We all know you can’t do the following because of ConcurrentModificationException
:
for (Object i : l) { if (condition(i)) { l.remove(i); } }
But this apparently works sometimes, but not always. Here’s some specific code:
public static void main(String[] args) { Collection<Integer> l = new ArrayList<>(); for (int i = 0; i < 10; ++i) { l.add(4); l.add(5); l.add(6); } for (int i : l) { if (i == 5) { l.remove(i); } } System.out.println(l); }
This, of course, results in:
Exception in thread "main" java.util.ConcurrentModificationException
Even though multiple threads aren’t doing it. Anyway.
What’s the best solution to this problem? How can I remove an item from the collection in a loop without throwing this exception?
I’m also using an arbitrary Collection
here, not necessarily an ArrayList
, so you can’t rely on get
.
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Answer
Iterator.remove()
is safe, you can use it like this:
List<String> list = new ArrayList<>(); // This is a clever way to create the iterator and call iterator.hasNext() like // you would do in a while-loop. It would be the same as doing: // Iterator<String> iterator = list.iterator(); // while (iterator.hasNext()) { for (Iterator<String> iterator = list.iterator(); iterator.hasNext();) { String string = iterator.next(); if (string.isEmpty()) { // Remove the current element from the iterator and the list. iterator.remove(); } }
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.
Source: docs.oracle > The Collection Interface
And similarly, if you have a ListIterator
and want to add items, you can use ListIterator#add
, for the same reason you can use Iterator#remove
— it’s designed to allow it.
In your case you tried to remove from a list, but the same restriction applies if trying to put
into a Map
while iterating its content.