为什么这似乎没有被初始化

Question

我有一个典型的 NullPointerException：

Exception in thread "Thread-0" java.lang.NullPointerException
at com.game.main.SpriteSheet.grabImage(SpriteSheet.java:17)
at com.game.main.Textures.getTextures(Textures.java:30)
at com.game.main.Textures.<init>(Textures.java:21)
at com.game.main.Game.init(Game.java:80)
at com.game.main.Game.run(Game.java:116)
at java.lang.Thread.run(Thread.java:745)

我很确定我知道 null 在哪里，我认为是这个方法（这也是 getImage 方法加载到我创建的 spritesheet.png 图像中你想要的任何网格中）

public BufferedImage grabImage(int column, int row, int width, int height){
        System.out.println("img is " + image); //<<<< How I tested for null.. May not be the best way, but image said to be null

        BufferedImage img = image.getSubimage((column * 32) - 32, (row * 32) - 32, width, height);

        return img;

}

我查看了这个类的所有内容，这是类：SpiteSheet.class

package com.game.main;

import java.awt.image.BufferedImage;

public class SpriteSheet {

private BufferedImage image;

public SpriteSheet(BufferedImage image){
    this.image = image;
    System.out.println("img is " + this.image); // <<<<<<<<< THIS ONE SHOWS "img is nullException in thread "Thread-0" " in the console
}

public BufferedImage grabImage(int column, int row, int width, int height){
    System.out.println("img is " + image); //<<<< How I tested for null.. May not be the best way, but image said to be null

    BufferedImage img = image.getSubimage((column * 32) - 32, (row * 32) - 32, width, height);

    return img; 
}
}

我想知道我将如何启动它？虽然我猜它已经是 image.getSubimage(code)，但我测试了 this.image，它说，那就是 NullPointerException……但它应该已经启动了。

可能涉及的更多类

• Textures.class

    package com.game.main;
  
    import java.awt.image.BufferedImage;
  
    public class Textures {
  
      public BufferedImage[] player = new BufferedImage[3];
      public BufferedImage[] missile = new BufferedImage[3];
      public BufferedImage[] enemy = new BufferedImage[3];
  
  
  
      private SpriteSheet ss;
      private Game game;
  
      public Textures(Game game){
          this.game = game;
          ss = new SpriteSheet(game.getSpriteSheet());
  
          getTextures(); // This is line 21
  
  
      }
  
      public void getTextures(){
  
  
  
      player[0] = ss.grabImage(1, 1, 32, 32); //This is line 30
      player[1] = ss.grabImage(1, 2, 32, 32);
      player[2] = ss.grabImage(1, 3, 32, 32);
  
      missile[0] = ss.grabImage(2, 1, 32, 32);
      missile[1] = ss.grabImage(2, 2, 32, 32);
      missile[2] = ss.grabImage(2, 3, 32, 32);
  
      enemy[0] = ss.grabImage(3, 1, 32, 32);
      enemy[1] = ss.grabImage(3, 2, 32, 32);
      enemy[2] = ss.grabImage(3, 3, 32, 32);
  
  
  }
  

  }

game.class 非常大，所以我将给出它所说的涉及它的主要方法（明白吗？init()，在其中...）

• 游戏类

   public void init(){
  
  requestFocus(); //Brings focus to the screen at the start
  BufferedImageLoader loader = new BufferedImageLoader();
  try{
  
      spriteSheet = loader.loadImage("/Sprite_Sheet.png");
      background = loader.loadImage("/Background.png");
  
  }catch(IOException e){
      e.printStackTrace();
  }
  
  addKeyListener(new KeyInput(this));
  
  tex = new Textures(this);
  p = new Player(300,300, tex);
  c = new Controller(tex);
  
  ea = c.getEntityA();
  eb = c.getEntityB();
  
  c.createEnemy(enemyCount);
  

  }

如果您需要我展示其他课程，我会很乐意这样做，因为最大和唯一的大（ish）课程是游戏

**编辑：** 这是 getSpriteSheet 方法

        `public BufferedImage getSpriteSheet(){ return spriteSheet; }` 

**EDIT1: ** 这是 BufferedImageLoader 类

 package com.game.main;

import java.awt.image.BufferedImage;
import java.io.IOException;

import javax.imageio.ImageIO;

public class BufferedImageLoader {

    private BufferedImage image;

    public BufferedImage loadImage(String path) throws IOException{

        image = ImageIO.read(getClass().getResource(path));
        return image;
    }

}

**EDIT2：**当我尝试从一开始就对其进行路径时，我只是收到此错误，我认为我在路径时也没有犯错误。所以我把它保存为 /Sprite_Sheet.png;

Exception in thread "Thread-0" java.lang.IllegalArgumentException: input == null!
    at javax.imageio.ImageIO.read(ImageIO.java:1388)
    at com.game.main.BufferedImageLoader.loadImage(BufferedImageLoader.java:14)
    at com.game.main.Game.init(Game.java:72)
    at com.game.main.Game.run(Game.java:117)
    at java.lang.Thread.run(Thread.java:745)

**EDIT3: ** 我检查了它是否只是 spriteSheet 加载了 null 的东西，而且背景没有返回 null

**找到答案：**我格式错误，虽然是 PNG，但显然不正确，所以我用不同的图像编辑器打开它并再次导出。

Answer 1

ss = new SpriteSheet(game.getSpriteSheet())被调用时，Game.spriteSheet成员似乎还没有被初始化，这是由Game.init()中的tex = new Textures(this)触发的。

奇怪的是，在Game.init() 中，spriteSheet 在调用new Texture(this) 之前被初始化。你能在Game.init() 中使用spriteSheet = loader.loadImage("/Sprite_Sheet.png") 的try/catch 块之后检查spriteSheet 是否为空。