remove elements from CopyOnWriteArrayList

问题:

I am getting an exception when I try to remove elements from CopyOnWriteArrayList using an iterator. I have noticed that it is documented

Element-changing operations on iterators themselves (remove, set, and add) are not supported. These methods throw UnsupportedOperationException.

(from http://download.oracle.com/javase/6/docs/api/java/util/concurrent/CopyOnWriteArrayList.html)

Now, surprisingly i can iterate it with foreach and use the remove() function . But then I get the famous bug - when trying to remove an item from a list using a for loop - you skip the element next to the removed element. any suggestions then?

回答1:

Iterate over the collection choosing all the elements you want to delete and putting those in a temporary collection. After you finish iteration remove all found elements from the original collection using method removeAll.

Would that work out for you? I mean, not sure if deletion logic is more complicated than that in your algorithm.

回答2:

EDIT: I'm an idiot. I missed the fact that this is a copy-on-write list so every removal means a new copy. So my suggestions below are likely to be suboptimal if there's more than one removal.

Same as for any other list whose iterator doesn't support remove, or anything where you're not using an iterator. There are three basic techniques that come to mind to avoid this bug:

1. Decrement the index after removing something (being careful not to do anything with the index until the next iteration). For this you'll obviously have to use a for(int i=0; i < ... style of for loop, so that you can manipulate the index.

2. Somehow repeat what the inside of the loop is doing, without literally going back to the top of the loop. Bit of a hack - I would avoid this technique.

3. Iterate over the list in reverse (from end to start, instead of from start to end). I prefer this approach as it's the simplest.

回答3:

Since this is a CopyOnWriteArrayList it is totally safe to remove elements while iterating with forEach. No need for fancy algorithms.

list.forEach(e -> {
    if (shouldRemove(e))
        list.remove(e);
});

EDIT: Well of course that works if you want to delete elements by reference, not by position.

回答4:

Ususlly you would iterate first gathering elemenet to be deleted in a separate list then delete them outside the for each loop (which is disguised iterator based loop anyway)

回答5:

Something like this:

int pos = 0;
while(pos < lst.size() ) {
  Foo foo = lst.get(pos);
  if( hasToBeRemoved(foo) ) {
    lst.remove(pos);
    // do not move position
  } else {
    pos++;
  }
}

回答6:

You could use Queue instead of List.

private Queue<Something> queue = new ConcurrentLinkedQueue<Something>();

It's thread safe and supports iterator.remove(). Be aware of the thread-safe behavior of Queue iterators, though (check the javadoc).

回答7:

the shortest and most efficient way:

List<String> list = new CopyOnWriteArrayList<>();
list.removeIf(s -> s.length() < 1);

internally it creates an temporary array with the same length and copies all elements where the predicate returns true.

keep in mind that if you use this method to actually iterate over the elements to perform some action, these actions cannot be performed in paralell anymore since the removeIf-call is atomic and will lock the traversal for other threads

回答8:

Below works fine with CopyOnWriteArrayList

for(String key : list) {
    if (<some condition>) {
        list.remove(key);
    }
}