Iterating through a list of a super class while allowing removal and cast as subtype - list

Question

I am writing an engine for older arcade games. Along with the engine I am writing Space Invaders. The idea is to leave the classes that comprise the engine generic enough to be adaptable to a wide variety of arcade games and then specific games like Space Invaders extend the engine and fill in the specific parts.
I have a game engine developed into two abstract classes that create a game loop and update sprites based on the elapsed time in between loop successions. In a class of the game engine, I have a resource manager that contains lists of Sprites for enemies, enemy projectiles, and player projectiles. The resource manager currently stores these lists as ArrayLists of type Sprite. The game engine then, in an update method, iterates through these lists of Sprites and updates the Sprites.
I then began working on Space Invaders and realized I needed to extend Sprite for my player ship (lives, player states) and also needed to extend Sprite for the aliens(points, alien states). I can still store my player as a Sprite in the resource manager and when I need to work with the player in the SpaceInvaders class I can call ResourceManager.getPlayer() which returns the player as a Sprite, but then I can cast it to a Player.
The problem arises when I try to work with the aliens. I made an Alien class that also extends Sprite. When I create the aliens, I create them as Alien objects and then pass them to the resource manager which stores them in the ArrayList of type Sprite. In my collision check method I iterate through the list of aliens as such:
for (Iterator<Sprite> iter = resources.getPlayerProjectiles().iterator();
iter.hasNext();) {
boolean shotRemove = false;
Sprite sprite = iter.next();
for (Iterator<Sprite> iterAlien = resources.getEnemies().iterator();
iterAlien.hasNext();) {
Sprite alien = iterAlien.next();
if (collision(sprite,alien)) {
shotRemove = true;
iterAlien.remove();
}
}
if (sprite.getY() < 100 || shotRemove)
iter.remove();
}
At this point I would like to use methods exclusive to Alien like writing score += alien.getPoints() below the shotRemove = true line. But the method only knows of alien as a Sprite because iterAlien is an iterator of type Sprite because resources.getEnemes() is a list of type Sprite. If I try to cast alien as an Alien I get a classCastException.
So in summary, I would like to be able to keep the lists in the resource manager as generic as possible (Sprite) because the engine only knows of Sprites, but be able to work with the lists when I iterate through them in game specific classes as the class which extends Sprite (in this case, Alien).
I have tried using generics like E extends Sprite in the resource manager, but I cannot get that to work. I have read a little bit about wildcards (?) but I don't think that is what I want to do. I would think E extends Sprite is what I want because then the list is of some generic type that extends Sprite and could be treated as either a Sprite (in the engine) or as the E (Alien in SpaceInvaders).

Answer 1

You can't guarantee that the Sprite is an Alien, so use instanceof, something like:
if (sprite instanceof Alien) {
Alien alien = (Alien)sprite;
// do something with alien
}
Generics can't help you further here.