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Riyo
Riyo wrote ...

2018/5/16

java.lang.NullPointerException error

Riyo Riyo

2018/5/16

#
I've tried giving Health to the alien in my game, but it constantly comes up with a java.lang.NullPointerException error.
java.lang.NullPointerException at Alien.HealthSystem(Alien.java:51) at Alien.act(Alien.java:18) at greenfoot.core.Simulation.actActor(Simulation.java:604) at greenfoot.core.Simulation.runOneLoop(Simulation.java:562) at greenfoot.core.Simulation.runContent(Simulation.java:221) at greenfoot.core.Simulation.run(Simulation.java:211)
?
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public void act() //Alien.class
   {
       movement();
       HealthSystem(); //First error happens here
   }
   private int health = 2;
   public void movement()
   {
       if(! getWorld().getObjects(Planet.class).isEmpty())
       {
           Actor planet = (Actor)getWorld().getObjects(Planet.class).get(0);
           int planetX = planet.getX();
           int planetY = planet.getY();
           turnTowards(planetX, planetY);
           move(2);
       }
       if(isTouching(Alien.class))
       {
           Teleport();
       }
   }
 
   public void Teleport()
   {
       int cordsX = getX();
       int cordsY = getY();
 
       setLocation(cordsX, (cordsY - Greenfoot.getRandomNumber(200)));
   }
 
   public void HealthSystem()
   {
       if(health == 0)
       {
           getWorld().removeObject(this);
           MyWorld world = (MyWorld)getWorld();
           world.getScore();
           System.out.println("This ran successfully"); //Doesn't run as of yet
           world.increaseScore(); //Next happens here
       }
       if(getOneIntersectingObject(Bullet.class) != null)
       {
           int score;
           MyWorld world = (MyWorld)getWorld();
           getWorld().removeObject(getOneIntersectingObject(Bullet.class));
           health = health - 1;
       }
   }
?
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public MyWorld()
   {   
       // Create a new world with 600x400 cells with a cell size of 1x1 pixels.
       super(1500, 1000, 1);
       setUpWorld();
   }
    
   public void setUpWorld()
   {
       addObject(new BackGround(), 750, 500);
       addObject(new Planet(), 0, 500);
       addObject(new Player(), 238, 493);
       addObject(new Alien(), 1200 ,Greenfoot.getRandomNumber(1000));
       addObject(new Alien(), 1200 ,Greenfoot.getRandomNumber(1000));
       addObject(new Alien(), 1200 ,Greenfoot.getRandomNumber(1000));
   }
    
   public int getScore()
   {
       return score;
   }
 
   public int level = 1;
 
   public void increaseScore() //Problem is supposedly here?
   {
       score++;
       showText("Score" + score, 300, 30);
   }
If possible could you help me understand it to a level of understanding so I could avoid these types of problems later/be able to handle them myself. Thanks. Curtis E.
danpost danpost

2018/5/16

#
Switch lines 35 and 36. You need to get the world reference before you remove the actor from the world. If the actor is not in a world, the getWorld method will return a null value.
Riyo Riyo

2018/5/16

#
Thanks, managed to sort the errors out and now can continue finishing off the game.
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