Java Reference
In-Depth Information
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15.2: Static Inner Classes
You first saw the use of inner classes for event handlers. Inner classes are useful in
that context, because their methods have the privilege of accessing private data
members of outer-class objects. The same is true for the
LinkedListIterator
inner class in the sample code for this section. The
iterator needs to access the
first
instance variable of its linked list.
However, the
Node
inner class has no need to access the outer class. In fact, it has
no methods. Thus, there is no need to store a reference to the outer list class with
each
Node
object. To suppress the outer-class reference, you can declare the inner
class as
static
:
public class LinkedList
{
. . .
private
static
class Node
{
. . .
}
}
The purpose of the keyword
static
in this context is to indicate that the
inner-class objects do not depend on the outer-class objects that generate them. In
particular, the methods of a static inner class cannot access the outer-class instance
variables. Declaring the inner class
static
is efficient, because its objects do not
store an outer-class reference.
However, the
LinkedListIterator
class cannot be a static inner class. It
frequently references the
first
element of the enclosing
LinkedList
.
15.3 Abstract and Concrete Data Types
There are two ways of looking at a linked list. One way is to think of the concrete
implementation of such a list as a sequence of node objects with links between them
(see
Figure 8
).
