Calling remove in foreach loop in Java

java loops iterator foreach
Michael Bobick picture Michael Bobick · Jul 28, 2009 · Viewed 455.2k times · Source

In Java, is it legal to call remove on a collection when iterating through the collection using a foreach loop? For instance:

List<String> names = ....
for (String name : names) {
   // Do something
   names.remove(name).
}

As an addendum, is it legal to remove items that have not been iterated over yet? For instance, 

//Assume that the names list as duplicate entries
List<String> names = ....
for (String name : names) {
    // Do something
    while (names.remove(name));
}

Answer

Mark picture Mark · Jul 28, 2009

To safely remove from a collection while iterating over it you should use an Iterator.

For example:

List<String> names = ....
Iterator<String> i = names.iterator();
while (i.hasNext()) {
   String s = i.next(); // must be called before you can call i.remove()
   // Do something
   i.remove();
}

From the Java Documentation :

The iterators returned by this class's iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Perhaps what is unclear to many novices is the fact that iterating over a list using the for/foreach constructs implicitly creates an iterator which is necessarily inaccessible. This info can be found here

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