I am currently having a small issue about ConcurrentModificationException(). I know there are alot of tutorials out there on how to fix this problem, but I only saw evryone had trouble removing entities. I am having trouble adding entities. I thaught I do the same thing as I did with destroying entities. I didnt knew I had this issue, because I was adding entities in the Game.java and not in an entitie. In that way I added the entities before the level was “ticking” through the arrayList of the entities. But now my problem is: I don’t know how to add entities after ticking throught them. This is what I tried:
public synchronized void tick() {
Iterator<Entity> it = getEntities().iterator();
while (it.hasNext()) {
Entity e = it.next();
e.tick();
if (e.destroyed) {
it.remove();
}
if (e.spawn) {
Entity s = e.getEntityToSpawn();
this.getEntities().add(s);
e.spawn = false;
}
}
}
But as you can see I am just again adding the entities while looping through the list. Does someone know how to fix this and only explain it. I need to learn java so I want to get the solusion by myself!
one easy why to fix this is by having two collections while iterating over all game objects. Caching those two collections or creating every tick a new one and throwing the older one away is another topic.
So you tick loop over your game objects would look like this.
Get a empty collection
iterate over your old collection
add all game objects which still live to the new collection
add any new game objects to the new collection
mark the new collection as the primary game object collection
You can simply remove the entities using iterator.remove(), and add new entities to a holding collection while your loop is running. Then after the loop is finished, add all the entities from the holding collection to the real collection.
Thanks for the help. If someone is interested in this code because (s)he is doing it also wrong:
List<Entity> nextEntities = new ArrayList<Entity>();
for (Entity e : getEntities()) {
e.tick();
if (!e.destroyed){
nextEntities.add(e);
}
if (e.spawn){
nextEntities.add(e.entityToSpawn);
}
}
There is only 1 problem when I am spawning entities now: The first and last entity are glitching out or something. I am using this code to spawn them:
package blockygames.game.entities;
import blockygames.game.gfx.Screen;
import blockygames.game.level.Level;
public abstract class Entity {
public int x, y;
protected Level level;
public boolean destroyed = false, spawn = false;
public Entity entityToSpawn;
public Entity(Level level) {
init(level);
}
public final void init(Level level) {
this.level = level;
}
public abstract void tick();
public boolean isDestroyed() {
return destroyed;
}
public Entity getEntityToSpawn() {
return entityToSpawn;
}
public int getX() {
return x;
}
public int getY() {
return y;
}
public abstract void render(Screen screen);
}
If someone has an idea how to fix this problem, because I don’t know what I am doing wrong. The bunny is speedhacking/crashing or some sort of glitch.
Read these lovely things from princec and theagentd.
Though it has to be said, considering the number of things that might be found in an ArrayList, removing things from the head of the queue is really often so fast you’d never ever notice it. It depends on how many things are likely to be in it…
Consider a real-life example of, hmm… Particles. So you’ve got 5,000 particles in a frame (a high figure, but not unlikely). They’re stored in an ArrayList. Each entry in the ArrayList takes 4 bytes, so that’s 20kb of RAM, fitting neatly in the L1 data cache on many if not all desktops.
Now, imagine the rather unlikely even that every single particle dies one frame, and you discover this as you iterate through the ArrayList (using an integer index, not an iterator, just for absolute efficiency’s sake). You have to shuffle 4,999 particles down 4 bytes. It’s all in the L1 cache so memory access is effectively free to the CPU; you spend 4,999 cycles moving your particles assuming the loop that copies the data is about as simple and efficient as it can be. Then you have to do it again on the next particle. 4998 cycles. Repeat to solve triangular number, approximately (5000x5000)/2, or in the end 12,500,000 clock cycles, without ever having to touch the L2 or L3 caches most likely, let alone system RAM, until the end of the operation.
That’s 12.5m cycles out of your 3.3 million or so you’ve typically got in a single video frame (2GHz core). Oh dear, you just spent 4 frames ditching a mere 5000 particles. Judder. Imagine if you had, somehow, 10000 particles. That’d be nearly a second wasted doing something utterly trivial and would present itself as a horrible jarring delay in your buttery smooth animation.
But what if, weirdly but possibly, the worst case occurred when even going backwards - every other particle died in one frame? Well, then you’d be compacting a slowly more complex list of particles even if you were scanning backwards. Scanning forwards would be exactly the same speed too.
So you’d be crafty and use the third and final technique which is to make a copy of the original arraylist, and copy the surviving particles into it each frame. This performs consistently no matter how many live or dead particles you have in any particular frame or the pattern of their expiration.
Anyway - for the OP - basically you need to do Just That. Each frame, scan your list of Interactables, and copy each live one into a second ArrayList, and then point your game at that ArrayList. Flip between two ArrayLists, alternating each frame, rather than creating (and expanding!) an ArrayList every frame, or that’ll be slow.
