使用Java線程創建測試碰撞檢測的對象

Question

我正在使用acm庫（acm.jar），同一個庫斯坦福大學的學生用它們的Java課程CS106A來創建圖形化應用程序更簡單。

acm.jar可以在這裏找到：http://jtf.acm.org/

下面的代碼將一個字符精靈圖形窗口。如果我按z，角色會自動移動他的弓箭，並開始通過使用Java線程垂直移動。迄今爲止沒有錯誤。

現在我想能夠在線程GImage object(linkArrow)上執行碰撞檢測。在我的程序發生了一個錯誤，當我試着這樣做：

arrowPoint = new GPoint(linkArrow.getX(),linkArrow.getY()); 
arrowObject = getElementAt(arrowPoint); 

我得到這個錯誤：

Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException 

您可以按Ctrl + F和類型的錯誤，看看我的程序的錯誤位置。

我已經做了碰撞檢測使用使用GPoint和getElementAt我的遊戲「突圍」的圖形矩形和橢圓形之前，從來沒有使用GPoint和getElementAt問題。

這是我的代碼：第一類是運行線程的主程序。第二類是線程。

 import java.awt.event.KeyEvent; 
     import acm.graphics.GImage; 
     import acm.program.GraphicsProgram; 
     import acm.util.RandomGenerator; 

     public class Link extends GraphicsProgram { 


      private static final double GRAVITY = 1; 
      public void init(){ 
       setSize(APPLICATION_WIDTH, APPLICATION_HEIGHT); 
       addLink(); 

       addKeyListeners(); 
     } 


      private void addLink(){ 
       linkCharacter = new GImage("link sprites/linkFaceRight/link_frame_1_face_right.png"); 
       add(linkCharacter,link_Location_XCoord,link_Location_YCoord); 
      } 

      public void keyPressed(KeyEvent e){ 
       /* Link's Movement 
       * 
       */ 
       char linkMoveRightKey = e.getKeyChar(); 
      if(linkMoveRightKey == 'z') 
        { 

        xSpeed = 0; 
        ySpeed = 0; 
        pause(arrowDELAY); 
        linkCharacter.move(xSpeed, ySpeed); 
        linkCharacter.setImage(linkAttackWithBow[linkFrame]); 

        linkFrame++; 
        callArrow(); 

// BUG 
      arrowPoint = new GPoint(linkArrow.getX(),linkArrow.getY()); 
      arrowObject = getElementAt(arrowPoint); 

      } 
    //     


        /* 
        * summon link's arrow 
        */ 
        } 

        if(linkFrame>=linkAttackWithBow.length){ 
         linkFrame = 0; 
        } 


      } 



      private void callArrow(){ 
       if(linkFrame == 2){ 
       linkArrow = new ArrowThread(SIZE, rgen.nextColor()); 
       add(linkArrow,linkCharacter.getX(),linkCharacter.getY()); 

       Thread arrowThread = new Thread(linkArrow); 
       arrowThread.start(); 




       } 
      } 

     private ArrowThread linkArrow; 
      private int SIZE = 400; 

      private RandomGenerator rgen = RandomGenerator.getInstance(); 
      private GImage gameBackgroundImage; 

      private GImage linkCharacter; 

      private double arrowDELAY = 28; 


      private int link_Location_XCoord = 50; 
      private int link_Location_YCoord = 50 ; 
      private final int APPLICATION_WIDTH = 1200; 
      private final int APPLICATION_HEIGHT = 800; 
     private String[] linkAttackWithBow = {"link sprites/linkAttackWithBow/Link_Bow_Attack_Frame_1.png","link sprites/linkAttackWithBow/Link_Bow_Attack_Frame_2.png","link sprites/linkAttackWithBow/Link_Bow_Attack_Frame_3.png","link sprites/linkAttackWithBow/Link_Bow_Attack_Frame_1.png","link sprites/linkAttackWithBow/Link_Bow_Attack_Frame_2.png","link sprites/linkAttackWithBow/Link_Bow_Attack_Frame_3.png","link sprites/linkAttackWithBow/Link_Bow_Attack_Frame_1.png","link sprites/linkAttackWithBow/Link_Bow_Attack_Frame_2.png"}; 

      private int linkFrame = 0; 
      private int xSpeed =5; //the number of pixels to move in x 
      private int ySpeed = 0; //0 so you only move horizontally 
     } 

這裏是我的Thread類產卵的箭頭

import java.awt.Color; 

import acm.graphics.GImage; 


public class ArrowThread extends GImage implements Runnable{ 

    public ArrowThread(int SIZE,Color color){ 
     super("link sprites/linkAttackWithBow/arrow.png", SIZE,SIZE); 


    } 

    public void run(){ 
     for(int i = 0; i < 1000/STEP;i++){ 
      pause(60); 

      move(arrowSpeedX,arrowSpeedY); 

     } 

    } 






    private static final int arrowSpeedX = 0; 
    private static final int arrowSpeedY = -5; 

    private static final double STEP = 5; 


} 

Answer 1

它看起來像問題是，成員linkArrow是第一幀上的空。看看檢查：

if(linkFrame == 2){ 
    linkArrow = new ArrowThread(SIZE, rgen.nextColor());