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HecticKiwi
HecticKiwi wrote ...

2018/4/3

Counter problems

HecticKiwi HecticKiwi

2018/4/3

#
Was trying to implement a counter for the asteroid tutorial but keep getting a java.lang.NullPointerException when running the scenario, as soon as the shot hits the asteroid it throws an error, below is the code for Space, Counter and Shot.
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public class Space  extends World
{
 
    private Counter theCounter;
 
    /**
     * Constructor for objects of class Space.
     */
    public Space()
    {   
        // Create a new world with 600x400 cells with a cell size of 1x1 pixels.
        super(600, 400, 1);
        addObject(new Rocket(), 300, 200);
        Counter theCounter = new Counter();
        addObject(theCounter, 5, 5);
    }
 
    public Counter getCounter()
    {
        return theCounter;
    }
     
    /**
     * Prepare the world for the start of the program. That is: create the initial
     * objects and add them to the world.
     */
    private void prepare()
    {
    }
     
    public void act()
    {
        if (Greenfoot.getRandomNumber(1000) < 3) {
            addObject(new Asteroid(), 0, 20);
        }
    }
}
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public class Counter  extends Actor
{
    private int totalCount = 0;
 
    public Counter()
    {
        setImage(new GreenfootImage("0", 20, Color.WHITE, Color.BLACK));
    }
 
    /**
     * Increase the total amount displayed on the counter, by a given amount.
     */
    public void bumpCount(int amount)
    {
        totalCount += amount;
        setImage(new GreenfootImage("" + totalCount, 20, Color.WHITE, Color.BLACK));
    }
}
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public class Shot  extends Actor
{
    private Rocket myShip;
 
    /**
     * Constructor for a Shot. You must specify the ship the shot comes from.
     */
    public Shot(Rocket myShip)
    {
        this.myShip = myShip;
    }
 
    /**
     * Act - do whatever the Shot wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public void act()
    {
        int ypos = getY();
        if (ypos > 0) {
            ypos = ypos - 5;
            setLocation(getX(), ypos);
            Actor rock = getOneIntersectingObject(Asteroid.class);
            if (rock != null) {
                // We've hit an asteroid!
                hitAnAsteroid();
                getWorld().removeObject(rock);
                getWorld().removeObject(this);
            }
        }
        else {
            // I reached the top of the screen
            getWorld().removeObject(this);
        }
    }
     
    /**
     * This method gets called (from the act method, above) when the shot hits an
     * asteroid. It needs to do only one thing: increase the score counter.
     * (Everything else, such as removing the asteroid which was hit, is dealt
     * with in the act method).
     */
    private void hitAnAsteroid()
    {
       Space spaceWorld = (Space) getWorld();
       Counter counter = spaceWorld.getCounter();
       counter.bumpCount(5);
        
       //What goes here????
       // We want to call the "bumpCount" method from the Counter class -
       // but how??!!
    }
}
Vercility Vercility

2018/4/3

#
Provide the error at least :( Also, im pretty sure y can't be <0 since you can't leave the world
danpost danpost

2018/4/3

#
Remove the declared type, Counter (the first word), from line 14 in your Space class. You do not want to create a new Counter variable there -- you just want to refer to the Counter field declared at line 4.
HecticKiwi HecticKiwi

2018/4/5

#
danpost wrote...
Remove the declared type, Counter (the first word), from line 14 in your Space class. You do not want to create a new Counter variable there -- you just want to refer to the Counter field declared at line 4.
Thanks so much, it worked for the tutorial but when implementing it into my project I have run into another error, "cannot find symbol - method getCounter()" on line 39 in the car class, not sure if it's because getCounter() is in MyWorld, any help would be appreciated.
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import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)
public class car extends Actor
{
 
    public void act()
    {
        move(4);
        if(Greenfoot.isKeyDown("up"))
        {
            move(8);
        }
         if(Greenfoot.isKeyDown("down"))
        {
            move(-12);
        }
        if (Greenfoot.isKeyDown("left"))
        {
            turn(-5);
        }
        if (Greenfoot.isKeyDown("right"))
        {
            turn(5);
        }
         
        Actor fruit;
        fruit = getOneObjectAtOffset(0, 0, Fruit.class);
        if (fruit != null) //when car contacts fruit, fruit disappears
        {
            World world;
            world = getWorld();
            world.removeObject(fruit);
            eatFruit(); //calling the eatFruit method
        }
     
    }
    private void eatFruit() //car eats the fruit and score increases
    {
       World newWorld = (MyWorld) getWorld();
       Counter counter = newWorld.getCounter();  //cannot find symbol getCounter(), is it because
       counter.bumpCount(5);                     //the getCounter method is in MyWorld?
    }
     
}
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import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)
public class MyWorld extends World
{
    private Counter theCounter;
    /**
     * Constructor for objects of class MyWorld.
     *
     */
    public MyWorld()
    {   
        super(600, 400, 1);
        GreenfootImage bg = new GreenfootImage("space-1.jpg");
        bg.scale(getWidth(), getHeight());
        setBackground(bg);
        prepare();
        theCounter = new Counter();
        addObject(theCounter, 5, 5);
    }
    public Counter getCounter()
    {
        return theCounter;
    }
    private void prepare()
    {
        car car = new car();
        addObject(car,76,50);
        Fruit fruit = new Fruit();
        addObject(fruit,467,74);
        Fruit fruit2 = new Fruit();
        addObject(fruit2,65,297);
        Fruit fruit3 = new Fruit();
        addObject(fruit3,240,206);
        Fruit fruit4 = new Fruit();
        addObject(fruit4,436,301);
        Rocket rocket = new Rocket();
        addObject(rocket,237,342);
        Fruit fruit5 = new Fruit();
        addObject(fruit5,393,200);
        Fruit fruit6 = new Fruit();
        addObject(fruit6,549,209);
        Fruit fruit7 = new Fruit();
        addObject(fruit7,530,359);
        Fruit fruit8 = new Fruit();
        addObject(fruit8,164,137);
        Rocket rocket2 = new Rocket();
        addObject(rocket2,132,237);
    }
}
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import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)
public class Counter extends Actor
{
    private int totalCount = 0;
 
    public Counter()
    {
        setImage(new GreenfootImage("0", 20, Color.WHITE, Color.BLACK));
    }
    public void bumpCount(int amount)
    {
        totalCount += amount;
        setImage(new GreenfootImage("" + totalCount, 20, Color.WHITE, Color.BLACK));
    }    
}
danpost danpost

2018/4/5

#
In car class change World to MyWorld at line 38.
HecticKiwi HecticKiwi

2018/4/7

#
Argh I knew it was something simple like that, I'd tried putting MyWorld in various places in eatFruit() but obviously hadn't tried that one. Thanks a bunch :)
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