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我知道这个问题已经被问了一百万次了，但我看过很多答案，但它们对我没有任何帮助。我不知道我做错了什么。

我相信我收到错误的原因是由于这里的代码：

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

我正在使用上面的代码来调用一个代码如下所示的类：

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) {}
}

最后，当尝试这样做时，我收到以下错误：

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)

在此先感谢您的帮助。

4

1 回答 1

1

编码：

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

堆栈跟踪中未提及。

堆栈跟踪中提到了这一点：

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)

所以错误在这里某处，但你没有显示行号：

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

那么game, getPosition(), p, 还是p.getPosition()null 呢？

实际上，如果行号与发布的内容一致，我们就可以解决。如果：

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

然后我们可以倒数到第 23 行，就是这一行：

    Player p = game.getPlayer();

所以我猜那game是空的。

编辑- 查看您的构造函数：

game = game;

这不会分配game给您班级的game字段。你需要这个：

this.game = game;
于 2013-04-06T20:48:00.493 回答