为什么这似乎并没有被初始化

Question

我有一个典型的NullPointerException异常：

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) 

，我敢肯定，我知道空是的，我认为这是这个方法（同样，这是的getImage方法加载在任何一个网格，你要我创建了spritesheet.png图像）

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; 

}

我看着周围大约在这个课堂上的一切，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; 
} 
} 

我想知道如何才能启动它？虽然我猜它已经是image.getSubimage（代码），我测试了this.image，它说，这是NullPointerException ...但它应该已经启动。

一些更多的类中可能涉及

• 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); 
  
  
  } 
  

  }

的game.class是相当大的，所以我会给它所说的主要方法是涉及它（获得它？init（），在其中......）

• Game.class

  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); 
  

  }

如果你需要我展示另一个类，我会很高兴，因为最大的和唯一的大这样做（ ish）一个是游戏

**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：**当我试图从一开始的路径，我只是得到这个错误，我不认为我在处理它时犯了一个错误。所以生病保持它/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，如果它只是在一些空spriteSheet加载，它是，背景没有给回空

**找到了答案：**我将其格式错误，虽然它是PNG，但它显然不对，所以我用另一个图像编辑器打开它并再次导出它。

Answer 1

看起来像Game.spriteSheet当ss = new SpriteSheet(game.getSpriteSheet())被调用时，成员尚未初始化，由Game.init()触发。

奇怪的是，在Game.init()中，spriteSheet在调用new Texture(this)之前被初始化。你可以检查spriteSheet在try/catch块之后不为空，spriteSheet = loader.loadImage("/Sprite_Sheet.png")在Game.init()之后。