Why CopyOnWriteArrayList's iterator allows remove() in enhanced-for loop, while its iterator does not support remove() operation?

Question

CopyOnWriteArrayList

The iterator does NOT support the remove method.

But why does it work in enhanced-for loop?

List<String> lst = new CopyOnWriteArrayList<>();
lst.add("one"); 
lst.add("two"); 
lst.add("three"); 


for (String str : lst) { 
    if (str.equals("one")) { 
        lst.remove("two");   // no ConcurrentModificationException      
    } 
}        

System.out.println(lst);   // [one, three] => removed "two" !

List<String> lst = new ArrayList<>(); would generate ConcurrentModificationException

Javadoc clearly states that CopyOnWriteArrayList.iterator() does not support remove() => it shall throw UnsupportedOperationException! I understand it is weakly consistent - say there is no ConcurrentModificationException if I add elements to CopyOnWriteArrayList after I obtained an iterator from that CopyOnWriteArrayList

P.S. Sorry, I was inattentive - I called remove() not on the iterator! I was confused by being inside enhanced-for (it uses an iterator implicitly).

Answer 1

CopyOnWriteArrayList iterator fail safe implementation supports modify actions.

When you iterate over a CopyOnWriteArrayList and CopyOnWriteArraySet the iterator uses a snapshot of the underlying list (or set) and does not reflect any changes to the list or set after the snapshot was created. The iterator will never throw a ConcurrentModificationException.

Read more at: https://markusjais.com/java-concurrency-understanding-copyonwritearraylist-and-copyonwritearrayset/

By the way, in classic List implementation like ArrayList() etc, you don't need to use iterator explicitly. Use list.removeIf(predicate) method.

Answer 2

If you try to use iterator to remove an element from CopyOnWriteArrayList. It will throw an exception. That's what javadoc is trying to say.

Code:

List<String> lst2 = new CopyOnWriteArrayList<String>();
lst2.add("one");
lst2.add("two");
lst2.add("three");

Iterator<String> iterator2 = lst2.iterator();
while (iterator2.hasNext()) {
    if (iterator2.next().equals("two")) {
        iterator2.remove();
    }
}
System.out.println(lst2.toString());

Output:

Exception in thread "main" java.lang.UnsupportedOperationException
    at java.util.concurrent.CopyOnWriteArrayList$COWIterator.remove(CopyOnWriteArrayList.java:1178)
    at MyClass.main(MyClass.java:29)

However if you do the same thing on Arraylist. It will work fine.

Source: ArrayList

Code:

List<String> lst = new ArrayList<String>();
lst.add("one");
lst.add("two");
lst.add("three");

Iterator<String> iterator = lst.iterator();
while (iterator.hasNext()) {
    if (iterator.next().equals("two")) {
        iterator.remove();
    }
}
System.out.println(lst.toString());

Output:

[one, three]

Though if you don't want to use iterator you can just use public boolean removeIf(Predicate<? super E> filter) which is just one line and gives the same output as above.

lst.removeIf(n -> (n.equals("two")));