Is there an accepted best practice in Java for deleting a list element while iterating over the list?

Question

I'm finding conflicting advice over the best way to avoid a ConcurrentModificationException while doing this:

    List<Apple> Apples = appleCart.getApples();
    for (Apple apple : Apples)
    {
        delete(apple);
    }

I'm leaning towards using an Iterator in place of a List and calling its remove method.

Does that make the most sense here?

Hovercraft Full Of Eels · Answer 1 · 2012-03-30T16:41:14.587

5

Yes, use an Iterator. Then you could use its remove method.

  for (Iterator<Apple> appleIterator = Apples.iterator(); appleIterator.hasNext();) {
     Apple apple = appleIterator.next();
     if (apple.isTart()) {
        appleIterator.remove();
     }
  }
}

edited Mar 30 '12 at 16:41

answered May 06 '11 at 18:11

Hovercraft Full Of Eels

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Andy Thomas · Answer 2 · 2011-05-06T18:24:17.450

2

If you're getting a ConcurrentModificationException, you likely have multiple threads.

So the full answer includes both using Iterator.remove() and synchronizing access to the collection.

For example (where lock is synchronized on by all threads that may modify the list):

synchronized ( lock ) {
   List<Apple> apples = appleCart.getApples();
   for ( Iterator<Apple> it = apples.iterator(); it.hasNext(); )
   {
      Apple a = it.next(); 
      if ( a.hasWorm() ) {
         it.remove();
      }
   }
}

edited May 06 '11 at 18:24

answered May 06 '11 at 18:17

Andy Thomas

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The code he posted will always throw ConcurrentModification regardless of concurrency issues. (Unless of course the list only has one element!) – Affe May 06 '11 at 18:40
In the OP's example, `ConcurrentModificationException` wouldn't involve multiple threads at all. It'd just be a result of removing from the list while iterating using its iterator. – ColinD May 06 '11 at 18:41

score 1 · Answer 3 · answered May 06 '11 at 18:12

1

List<Apple> apples = appleCart.getApples();
for (Iterator<Apple> appleIterator = apples.iterator(); appleIterator.hasNext();)
{
   Apple apple = appleIterator.next();
   if ( apple.isYucky() ) {
     appleIterator.remove();
   }
}

answered May 06 '11 at 18:12

Clint

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1+ for thinking up the exact same code as me at the same time! :) – Hovercraft Full Of Eels May 06 '11 at 18:14
We have all been around this block. – Clint May 06 '11 at 18:15

score 0 · Answer 4 · answered Oct 25 '18 at 21:19

0

Since Java 8 you can now do this: apples.removeIf(apple -> apple.equals(this))

answered Oct 25 '18 at 21:19

Sam Orozco

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score 0 · Answer 5 · answered May 06 '11 at 18:15

You could keep a list of items to remove and then remove them after the loop:

List<Apple> apples = appleCart.getApples();
List<Apple> badApples = new ArrayList<Apple>();
for (Apple apple : apples) {
    if (apple.isBad()) {
        badApples.add(apple);
    } else {

    eat(apple);
}

apples.removeAll(badApples);

Is there an accepted best practice in Java for deleting a list element while iterating over the list?

5 Answers5

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