Grouping objects - Objects First with Java: A Practical Introduction Using BlueJ - page 125

The most recent solution, using an Iterator , is available for all collections in the Java class
library and thus is an important code pattern that we shall use again in later projects.
4.12.2 Removingelements
Another important consideration when choosing which looping structure to use comes in when
we have to remove elements from the collection while iterating. An example might be that we
want to remove all tracks from our collection that are by an artist we are no longer interested in.
We can quite easily write this in pseudo-code:
for each track in the collection {
if track.getArtist() is the out-of-favor artist:
collection.remove(track)
}
It turns out that this perfectly reasonable operation is not possible to achieve with a
for-each loop. If we try to modify the collection using one of the collection's remove
methods while in the middle of an iteration, the system will report an error (called a
ConcurrentModificationException ). This happens because changing the collection in
the middle of an iteration has the potential to thoroughly confuse the situation. What if the re-
moved element was the one we were currently working on? If it has now been removed, how
should we find the next element? There are no generally good answers to these potential prob-
lems, so using the collection's remove method is just not allowed during an iteration with the
for-each loop.
The proper solution to removing while iterating is to use an Iterator . Its third method (in
addition to hasNext and next ) is remove . It takes no parameter and has a void return type.
Calling remove will remove the item that was returned by the most recent call to next . Here is
some sample code:
Iterator<Track> it = tracks.iterator();
while(it.hasNext()) {
Track t = it.next();
String artist = t.getArtist();
if(artist.equals(artistToRemove)) {
it.remove();
}
}
Once again, note that we do not use the tracks collection variable in the body of the loop.
While both ArrayList and Iterator have remove methods, we must use the Iterator 's
remove method, not the ArrayList 's.
Using the Iterator 's remove is less flexible: we cannot remove arbitrary elements, but only
the last element we retrieved from the Iterator 's next method. On the other hand, using the
Iterator 's remove is allowed during an iteration. Because the Iterator itself is informed
of the removal (and does it for us), it can keep the iteration properly in sync with the collection.
Such removal is not possible with the for-each loop, because we do not have an Iterator
there to work with. In this case, we need to use the while loop with an Iterator .