超类类型派生类对象的返回值,直接给超类类型变量赋值报错.

Question

以下是部分代码:
Airplane , BigAirplane, Bee都继承超类FlyingObject
用注释里面代码会在其他部分报错,提示paint方法里面enemies 空指针异常...
主要就是加粗的那2段代码,注释掉的那段会出现enemies可能得到null,这是为什么呢

public class World extends JPanel
{
Sky sky=new Sky();
Hero hero= new Hero();
FlyingObject enemies[]= {};
Bullet bullets[]= {};

    public FlyingObject nextOne()
    {
        Random rand=new Random();
        int type=rand.nextInt(100);
        if(type<45)return new Airplane();
        else if(type<80)return new BigAirplane();
        else return new Bee();
    }


    public void enterAction()
    {

** /*
enemies=Arrays.copyOf(enemies, enemies.length+1);
enemies[enemies.length-1]=nextOne();
*/

            FlyingObject obj=nextOne();
            enemies=Arrays.copyOf(enemies, enemies.length+1);
            enemies[enemies.length-1]=obj;**

    }

    public void stepAction()
    {
        sky.step();
        hero.step();
        for(int i=0;i<enemies.length;i++)
            enemies[i].step();
        for(int i=0;i<bullets.length;i++)
            bullets[i].step();
    }

private void action()
{
    Timer timer=new Timer();
    timer.schedule(new TimerTask() {
        public void run()
        {
            enterAction();
            stepAction();
            repaint();
        }

    }, 10,10);
}

public void paint (Graphics g)
{
    sky.paint(g);
    hero.paint(g);

    for(int i=0;i<enemies.length;i++)
        enemies[i].paint(g);

    for(int i=0;i<bullets.length;i++)
        bullets[i].paint(g);

}
public static void main(String[] args) 
{
        World world=new World();
        JFrame f=new JFrame();
        f.add(world);

        f.setSize(400,700);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setVisible(true);     

        world.action();

}

}

Answer 1

https://blog.csdn.net/lmb1612977696/article/details/64203307