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From: Robert Klemme <shortcutter@googlemail.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: iteration blues
Date: Fri, 04 Nov 2011 10:34:53 +0100
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On 11/03/2011 04:37 PM, bob wrote:
> So, I wrote this code for some particle effects:
>
> package com.coolfone.particles;
>
> import java.util.Iterator;
> import java.util.Vector;
>
> import javax.microedition.khronos.opengles.GL10;
>
> public class FireManager {
> 	static Vector<Particle>  particles = new Vector<Particle>();
>
> 	public static void startfire(float x, float y) {
> 		for (int ctr = 0; ctr<  100; ctr++) {
> 			Particle p = new Particle();
> 			p.x = (float) (x + Math.random()-.5);
> 			p.y = (float) (y + Math.random()-.5);
> 			p.dx = (float) (Math.random()-.5)/4f;
> 			p.dy = (float) (Math.random()-.5)/4f;
> 			p.timeleft = (int) (Math.random() * 50 + 50);
> 			particles.add(p);
> 		}
> 	}
>
> 	public static void burnfire() {
> 		Iterator<Particle>  i = particles.iterator();
> 		Vector<Particle>  removelist = new Vector<Particle>();
> 		while (i.hasNext()) {
> 			Particle p = i.next();
> 			p.move();
> 			p.timeleft--;
> 			if (p.timeleft == 0) removelist.add(p);
>
> 		}
> 		particles.removeAll(removelist);
>
> 	}
>
> 	public static void drawfire(GL10 gl) {
> 		Iterator<Particle>  i = particles.iterator();
> 		while (i.hasNext()) {
> 			Particle p = i.next();
> 			p.draw(gl);
> 		}
> 	}
>
> }
>
> I'm concerned about inefficiency in the burnfire function.  Does
> anyone know how to rewrite this quickly if particles was a linked
> list?  The main issue is that I'm not sure if removing items during
> iteration messes up the iterator.

I'm surprised nobody seems to mention Iterator.remove().

public static void burnfire() {
   for (final Iterator<Particle> i = particles.iterator(); i.hasNext();) {
     final Particle p = i.next();
     p.move();
     p.timeleft--;  // Direct access to member, bad!

     if (p.timeleft == 0) {
       iter.remove();
     }
   }
}

This can be used regardless of container type.  Efficiency depends on 
the ratio of removed elements.  If you remove much and do not need 
indexed access (i.e. via List.get(int)) you can use a LinkedList. 
Otherwise use ArrayList as indicated already.  There is no point in 
using Vector these days any more.

And btw, do not be concerned about performance, measure it.  Results may 
be surprising.

Kind regards

	robert