Question

I’m trying to write a game, so every frame I call the doDraw() method where I’m using an iterator to loop through all GameObjects and print them all on screen:

Iterator<GameObject> itr = mObjList.iterator();
    while (itr.hasNext()) {
        GameObject obj = itr.next(); // this line gives me the error
        ...
        // print object
    }

The only method that adds item to the list, is this:

public void click(int x, int y) {
    // adds new object to the list on a click event
    mObjList.add(new GameObject(x, y));
}

Most of the times it works. But sometimes I get this error:

java.util.ConcurrentModificationException

From the line with “itr.next()”. From what I’ve googled, I figured this is because the click() event sometimes happen before the draw() finishes drawing every object, so it’s changing the list while the iterator is using it. I suppose this is what’s wrong?

But I’m not experienced with threads. How could I possibly fix this? Maybe I’m doing this whole thing wrong and I should use a completely different method to print all objects on screen?

Answer 1

If the expected number of reads and traversals greatly outnumbers the number of updates to the list, use a CopyOnWriteArrayList.

Otherwise, synchronize on the list (or a dedicated mutex object) when iterating and mutating:

private List<GameObject> mObjList = /* whatever */;
private final Object mListMutex = new Object();

// snip...

synchronized (mListMutex) {
    for (GameObject obj : mObjList) {
        // do your thang
    }
}

// snip...
public void click(int x, int y) {
    GameObject obj = new GameObject(x, y);
    synchronized (mListMutex) {
        mObjList.add(obj);
    }
}