野兔和烏龜種小程序，我怎樣才能讓我的代碼正常工作？

Question

我一直對我的項目，這是一個有點難以知道我可以在此解決，所以我可以使它工作，我是一個初學者，只想讓您知道..

這裏的錯誤（S）消息：

Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException: String is null 
    at sun.java2d.SunGraphics2D.drawString(SunGraphics2D.java:2817) 
    at project2.project2.paint(project2.java:101) 
    at sun.awt.RepaintArea.paintComponent(RepaintArea.java:264) 
    at sun.awt.RepaintArea.paint(RepaintArea.java:240) 
    at sun.awt.windows.WComponentPeer.handleEvent(WComponentPeer.java:347) 
    at java.awt.Component.dispatchEventImpl(Component.java:4937) 
    at java.awt.Container.dispatchEventImpl(Container.java:2287) 
    at java.awt.Component.dispatchEvent(Component.java:4687) 
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:729) 
    at java.awt.EventQueue.access$200(EventQueue.java:103) 
    at java.awt.EventQueue$3.run(EventQueue.java:688) 
    at java.awt.EventQueue$3.run(EventQueue.java:686) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87) 
    at java.awt.EventQueue$4.run(EventQueue.java:702) 
    at java.awt.EventQueue$4.run(EventQueue.java:700) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76) 
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:699) 
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242) 
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161) 
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150) 
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146) 
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138) 
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:91) 

這裏是我的代碼：

package project2; 

import java.awt.*; 
import java.applet.*; 

public class project2 extends Applet 
{ 
    Image tortoise, hare; 
    int tortX = 250, hareX = 250; 
    final int tortY = 100, hareY = 300, WIDTH = 15, HEIGHT = 50; 
    int turn; String turnNum; 
    int move; String tMove, hMove; 
    public void init() 
    { 
     tortoise = getImage(getDocumentBase(), "images/tortoise.gif"); 
     hare = getImage(getDocumentBase(), "images/hare.gif"); 
     move = 0; turn = 0; 
    } 
    public void control() 
    { 
     while ((tortX < 985) || (hareX < 985)) 
     { 
      move = (int)(10 * Math.random()); 
      switch (move) 
      { 
      case 1: 
      case 2: 
       tortX += (3 * WIDTH); 
       hareX += (9 * WIDTH); 
       tMove = "Fast Plod"; hMove = "Big Hop"; 
       break; 
      case 3: 
      case 4: 
      case 5: 
       tortX += (3 * WIDTH); 
       hareX += WIDTH; 
       tMove = "Fast Plod"; hMove = "Small Hop"; 
       break; 
      case 6: 
       tortX += WIDTH; 
       if (hareX == 250) {} // Agit Nihil 
       else if (hareX <= (250 + (11 * WIDTH))) 
        hareX = 250; 
       else 
        hareX -= (12 * WIDTH); 
       tMove = "Slow Plod"; hMove = "Big Slip"; 
       break; 
      case 7: 
      case 8: 
       tortX += (1 * WIDTH); 
       if (hareX == 250) {} // Agit Nihil 
       else if (hareX <= (250 + (WIDTH))) 
        hareX = 250; 
       else 
        hareX -= (2 * WIDTH); 
       tMove = "Slow Plod"; hMove = "Small Slip"; 
       break; 
      case 9: 
      case 10: 
       if (tortX == 250) {} // Agit nihil 
       else if (tortX <= (250 + (5 * WIDTH))) 
        tortX = 250; 
       else 
        tortX -= (6 * WIDTH); 
       tMove = "Slip"; hMove = "Fall Asleep."; 
       break; 
       // Cuniculus dormit, agit nihil . 
      } 
      turn++; turnNum = (turn + ""); 
      repaint(); 
      for (int i = 1; i <= 10; i++) 
      { 
       delay(); 
      } 
     } 
     tortX = 985; hareX = 985; 
     repaint(); 
    } 
    public void paint(Graphics screen) 
    { 
     drawRace(screen); 
     if (tortX >= 985) 
     { 
      screen.setFont(new Font("Times New Roman", Font.ITALIC, 48)); 
      screen.drawString("Tortoise Wins", 650, 240); 
      clearCurrent(screen); 
      fillNext(screen); 
     } 
     else if (hareX >= 985) 
     { 
      screen.setFont(new Font("Times New Roman", Font.ITALIC, 48)); 
      screen.drawString("Tortoise Wins", 650, 240); 
      clearCurrent(screen); 
      fillNext(screen); 
     } 
     else 
     {  
      screen.drawString(("Turn " + turnNum), 621, 55); 
      screen.setFont(new Font("Times New Roman", Font.ITALIC, 12)); 
      screen.drawString(tMove, 59, 65); screen.drawString(hMove, 66, 255); 
      clearCurrent(screen); 
      fillNext(screen); 
     } 
     stop(); 
    } 
    public void clearCurrent(Graphics s) 
    { 
     s.clearRect(tortX+1, tortY+1, WIDTH-1, HEIGHT-1); 
     s.clearRect(hareX+1, hareY+1, WIDTH-1, HEIGHT-1); 
    } 
    public void fillNext(Graphics s) 
    { 
     s.fillRect(tortX+1, tortY+1, WIDTH-1, HEIGHT-1); 
     s.fillRect(hareX+1, hareY+1, WIDTH-1, HEIGHT-1); 
    } 
    public void drawRace(Graphics s) 
    { 
     // Initium 
     s.drawRect(250, 100, 750, 50); 
     s.drawRect(250, 300, 750, 50); 
     int lineX = 265, lineYi = 100, lineYf = 150; 
     for (int i = 1; i <= 98; i++) 
     { 
      if (lineX == 1000) 
      { 
       lineX = 265; lineYi = 300; lineYf = 350; 
      } 
      s.drawLine(lineX, lineYi, lineX, lineYf); 
      lineX += 15; 
     } 
     s.fillRect(tortX+1, tortY+1, WIDTH-1, HEIGHT-1); 
     s.fillRect(hareX+1, hareY+1, WIDTH-1, HEIGHT-1); 
     s.drawImage(tortoise, 59, 80, this); 
     s.drawImage(hare, 66, 271, this); 
     s.setFont(new Font("Times New Roman", Font.BOLD, 24)); 
     s.drawString("Race", 250, 55); 
    } 
    public void delay() 
    { 
     for (int i = 0; i < 90000000; i++) 
     { 
     } 
    } 
    public void stop() 
    { 
    } 
} 

Answer 1

它看起來像tMove或hMove （或兩者都是）null。它在control()中的switch聲明可能會在某些情況下留下這些未分配的聲明。要麼確保tMove和hMove肯定是被繪製在屏幕前或分配（也許更好，因爲不能保證你的applet是畫之前control()將被調用）替換此代碼：

screen.drawString(tMove, 59, 65); 
screen.drawString(hMove, 66, 255); 

與此：

if (tMove != null) { 
    screen.drawString(tMove, 59, 65); 
} 
if (hMove != null) { 
    screen.drawString(hMove, 66, 255); 
}