

  for (int i = 0; i<width*height-1; i++){
  pixels[i] = screen.pixels[i];
  }

  Graphics g = bs.getDrawGraphics();
  g.drawImage(img,0,0,width,height,null);
  g.dispose();
  bs.show();
  }


 public static void main(String[] args){
 Display_3d game = new Display_3d();
 JFrame frame    = new JFrame();
 frame.add(game);
 frame.pack();
 frame.setTitle(title);
 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 frame.setSize(width,height);
 frame.setLocationRelativeTo(null);
 frame.setResizable(false);
 frame.setVisible(true);

 System.out.println("Running...");

 game.start();

 }
 }

我的渲染类

  public class Render{
  public final int width;
  public final int height;
  public final int[] pixels;

  public Render (int width, int height){
  this.width = width;
  this.height = height;
  pixels = new int[width * height];
}

public void draw(Render render, int xOffset, int yOffset){
  for (int y = 0; y<render.height; y++){
    int yPix = y + yOffset;

  for (int x = 0; x<render.width; x++){
    int xPix = x + xOffset;
    pixels[xPix + yPix * width] = render.pixels[x+y * render.width];

  }
  }
  }
  public static void main (String[] args){}
 }

我的屏幕类

 import java.util.Random;
 public class Screen extends Render{
 private Render test;

 public Screen(int width, int height){
   super(width,height);
   Random random = new Random();
   test = new Render(256,256);

   for (int i = 0; i <256*256; i++){
    test.pixels[i] = random.nextInt();
   }
  }

  public void render() {
    draw(test,0,0);
  }

  public static void main (String[] args){}
}

Answer 1

在构造函数中创建局部变量

public Display_3d(){
  Screen screen = new Screen(width,height); //<-------------
  img = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
  pixels = ((DataBufferInt)img.getRaster().getDataBuffer()).getData();
}

而是分配给字段

public Display_3d(){
  this.screen = new Screen(width,height); //<-------------
  img = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
  pixels = ((DataBufferInt)img.getRaster().getDataBuffer()).getData();
}