for (Iterator<Attack> i = projectiles.iterator(); i.hasNext();) {
    Attack a = i.next();
    if (a.exists() {
        a.update(bg);
    } else {
        i.remove(); // this will remove the current element from the iterator
    }
}

import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

public class Keyboard {

    private static boolean[] pressed = new boolean[128];

    public static boolean isPressed(int key) {
        return pressed[key];
    }
 
   public static KeyListener listener = new KeyAdapter() {

        @Override
        public void keyPressed(KeyEvent e) {
            int code = e.getKeyCode();
            if (code < pressed.length) {
                pressed[code] = true;
            }
        }

        @Override
        public void keyReleased(KeyEvent e) {
            int code = e.getKeyCode();
            if (code < pressed.length) {
                pressed[code] = false;
            }
        }
    };
}

panel.addKeyListener(KeyBoard.listener);

if (Keyboard.isKeyPressed(KeyEvent.VK_LEFT)) {
    ...
}
if (Keyboard.isKeyPressed(KeyEvent.VK_DOWN)) {
    ...
}

while (not finished)
  1. figure out what the player wants to do by looking at these key variables
  2. calculate what the game entities want to do, by applying AI logic
  3. update the scene, checking for collisions etc
  4. render everything