How to Safely Remove Elements from a Java Collection While Iterating?

Safely Removing Elements during Iteration in Java

Q: Is it permissible to remove elements from a collection while iterating over it using a foreach loop in Java?

A: Using a foreach loop for removal is not advisable. It can result in unexpected behavior as the underlying iterator is not exposed and directly accessible for modification.

Example:

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

This code may fail with a java.util.ConcurrentModificationException as the remove operation modifies the collection underneath while the foreach loop is in progress.

Q: Can we remove items that have not been iterated over yet?

A: No. Removing elements not yet processed in the loop can lead to an "index out of bounds" error. Example:

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

To safely remove elements while iterating, an Iterator should be used:

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();
}

The Java documentation emphasizes that iterators returned by the iterator and listIterator methods are "fail-fast." Any structural modifications to the list outside the iterator's remove or add methods will result in a ConcurrentModificationException.

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