線程「AWT-EventQueue-0」塔防的例外

Question

所以我正在製作塔防遊戲，當我運行代碼時，我想在那裏顯示一個網格，但我所得到的只是一個空白窗口，控制檯：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException 
    at Room.draw(Room.java:41) 
    at Screen.paintComponent(Screen.java:42) 
    at javax.swing.JComponent.paint(Unknown Source) 
    at javax.swing.JComponent.paintToOffscreen(Unknown Source) 
    at javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(Unknown Source) 
    at javax.swing.RepaintManager$PaintManager.paint(Unknown Source) 
    at javax.swing.RepaintManager.paint(Unknown Source) 
    at javax.swing.JComponent._paintImmediately(Unknown Source) 
    at javax.swing.JComponent.paintImmediately(Unknown Source) 
    at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source) 
    at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source) 
    at javax.swing.RepaintManager.prePaintDirtyRegions(Unknown Source) 
    at javax.swing.RepaintManager.access$700(Unknown Source) 
    at javax.swing.RepaintManager$ProcessingRunnable.run(Unknown Source) 
    at java.awt.event.InvocationEvent.dispatch(Unknown Source) 
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source) 
    at java.awt.EventQueue.access$200(Unknown Source) 
    at java.awt.EventQueue$3.run(Unknown Source) 
    at java.awt.EventQueue$3.run(Unknown Source) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) 
    at java.awt.EventQueue.dispatchEvent(Unknown Source) 
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source) 
    at java.awt.EventDispatchThread.run(Unknown Source) 

這裏是我的代碼：

框架類：

import javax.swing.*; 
import java.awt.*; 

public class Frame extends JFrame { 

    public static String title = "Tower Defence"; 
    public static Dimension size = new Dimension(700,550); 

    public Frame() 
    { 
     setTitle(title); 
     setSize(size); 
     setResizable(false); 
     setLocationRelativeTo(null); 
     setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 

     init(); 

    } 

    public void init() 
    { 
     setLayout (new GridLayout(1, 1, 0, 0)); 

     Screen screen = new Screen(); 
     add(screen); 
     setVisible(true); 
    } 

    public static void main(String args[]) 
    { 
     Frame frame = new Frame(); 

    } 

} 

Screen類：

import java.awt.Graphics; 

import javax.swing.*; 


public class Screen extends JPanel implements Runnable 
{ 

    public Thread thread = new Thread(this); 


    public static int myWidth, myHeight; 

    public static boolean isFirst = true; 

    public static Room room; 

    public Screen() 
    { 
     thread.start(); 
    } 

    public void define() 
    { 
     room = new Room(); 
    } 



    public void paintComponent (Graphics g) 
    { 
     if(isFirst) 
     { 
      myWidth = getWidth(); 
      myHeight = getHeight(); 
      define(); 
      isFirst = false; 
     } 

     g.clearRect(0,0, getWidth(), getHeight()); 
     room.draw(g); //Drawing the room. 
    } 

    public void run() 
    { 
     while(true) 
     { 
      if(!isFirst) 
      { 
       room.physics(); 
      } 

      repaint(); 


      try 
      { 
       Thread.sleep(1); 

      } catch(Exception e){} 
     } 
    } 
} 

室等級：

import java.awt.*; 


public class Room 
{ 
public int worldWidth = 12; 
public int worldHeight = 8; 
public int blockSize = 52; 

public Block[][] block; 

    public Room() 
    {  

    } 

    public void define() 
{ 
     block = new Block[worldWidth][worldHeight]; 

    for(int y=0;y<block.length;y++) 
    { 
     for(int x=0;x<block[0].length;x++) 
     { 
      block[y][x] = new Block(((Screen.myWidth/2)) - ((worldWidth*blockSize/2)) + (x * blockSize), y * blockSize, blockSize, blockSize, 0, 0); 
     } 
    } 
} 


    public void physics() 
    { 


    } 



    public void draw(Graphics g) 
    { 
     for(int y=0;y<block.length;y++) 
     { 
      for(int x=0;x<block[0].length;x++) 
      { 
       block[y][x].draw(g); 
      } 
     } 
    } 
} 

塊類：

import java.awt.*; 

public class Block extends Rectangle 
{ 
    public int groundID; 
    public int airID; 

    public Block(int x, int y, int width, int height, int groundID, int airID) 
    { 
     setBounds(x,y,width,height); 
     this.groundID = groundID; 
     this.airID = airID; 
    } 

    public void draw(Graphics g) 
    { 
     g.drawRect(x, y, width, height); 
    } 
} 

提前感謝！

Answer 1

在Room類中，block變量（或其元素之一）爲null。由於錯誤在這條線發生：

public void draw(Graphics g) 
    { 
>  for(int y=0;y<block.length;y++) 
     { 
      for(int x=0;x<block[0].length;x++) 
      { 
       block[y][x].draw(g); 
      } 
     } 
    } 

我說你已經不叫define()（這似乎初始化block變量）由draw()方法被調用的時候你的房間。