Declarations, Initialization, and Scoping - SCJP Sun Certified Programmer for Java Platform SE6 - page 153

A member inner class can be declared public , private , protected or have the default
access.
A member inner class can extend any class and implement any number of interfaces.
A member inner class can be abstract or final .
An inner class cannot declare static fields or methods.
Most importantly, a member inner class has access to the members of the outer class,
even the private members.
That last property is what makes inner classes so useful and benefi cial. A member inner
class has access to its outer class members without using any special syntax.
Let's look at a simple example to get started. Here is a class named Outer that contains a
protected member inner class named Inner :
1. public class Outer {
2. private String greeting;
3.
4. protected class Inner {
5. public int repeat = 3;
6. public void go() {
7. for(int i = 1; i <= repeat; i++) {
8. System.out.println(greeting);
9. }
10. }
11. }
12.}
An inner class declaration is like any other top-level class. It can declare fi elds, methods,
constructors, and so on. The Inner class has a fi eld named repeat and a method named go .
However, what makes Inner unique is it can access the members of Outer .
The important line of code here to focus on is line 8 when the Inner class displays the
private greeting fi eld of the Outer class. For line 8 to make sense, there has to be a unique
greeting associated with the instance of Inner . Otherwise, it is not clear which greeting
reference to display. What makes this possible are the following properties of inner classes:
An inner class object is associated with exactly one outer class object. This association
is made when the inner object is instantiated with the new keyword.
You cannot instantiate an instance of an inner class without a corresponding outer
class instance.
The syntax for instantiating an inner class is to use a reference with the new operator.
For example:
Outer a = new Outer();
Outer.Inner b = a.new Inner();