使用Java线程创建测试碰撞检测的对象

Question

我正在使用acm库（acm.jar），同一个库斯坦福大学的学生用它们的Java课程CS106A来创建图形化应用程序更简单。

acm.jar可以在这里找到：http://jtf.acm.org/

下面的代码将一个字符精灵图形窗口。如果我按z，角色会自动移动他的弓箭，并开始通过使用Java线程垂直移动。迄今为止没有错误。

现在我想能够在线程GImage object(linkArrow)上执行碰撞检测。在我的程序发生了一个错误，当我试着这样做：

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

我得到这个错误：

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

您可以按Ctrl + F和类型的错误，看看我的程序的错误位置。

我已经做了碰撞检测使用使用GPoint和getElementAt我的游戏“突围”的图形矩形和椭圆形之前，从来没有使用GPoint和getElementAt问题。

这是我的代码：第一类是运行线程的主程序。第二类是线程。

 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 
     } 

这里是我的Thread类产卵的箭头

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

它看起来像问题是，成员linkArrow是第一帧上的空。看看检查：

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