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Suavessence
Suavessence wrote ...

2021/1/13

Referencing int from World to Actor - NullPointerException

Suavessence Suavessence

2021/1/13

#
Hello, I am trying to reference the Space world "time" int from the PointOrbs Actor class. I have attempted to use a getTime() function within the PointOrbs constructor, but am getting a null pointer exception once the program attempts to create a PointOrbs object. The objective is to set the speed of the orbs according to a fraction of time. Here is the Space class:
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import greenfoot.*;
public class Space extends World
{
    private int time;
 
    public Space()
    {   
            GreenfootImage background = getBackground();
            background.setColor(Color.BLACK);
            background.fill();
            prepare();
            time = 0;
        }
 
    public void act()
        {
            countTime();
            if (Greenfoot.getRandomNumber(1000) < 2)
            {
                    addObject(new PointOrbs(), Greenfoot.getRandomNumber(460), -60);
            }
    }
 
    public int getTime()
    {
        return time;
    }
 
    private void countTime()
    {
            if (lives != 0)
            {
                    time++;
            }
    }
}
Here is the PointOrbs class:
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import greenfoot.*; 
public class PointOrbs extends Actor
{
    private int speed;
 
    public PointOrbs()
    {
        Space space = (Space)getWorld();
        speed = 1 + (space.getTime()/100);
    }
     
    public void act()
    {
        setLocation(getX(), getY()+speed);
        turn(1);
         
        if (getY() == 599)
        {
            Space space = (Space)getWorld();
            space.removeObject(this);
        }
    }  
}
Here is the error: java.lang.NullPointerException at PointOrbs.<init>(PointOrbs.java:9) at Space.act(Space.java:20) at greenfoot.core.Simulation.actWorld(Simulation.java:573) at greenfoot.core.Simulation.runOneLoop(Simulation.java:506) at greenfoot.core.Simulation.runContent(Simulation.java:193) at greenfoot.core.Simulation.run(Simulation.java:183)
danpost danpost

2021/1/13

#
Suavessence wrote...
I am trying to reference the Space world "time" int from the PointOrbs Actor class. I have attempted to use a getTime() function within the PointOrbs constructor, but am getting a null pointer exception once the program attempts to create a PointOrbs object.
In PointOrbs class, change line 6 to be:
?
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protected void addedToWorld(World world)
The actor must be in a world for getWorld to return a World object instead of null (the actor is not in any world during its creation -- or, while being constructed). You could also change line 8 to be:
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Space space = (Space) world;
since you now already have a local reference to the world it is placed in.
Suavessence Suavessence

2021/1/19

#
Great, the solution is what I needed. Thanks!
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