在“ AWT-EventQueue-0”线程错误中获取异常

Question

i know this question has been asked like a million times but i have looked on a lot of the answers and they in no way help me. i have no idea what i am doing wrong.

I believe the reason i am getting the error is due to this code here:

        RollerBall roller = new RollerBall(game);
        roller.setPosition(new Vec2(-50,-120));

I am using the above code to call a class which code is shown below:

package game;

import city.soi.platform.*;
import fsm.FSM;

public class RollerBall extends Body implements StepListener {

    public static final float RANGE = 150;

    private Game game;
    private FSM<RollerBall> fsm;

    public RollerBall(Game game) {
        super(game.getWorld());
        game = game;
        fsm = new FSM<RollerBall>(this);
        fsm.start(new StandStillState());
        getWorld().addStepListener(this);
    }


    public boolean inRangeLeft() {
        Player p = game.getPlayer();
        float gap = getPosition().x - p.getPosition().x;
        return gap < RANGE && gap > 0;
    }

    public boolean inRangeRight() {
        Player p = game.getPlayer();
        float gap = getPosition().x - p.getPosition().x;
        return gap > -RANGE && gap < 0;
    }

    public boolean inRange() {
        return inRangeLeft() || inRangeRight();
    }

    public void preStep(StepEvent e) {
        fsm.update();
    }

    public void postStep(StepEvent e) {}
}

Finally, when trying to do this i am getting the following error:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at game.RollerBall.inRangeLeft(RollerBall.java:23)
    at game.StandStillState.update(StandStillState.java:10)
    at fsm.FSM.update(FSM.java:47)
    at game.RollerBall.preStep(RollerBall.java:39)
    at city.soi.platform.World.preStep(World.java:495)
    at city.soi.platform.World.step(World.java:328)
    at city.soi.platform.World$1.actionPerformed(World.java:206)
    at javax.swing.Timer.fireActionPerformed(Timer.java:312)
    at javax.swing.Timer$DoPostEvent.run(Timer.java:244)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:721)
    at java.awt.EventQueue.access$200(EventQueue.java:103)
    at java.awt.EventQueue$3.run(EventQueue.java:682)
    at java.awt.EventQueue$3.run(EventQueue.java:680)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:691)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:244)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:163)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:151)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:147)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:139)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:97)

Thanks in advance for your help.

Answer 1

The code:

    RollerBall roller = new RollerBall(game);
    roller.setPosition(new Vec2(-50,-120));

is not mentioned in the stack trace.

This is mentioned in the stack trace:

at game.RollerBall.inRangeLeft(RollerBall.java:23)
at game.StandStillState.update(StandStillState.java:10)
at fsm.FSM.update(FSM.java:47)
at game.RollerBall.preStep(RollerBall.java:39)

So the error is in here somewhere, but you have not shown line numbers:

public boolean inRangeLeft() {
    Player p = game.getPlayer();
    float gap = getPosition().x - p.getPosition().x;
    return gap < RANGE && gap > 0;
}

So is game , getPosition() , p , or p.getPosition() null?

Actually, if the line numbers are consistent with the posting we can work it out. If:

game.RollerBall.preStep(RollerBall.java:39)

Then we can count back to line 23 and it's this line:

    Player p = game.getPlayer();

so I'm guessing that game is null.

EDIT - Looking at your constructor:

game = game;

This won't assign game to your class's game field. You need this:

this.game = game;