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POUsing Java Threads to create an object that test collision detection
 

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  1. POUsing Java Threads to create an object that test collision detection
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    <p>I am using the acm library(acm.jar), same library Stanford students used for their Java course CS106A to create graphical application easier.</p> <p>acm.jar can be found here: <a href="http://jtf.acm.org/" rel="nofollow">http://jtf.acm.org/</a></p> <p>The following code adds a character sprite to a graphics window. If I press z , the character animates itself moving his bow and arrows starts to spawn moving vertically through the use of Java Threads. There is no errors so far.</p> <p>Now I want to be able to able to perform collision detection on a threaded <code>GImage object(linkArrow)</code>. An error happened in my program when I tried doing this:</p> <pre><code>arrowPoint = new GPoint(linkArrow.getX(),linkArrow.getY()); arrowObject = getElementAt(arrowPoint); </code></pre> <p>I get this error:</p> <pre><code>Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException </code></pre> <p>You can ctrl+f and type bug to see the error location of my program. </p> <p>I have done collision detection before using graphical rectangles and oval using the <code>GPoint</code> and <code>getElementAt</code> for my game "BreakOut" and never had a problem using <code>GPoint</code> and <code>getElementAt</code>.</p> <p>Here is my code: first class is main program that runs the thread. The second class is the thread.</p> <pre><code> 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&gt;=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 } </code></pre> <p>Here is my Thread class to spawn the arrows</p> <pre><code>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 &lt; 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; } </code></pre>
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