具有初始化变量的空指针异常

Question

我正在 greenfoot 中开发这个小行星游戏，我的代码不断吐出

    java.lang.NullPointerException.

代码如下。第 50 行出现错误：

    count = world.numberOfObjects();.

我知道有几篇关于此错误的帖子，但我对此错误所做的研究表明，当您尝试使用指向 null 的引用时会发生此错误。但是，我已经初始化了我的变量，所以我不确定为什么会这样。我对编程还很陌生，所以我可能只是没有正确理解这一点。任何帮助都会很棒。谢谢！

    public class Rocket extends SmoothMover
  {
private static final int gunReloadTime = 5;         // The minimum delay between firing the gun.
private static final int protonReloadTime = 500;    // The minimum delay between proton wave bursts.

private int reloadDelayCount;               // How long ago we fired the gun the last time.
private int protonDelayCount;               // How long ago we fired the proton wave the last time.

private int count = 0; //count the number of asteroids  

private GreenfootImage rocket = new GreenfootImage("rocket.png");    
private GreenfootImage rocketWithThrust = new GreenfootImage("rocketWithThrust.png");

/**
 * Initialise this rocket.
 */
public Rocket()
{
    reloadDelayCount = 5;
    protonDelayCount = 500;
    addToVelocity(new Vector(13, 0.7));    // initially slowly drifting
}

/**
 * Do what a rocket's gotta do. (Which is: mostly flying about, and turning,
 * accelerating and shooting when the right keys are pressed.)
 */
public void act()
{
    move();
    checkKeys();
    checkCollision();
    reloadDelayCount++;
    protonDelayCount++;

    Space space = (Space)getWorld(); //call space objects
    World world = getWorld(); //call world objects

    count = world.numberOfObjects(); //ERROR IS HERE 

    /**
     * Add an asteroid when the starting ones are cleared
     */
    if(count <=2)
    {
        space.add();
    }
}

/**
 * Check whether there are any key pressed and react to them.
 */
private void checkKeys() 
{
    ignite(Greenfoot.isKeyDown("up"));

    if (Greenfoot.isKeyDown("left")) 
    {
        turn(-5);
    }
    if (Greenfoot.isKeyDown("right")) 
    {
        turn(5);
    }
    if (Greenfoot.isKeyDown("space")) 
    {
        fire();
    }
    if (Greenfoot.isKeyDown("z")) 
    {
        startProtonWave();
    }
}

/**
 * Check whether we are colliding with an asteroid.
 */
private void checkCollision() 
{
    Asteroid a = (Asteroid) getOneIntersectingObject(Asteroid.class);
    if (a != null) 
    {
        Space space = (Space) getWorld();
        space.addObject(new Explosion(), getX(), getY());
        space.removeObject(this);
        space.gameOver();
    }
}

/**
 * Should the rocket be ignited?
 */
private void ignite(boolean boosterOn) 
{
    if (boosterOn) 
    {
        setImage(rocketWithThrust);
        addToVelocity(new Vector(getRotation(), 0.3));
    }
    else 
    {
        setImage(rocket);        
    }
}

/**
 * Fire a bullet if the gun is ready.
 */
private void fire() 
{
    if (reloadDelayCount >= gunReloadTime) 
    {
        Bullet bullet = new Bullet (getVelocity(), getRotation());
        getWorld().addObject (bullet, getX(), getY());
        bullet.move ();
        reloadDelayCount = 0;
    }
}

/**
 * Release a proton wave (if it is loaded).
 */
private void startProtonWave() 
{
    if (protonDelayCount >= protonReloadTime) 
    {
        ProtonWave wave = new ProtonWave();
        getWorld().addObject (wave, getX(), getY());
        protonDelayCount = 0;
    }
}

}

Answer 1

getWorld() 方法returns null，你可以调试它，或者打印世界来确认。如果 getWorld() 方法可能 returns null，您应该在使用 world 变量之前检查是否为 null。

Answer 2

这是对该对象的简单 null 检查。

       Space space = (Space)getWorld(); //call space objects
       World world = getWorld(); //call world objects
          if(world != null){
             count = world.numberOfObjects();
          }else{
                 //handle exception
          }

具有初始化变量的空指针异常

Null Pointer Exception with initialized variable

java

greenfoot