java - 为什么线程创建的数组会返回空指针异常？

Question

这是我的代码

protected static Objects[] theObject;

private static Runnable objectDeployment = () ->    {
    MainThread.theObject = new Objects[noOfObjects];
    for(int i=0; i<noOfObjects; i++)    {
        theObject[i] = new Objects();
        theObject[i].setEnableMovement(true);
        theObject[i].start();
    }
};

private static RAMDS[] track;

private static Runnable scanner = () -> {
    int no = noOfObjects;
    track = new RAMDS[no];
    for(int i=0; i<no; i++) {
        int[] position = MainThread.theObject[i].getPosition();
        int magnitude = MainThread.theObject[i].getMagnitude();

        //check to see if there is an object within range
        if(position[0] > 0 && position[0] < Wireframe.xlimit )
            if(position[1] > 0 && position[1] < Wireframe.ylimit )
                if(magnitude > 0)   {
                    track[i] = new RAMDS(); 
                    track[i].objectId(i);   
                    track[i].start();           
                }
    }
};

public static void main(String[] args) {
    // TODO Auto-generated method stub
    noOfObjects = getNoOfObjects();
    new Thread(objectDeployment).start();
    new Thread(scanner).start();
}

}

因此，当线程 2 尝试访问本应由线程 1 创建的 object[i] 数组时，我收到空指针异常。我做错了什么？

这是我获取 noOfObjects 的方法

private static int getNoOfObjects() {
    int deployedObjects = 0;
    @SuppressWarnings("resource")
    Scanner in = new Scanner(System.in);
    System.out.print("How many objects should be deployed?");
    deployedObjects =in.nextInt();
    return deployedObjects;
}

Answer 1

您需要等待 objectDeployment 线程完成。你有一个竞争条件。当扫描器线程尝试使用对象[i]时，未分配它。

编辑: 将 objectDeployment 拆分为同步的 init 阶段，以及启动对象的 objectStart。顺便说一句，我认为对象的名称非常困惑。

protected static Objects[] theObject;

private static Runnable objectDeployment = () ->    {
    MainThread.theObject = new Objects[noOfObjects];
    for(int i=0; i<noOfObjects; i++)    {
        theObject[i] = new Objects();
        theObject[i].setEnableMovement(true);
    }
};

private static Runnable objectStart = () ->    {

    for(int i=0; i<noOfObjects; i++)    {
        theObject[i].start();
    }
};

private static RAMDS[] track;

private static Runnable scanner = () -> {
    int no = noOfObjects;
    track = new RAMDS[no];
    for(int i=0; i<no; i++) {
        int[] position = MainThread.theObject[i].getPosition();
        int magnitude = MainThread.theObject[i].getMagnitude();

        //check to see if there is an object within range
        if(position[0] > 0 && position[0] < Wireframe.xlimit )
            if(position[1] > 0 && position[1] < Wireframe.ylimit )
                if(magnitude > 0)   {
                    track[i] = new RAMDS(); 
                    track[i].objectId(i);   
                    track[i].start();           
                }
    }
};

public static void main(String[] args) {
    // TODO Auto-generated method stub
    noOfObjects = getNoOfObjects();
    objectDeployment.run()
    new Thread(objectStart).start();
    new Thread(scanner).start();
}

}