Remove element from ArrayList internally

Question

I have the following code:

class Action {

    public void step(Game game) {
        //if some condition met, 
        // then remove self from action stack
        game.actionStack.remove(this);

}


class Game (

    public ArrayList<Action> actionStack;

    public Game() {
        actionStack = new Arraylist<Action>();
        actionStack.add(new Action());

        while (true) {
            for (Action action : this.actionStack) {
                action.step(this);
            }
        }
    }
}

An exception gets thrown when game.actionStack.remove(this); occurs. Is there a way to remove the element safely from inside the Action class like I want?

If you try to remove element from array, you may get `ConcurrentModificationException `. You need to use `Iterator` remove for this — Hüseyin BABAL, Jan 15 '14 at 07:05
possible duplicate of [ConcurrentModificationException for ArrayList](http://stackoverflow.com/questions/3184883/concurrentmodificationexception-for-arraylist) — Dennis Meng, Jan 15 '14 at 07:08

score 4 · Accepted Answer · answered Jan 15 '14 at 07:09

I'm guessing you're getting a ConcurrentModificationException because you're calling the list remove method while iterating it. You can't do that.

An easy fix is to work on a copy of the array when iterating:

for (Action action : new ArrayList<>(this.actionStack)) {
    action.step(this);
}

A slightly more efficient fix is to use an explicit Iterator and call its remove method. Perhaps have step() return a boolean indicating whether it wants to remain in the list for the next step or not:

for (Iterator<Action> it = this.actionStack.iterator(); it.hasNext();) {
    Action action = it.next();
    if (!action.step(this)) {
        it.remove();
    }
}

score 1 · Answer 2 · answered Jan 15 '14 at 07:31

From : the java tutorial we get the following:

Iterators

...

Note that Iterator.remove is the only safe way to modify a collection during iteration; the behavior is unspecified if the underlying collection is modified in any other way while the iteration is in progress.

Use Iterator instead of the for-each construct when you need to:

Remove the current element. The for-each construct hides the iterator, so you cannot call remove. Therefore, the for-each construct is not usable for filtering.

Iterate over multiple collections in parallel.

The following method shows you how to use an Iterator to filter an arbitrary Collection — that is, traverse the collection removing specific elements.

static void filter(Collection<?> c) {
    for (Iterator<?> it = c.iterator(); it.hasNext(); )
        if (!cond(it.next()))
            it.remove();
}

This simple piece of code is polymorphic, which means that it works for any Collection regardless of implementation. This example demonstrates how easy it is to write a polymorphic algorithm using the Java Collections Framework.

Hüseyin BABAL · Answer 3 · 2014-01-15T07:53:20.273

0

Note: I assume, you implemented equals and hashCode methods for your class You need to use iterator to remove like below;

class Game (

    public ArrayList<Action> actionStack;

    public Game() {
        actionStack = new Arraylist<Action>();
        actionStack.add(new Action());

        while (true) {
            for (Iterator<Action> it = this.actionStack.iterator(); it.hasNext(); ) {
               it.remove();
            }
        }
    }
}

Edit: step function is doing simple remove job. I move it to Game constructor

edited Jan 15 '14 at 07:53

answered Jan 15 '14 at 07:09

Hüseyin BABAL

15,400
4
51
73

This won't work because you're still concurrently modifying the list. You have to remove the element through the *same* iterator as is used in the main loop. – Boann Jan 15 '14 at 07:34

Kainda · Answer 4 · 2014-01-15T10:44:24.127

0

I suspect that you are getting a Concurrent Modification Exception. I would suggest you do it like this

class Action {

    public void step(Game game) {
        //if some condition met, 
        // then remove self from action stack
        List<Action> tmpActionList = new List<Action>();
        tmpActionList = game.actionStack
        tmpActionList.remove(this);
        game.actionStack = tmpActionList;
    }
}

Let me know if it works.

edited Jan 15 '14 at 10:44

answered Jan 15 '14 at 07:18

Kainda

525
6
17

1

This won't work because tmpActionList is not a copy. It's a pointer to the same list object. – Boann Jan 15 '14 at 07:34

Remove element from ArrayList internally

4 Answers4