Iterator allows to remove elements during iteration

With a for each loop you can iterate easily through a collection but you cannot remove elements during iteration.

List<String> list = new ArrayList<String>();
list.add("Max");
list.add("bob");
list.add("Alex");

// for each loop:
// execution this loop ends with a
// java.util.ConcurrentModificationException
for (String element : list) {
   list.remove(element);
}

A java.util.Iterator is a suitable solution for the above situation, because it allows you to remove elements from a collection during iteration.

Iterator<String> iterator = list.iterator(); 

while (iterator.hasNext()) {
   iterator.next()); 
   iterator.remove();
}

Iterator interface has three methods:

boolean hasNext()
E next()
void remove()

and allows for iteration in a one direction. If you want to iterate through a list in both directions you should use a ListIterator (implements Iterator) which has additional methods:

boolean hasPrevious() 
E previous()
int previousIndex() 
int nextIndex()