Everything works fine until i die and it shows me error. how do i fix this error;
" am i a using generic list<> correctly or not to get remaining Asteroid object in background.
code is between line 131-141 in rocket class
1 2 3 4 5 6 7 | java.lang.NullPointerException at Rocket.leftObject(Rocket.java:136) at Rocket.act(Rocket.java:46) at greenfoot.core.Simulation.actActor(Simulation.java:567) at greenfoot.core.Simulation.runOneLoop(Simulation.java:530) at greenfoot.core.Simulation.runContent(Simulation.java:193) at greenfoot.core.Simulation.run(Simulation.java:183) |
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 | //Rocket classimport greenfoot.*;import java.util.List;/** * A rocket that can be controlled by the arrowkeys: up, left, right. * The gun is fired by hitting the 'space' key. 'z' releases a proton wave. * * @author Poul Henriksen * @author Michael Kölling * * @version 1.0 */public class Rocket extends SmoothMover{ private static final int gunReloadTime = 5; // The minimum delay between firing the gun. private static final int protonReloadTime = 500; // The minimum delay between proton wave bursts. private int reloadDelayCount; // How long ago we fired the gun the last time. private int protonDelayCount; // How long ago we fired the proton wave the last time. private int count=0; private GreenfootImage rocket = new GreenfootImage("rocket.png"); private GreenfootImage rocketWithThrust = new GreenfootImage("rocketWithThrust.png"); /** * Initialise this rocket. */ public Rocket() { reloadDelayCount = 5; protonDelayCount = 500; addToVelocity(new Vector(13, 0.7)); // initially slowly drifting } /** * Do what a rocket's gotta do. (Which is: mostly flying about, and turning, * accelerating and shooting when the right keys are pressed.) */ public void act() { move(); checkKeys(); checkCollision(); reloadDelayCount++; protonDelayCount++; leftObject(); } /** * Check whether there are any key pressed and react to them. */ private void checkKeys() { ignite(Greenfoot.isKeyDown("up")); if (Greenfoot.isKeyDown("left")) { turn(-5); } if (Greenfoot.isKeyDown("right")) { turn(5); } if (Greenfoot.isKeyDown("space")) { fire(); } if (Greenfoot.isKeyDown("z")) { startProtonWave(); } } /** * Check whether we are colliding with an asteroid. */ private void checkCollision() { Asteroid a = (Asteroid) getOneIntersectingObject(Asteroid.class); if (a != null) { Space space = (Space) getWorld(); space.addObject(new Explosion(), getX(), getY()); space.removeObject(this); space.gameOver(); } } /** * Should the rocket be ignited? */ private void ignite(boolean boosterOn) { if (boosterOn) { setImage(rocketWithThrust); addToVelocity(new Vector(getRotation(), 0.3)); } else { setImage(rocket); } } /** * Fire a bullet if the gun is ready. */ private void fire() { if (reloadDelayCount >= gunReloadTime) { Bullet bullet = new Bullet (getVelocity(), getRotation()); getWorld().addObject (bullet, getX(), getY()); bullet.move (); reloadDelayCount = 0; } } /** * Release a proton wave (if it is loaded). */ private void startProtonWave() { if (protonDelayCount >= protonReloadTime) { ProtonWave wave = new ProtonWave(); getWorld().addObject (wave, getX(), getY()); protonDelayCount = 0; } }private void leftObject(){ Space space =(Space)getWorld(); World world =getWorld(); List<Asteroid> left =world.getObjects(Asteroid.class); if (left.isEmpty()) { space.add(); } }} |
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 | import greenfoot.*;/** * Space. Something for rockets to fly in. * * @author Michael Kölling * @version 1.2 */public class Space extends World{ private Counter scoreCounter; private int startAsteroids = 2;//Game will start with 2 asteroids /** * Create the space and all objects within it. */ public Space() { super(600, 500, 1); GreenfootImage background = getBackground(); background.setColor(Color.BLACK); background.fill(); createStars(300); Rocket rocket = new Rocket(); addObject(rocket, getWidth()/2 + 100, getHeight()/2); addAsteroids(startAsteroids); scoreCounter = new Counter("Score: "); addObject(scoreCounter, 60, 480); Explosion.initializeImages(); ProtonWave.initializeImages(); } /** * Add a given number of asteroids to our world. Asteroids are only added into * the left half of the world. */ private void addAsteroids(int count) { for(int i = 0; i < count; i++) { int x = Greenfoot.getRandomNumber(getWidth()/2); int y = Greenfoot.getRandomNumber(getHeight()/2); addObject(new Asteroid(), x, y); } } /** * Crete a given number of stars in space. */ private void createStars(int number) { GreenfootImage background = getBackground(); for(int i=0; i < number; i++) { int x = Greenfoot.getRandomNumber( getWidth() ); int y = Greenfoot.getRandomNumber( getHeight() ); int color = 120 - Greenfoot.getRandomNumber(100); background.setColor(new Color(color,color,color)); background.fillOval(x, y, 2, 2); } } /** * This method is called when the game is over to display the final score. */ public void gameOver() { addObject(new ScoreBoard(scoreCounter.getValue()), getWidth()/2, getHeight()/2); } /** * This is for to update the score */ public void countScore(int change) { scoreCounter.add(change); } public void add() { startAsteroids++; addAsteroids(startAsteroids); }} |
