The collections api - Data Structures and Problem Solving Using Java - page 241

API takes a strict view: Any external structural modification of the container
(adds, removes, and so on) will result in a ConcurrentModificationException by
the iterator methods when one of the methods is called. In other words, if we
have an iterator, and then an object is added to the container, and then we invoke
the next method on the iterator, the iterator will detect that it is now invalid, and
an exception will be thrown by next .

This means that it is impossible to remove an object from a container
when we have seen it via an iterator, without invalidating the iterator. This is
one reason why there is a remove method in the iterator class. Calling the itera-
tor remove causes the last seen object to be removed from the container. It
invalidates all other iterators that are viewing this container, but not the itera-
tor that performed the remove . It is also likely to be more efficient than the con-
tainer's remove method, at least for some collections. However, remove cannot
be called twice in a row. Furthermore, remove preserves the semantics of next
and hasNext , because the next unseen item in the iteration remains the same.
This version of remove is listed as an optional method, so the programmer
needs to check that it is implemented. The design of remove has been criticized
as poor, but we will use it at one point in the text.
Figure 6.10 provides a sample specification of the Iterator interface.
(Our iterator class extends the standard java.util version in order to allow

The Iterator
methods throw an
exception if its
container has
been structurally
modified.

1 package weiss.util;
2
3 /**
4 * Iterator interface.
5 */
6 public interface Iterator<AnyType> extends java.util.Iterator<AnyType>
7 {
8 /**
9 * Tests if there are items not yet iterated over.
10 */
11 boolean hasNext( );
12
13 /**
14 * Obtains the next (as yet unseen) item in the collection.
15 */
16 AnyType next( );
17
18 /**
19 * Remove the last item returned by next.
20 * Can only be called once after next.
21 */
22 void remove( );
23 }

figure 6.10
A sample specification of Iterator