为什么我的 ArrayList 项目在消失时不会自行删除？

Question

I am trying to get my Asteroids game in Java to have the ArrayList container be removed once it's off screen. I figured out how to stop having it print when off screen, but can see in my console the array continues to grow. Not sure where to go from here.

I think the way I can get them to be removed when off screen is by either using the remove or set feature with arrayLists. Visually everything is disappearing right, but in the console my ArrayList is still growing. I thought about setting a limit of how long it can be, but not sure if there is a better way than that.

import java.awt.*;
import java.awt.event.*;
import java.util.ArrayList;
import java.util.Iterator;

public class AsteroidGame extends Frame {

    private int FrameWidth = 500;
    private int FrameHeight = 400;

    static public void main(String[] args) {
        AsteroidGame world = new AsteroidGame();
        world.show();
        world.run();
    }

    public AsteroidGame() {
        setTitle("Asteroid Game0");
        setSize(FrameWidth, FrameHeight);
        addKeyListener(new keyDown());
        addWindowListener(new CloseQuit());
    }

    public void run() {
        while (true) {
            movePieces();
            repaint();
            try {
                Thread.sleep(100);

            } catch (Exception e) {
            }
        }
    }

    private ArrayList asteroids = new ArrayList();
    private ArrayList rockets = new ArrayList();
    private Station station = new Station(FrameWidth / 2, FrameHeight - 20);

    public void paint(Graphics g) {
        station.paint(g);
        Iterator astIt = asteroids.iterator();
        while (astIt.hasNext()) {
            Asteroid rock = (Asteroid) astIt.next();
            if (rock.y >= 400 || rock.x >= 500){
                rock = null;
            } else {
                rock.paint(g);
            }

        }
        Iterator rocIt = rockets.iterator();
        while (rocIt.hasNext()) {
            Rocket rock = (Rocket) rocIt.next();
            if (rock.y >= 400 || rock.x >= 500) {
                rock = null;
            } else {
                rock.paint(g);
            }
        }

    }

    public void movePieces() {
        if (Math.random() < 0.3) {
            Asteroid newRock = new Asteroid(FrameWidth * Math.random(), 20, 10 * Math.random() - 5, 3 + 3 * Math.random());
            if (newRock.y >= 500 || newRock.x >= 500){

                asteroids.remove(0);

            } else{
                asteroids.add(newRock);
            }
            System.out.println(asteroids.size());

        }

        Iterator astIt = asteroids.iterator();
        while (astIt.hasNext()) {
            Asteroid rock = (Asteroid) astIt.next();
            if (rock.y >= 400 || rock.x >= 500) {
                rock = null;
            } else {
                rock.move();
                station.checkHit(rock);
            }


        }
        Iterator rocIt = rockets.iterator();
        while (rocIt.hasNext()) {
            Rocket rock = (Rocket) rocIt.next();
            if (rock.y >= 400 || rock.x >= 500) {
                rock = null;
            } else {
                rock.move(asteroids);
            }

        }

    }

    private class gameMover extends Thread {

        public void run() {
            while (true) {
                movePieces();
                repaint();
                try {
                    sleep(100);

                } catch (Exception e) {
                }
            }
        }
    }

Answer 1

Change:

rock = null;

to:

astIt.remove();

Assigning null to the variable that has been assigned the value of an element of a List does absolutely nothing to either the List or the element in it; it only affects the value that the variable holds .

---

As an aside, nice variable name choice of rock - it is appropraite for both types of object - either an abbreviation of "rocket" or a reasonable synonym for an astroid.

Answer 2

Change rock = null; to asteroids.remove(rock); or astIt.remove(); and it should be fine, also no need to set the variable to null as the garbage collector will take care of it for you.
EDIT
Actually asteroids.remove(rock); will throw an exception as said in the comments of this answer, so nevermind it and use the other.

Also I think when in movePieces() you create a new rock and you check if this new rock is outside the screen, I don't think removing the first asteroid in the ArrayList is correct as you will not add the new rock (which may be right if the rock can actually randomly spawn outside the screen) but you will also remove a maybe fine working asteroid from the ArrayList (and thus from the game and the screen).

So, personally I would change that part of code to:

if (Math.random() < 0.3) {
        Asteroid newRock = new Asteroid(FrameWidth * Math.random(), 20, 10 * Math.random() - 5, 3 + 3 * Math.random());
        if (!(newRock.y >= 500 || newRock.x >= 500)){

            asteroids.add(newRock);
        }
        System.out.println(asteroids.size());

    }

But tell me if this works for you.