Inner classes and implementation of ArrayList - Data Structures and Problem Solving Using Java - page 576

1 package weiss.ds;
2
3 public class MyContainer
4 {
5 private Object [ ] items;
6 private int size = 0;
7 // Other methods for MyContainer not shown
8
9 public Iterator iterator( )
10 { return new LocalIterator( this ); }
11
12 // The iterator class as a nested class
13 private static class LocalIterator implements Iterator
14 {
15 private int current = 0;
16 private MyContainer container;
17
18 private LocalIterator( MyContainer c )
19 { container = c; }
20
21 public boolean hasNext( )
22 { return current < container.size; }
23
24 public Object next( )
25 { return container.items[ current++ ]; }
26 }
27 }

figure 15.5
Iterator design using
nested class

iterators and inner classes

15.2

An inner class is
similar to a nested
class in that it is
a class inside
another class and is
declared using the
same syntax as a
nested class,
except that it is not
a static class. An
inner class always
contains an implicit
reference to the
outer object that
created it.

In Section 15.1, we used a nested class to further hide details. In addition to
nested classes, Java provides inner classes. An inner class is similar to a
nested class in that it is a class inside another class and is treated as a member
of the outer class for visibility purposes. An inner class is declared using the
same syntax as a nested class, except that it is not a static class. In other
words, the static qualifier is missing in the inner class declaration.
Before getting into the inner class specifics, let us look at the problem that
they are designed to solve. Figure 15.6 illustrates the relationship between the
iterator and container classes that were written in the previous section. Each
instance of the LocalIterator maintains a reference to the container over
which it is iterating and a notion of the iterator's current position. The rela-
tionship that we have is that each LocalIterator must be associated with
exactly one instance of MyContainer . It is impossible for the container refer-
ence in any iterator to be null , and the iterator's existence makes no sense
without knowing which MyContainer object caused its creation.