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CNIAngel
PostPosted: Wed Apr 18, 2012 2:50 pm 
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Joined: Wed Feb 29, 2012 7:07 pm
Posts: 9
Hello all. My problem at this time is trying to create a firing system so that a new bullet is created while the button is down. I currently have one bullet rendering and moving (though not continuous movement from one press). So I wanted to have it so that each time the update reads the key pressed, it creates a new bullet.

Code:
@Override
   public void init(GameContainer gc, StateBasedGame sbg)
         throws SlickException {
      
      gc.setTargetFrameRate(60);
      screen = new Image("res/gamescreen.png");
      
      player = new Entity("player");
      player.AddComponent( new ImageRenderComponent("PlayerRender", new Image("res/player.png")) );
      player.setPosition(new Vector2f(300, 176));
      
      bullet = new Entity("bullet");
      bullet.AddComponent(new ImageRenderComponent("BulletRender", new Image("res/bullet.png")));
      bullet.setPosition(new Vector2f(315, 197));
      bullet.AddComponent(new BulletFiringComponent("BulletFire"));
   }


Here I init the player and the bullet. I'm perplexed by having the bullet entity being stored in an arraylist, then having a new one created that works correctly. I'll also post my component that handles the bullet firing motion.

Code:
package entities;

import java.util.ArrayList;

import org.newdawn.slick.GameContainer;
import org.newdawn.slick.Input;
import org.newdawn.slick.geom.Vector2f;
import org.newdawn.slick.state.StateBasedGame;

public class BulletFiringComponent extends Component {

   float direction;
   ArrayList<Entity> bullets;
   
   public BulletFiringComponent (String id)  {
      
      this.id = id;
   }
   
   
   @Override
   public void update(GameContainer gc, StateBasedGame sbg, int delta) {

      Vector2f position = owner.getPosition();
      Vector2f start = new Vector2f(315, 197);
      Input input = gc.getInput();
      
      if (input.isKeyDown(Input.KEY_LEFT)) {
         
         
         position.x -= 6f;
         
      } else if (input.isKeyDown(Input.KEY_RIGHT))  {
                     
         
         position.x += 6f;
      }
      
      if (input.isKeyPressed(Input.KEY_B))  {
         
         owner.setPosition(start);
      }
   }
   
}


If anyone can explain what I need to do I would appreciate it greatly. :)
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davedes
PostPosted: Wed Apr 18, 2012 5:42 pm 
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Joined: Sat Jan 27, 2007 7:10 pm
Posts: 1469
My game would create a new BulletEntity for each bullet (allowing each bullet has its own velocity, position, etc). Shooting a bullet would simply add it to my large list of (all) entities, which was updated every frame. Once the bullet hit a player or fell off the screen, it would be removed from the large list of entities (for the entities array, I used this technique).

I don't know what the deal is with your bullet firing component, or "owner", or whatever (ideally each bullet should be independent from each other; unless you want all the bullets to be translated when the user presses B). But basically you will need a new entity object per bullet.

And you'll probably want to delay it so that bullets aren't being fired every frame, e.g. see here.
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trunksbomb
PostPosted: Tue Apr 24, 2012 7:40 am 
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Joined: Mon Apr 16, 2012 5:04 pm
Posts: 14
Sorry to hijack the thread, but this is relevant and I don't feel like it needs its own thread.

dave, you mention how you handle ArrayLists. I instead use an iterator and a second "to remove" list. It's a little something like this in update():

Code:
Iterator<Entity> iter = entities.iterator();
while (iter.hasNext()) {
   Entity e = iter.next();
   
   //update the entity

   if (someCondition) entitiesToRemove.add(e);
}

//After all entities have been processed, remove expired entities

Iterator<Entity> iter2 = entitiesToRemove.iterator();
while (iter2.hasNext()) {
   Entity e = iter2.next();
   entities.remove(e);
}

entitiesToRemove.clear();


How would the effectiveness of this compare to your method? It seems roughly the same.
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davedes
PostPosted: Tue Apr 24, 2012 5:27 pm 
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Slick Zombie

Joined: Sat Jan 27, 2007 7:10 pm
Posts: 1469
The "efficiency" we're talking about here is pretty negligible. In most 2D games, your bottleneck will not be because of operations like these, but because of fill-rate limitation, texture binds, complex pathfinding/AI/etc.

With that said, your method is inefficient for a few reasons:
  • For an ArrayList, it's better to iterate through the indices as if it was an array. It should be slightly faster than creating a new Iterator.
  • Your implementation requires iterating over two lists.
  • Your implementation depends on ArrayList.remove(T), which is pretty slow. The method iterates over the list to find the object's index, then shifts all subsequent elements to the left (using System.arraycopy).
  • Your list might grow while updating an entity (e.g. to spawn a bullet), and since you're using iterators you'll run into ConcurrentModificationExceptions.
  • Ideally you should check "someCondition" twice to avoid updating a dead entity (once before updating the entity, and again after updating) -- e.g. if entity A 'kills' entity B, then entity B will not be updated on next iteration.
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trunksbomb
PostPosted: Tue Apr 24, 2012 6:18 pm 
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Joined: Mon Apr 16, 2012 5:04 pm
Posts: 14
All fair and well. Although I haven't hit any concurrent errors yet, but I guess that's just been because of the scope of my games so far. I see how it can be an issue though. Thanks!
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Epicbo
PostPosted: Wed May 02, 2012 12:36 am 
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Joined: Sun Dec 25, 2011 5:48 pm
Posts: 34
ConcurrentModificationExceptions shouldn't be a problem unless you use several threads which, in my opinion, is something you should avoid unless you really have to.
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