The collections api - Data Structures and Problem Solving Using Java - page 235

Figure 6.5 shows MyContainer . In the revised MyContainer , the iterator
method returns a reference to an Iterator object; the actual type turns out to
be a MyContainerIterator . Since MyContainerIterator IS-A Iterator , this is safe
to do.

Because iterator creates and returns a new Iterator object, whose actual
type is unknown, it is commonly known as a factory method . The iterator
interface, which serves simply to establish the protocol by which all sub-
classes of Iterator can be accessed, is shown in Figure 6.6. There are only
two changes to the implementation of MyContainerIterator , shown in
Figure 6.7 and both changes are at line 5. First, the implements clause has been
added. Second, MyContainerIterator no longer needs to be a public class.

A factory method
creates a new
concrete instance
but returns it using
a reference to the
interface type.

Figure 6.8 demonstrates how the inheritance-based iterators are used. At
line 9, we see the declaration of itr : It is now a reference to an Iterator .
Nowhere in main is there any mention of the actual MyContainerIterator type.
The fact that a MyContainerIterator exists is not used by any clients of the
MyContainer class. This is a very slick design and illustrates nicely the idea of
hiding an implementation and programming to an interface . The implementa-
tion can be made even slicker by use of nested classes, and a Java feature
known as inner classes . Those implementation details are deferred until
Chapter 15.

Nowhere in main is
there any mention
of the actual itera-
tor type.

1 package weiss.ds;
2
3 public class MyContainer
4 {
5 Object [ ] items;
6 int size;
7
8 public Iterator iterator( )
9 { return new MyContainerIterator( this ); }
10
11 // Other methods not shown.
12 }

figure 6.5
The MyContainer class,
design 2

1 package weiss.ds;
2
3 public interface Iterator
4 {
5 boolean hasNext( );
6 Object next( );
7 }

figure 6.6
The Iterator
interface, design 2