Introducing Nested and Inner Classes
It is possible to define a class within another class; such classes are known as nested classes. The scope of a nested class is bounded by the scope of its enclosing class. Thus, if class B is defined within class A, then B is known to A, but not outside of A.
A nested class has access to the members, including private members, of the class in which it is nested. However, the enclosing class does not have access to the members of the nested class.
There are two types of nested classes: static and non-static.
A static nested class is one which has the static modifier applied. Because it is static, it must access the members of its enclosing class through an object. That is, it cannot refer to members of its enclosing class directly. Because of this restriction, static nested classes are seldom used. The most important type of nested class is the inner class. An inner class is a non-static nested class. It has access to all of the variables and methods of its outer class and may refer to them directly in the same way that other non-static members of the outer class do. Thus, an inner class is fully within the scope of its enclosing class.
The following program illustrates how to define and use an inner class. The class named Outer has one instance variable named outer_x, one instance method named test( ), and defines one inner class called Inner.
// Demonstrate an inner class.
class Outer {
int outer_x = 100;
182
void test() {
Inner inner = new Inner();
inner.display();
}
// this is an inner class
class Inner {
void display() {
System.out.println("display: outer_x = " + outer_x);
}
}
}
class InnerClassDemo {
public static void main(String args[]) {
Outer outer = new Outer();
outer.test();
}
}
Output from this application is shown here:
display: outer_x = 100
In the program, an inner class named Inner is defined within the scope of class Outer. Therefore, any code in class Inner can directly access the variable outer_x. An instance method named display( ) is defined inside Inner. This method displays outer_x on the standard output stream. The main( ) method of InnerClassDemo creates an instance of class Outer and invokes its test( ) method. That method creates an instance of class Inner and the display( ) method is called.
It is important to realize that class Inner is known only within the scope of class Outer. The Java compiler generates an error message if any code outside of class Outer attempts to instantiate class Inner. Generalizing, a nested class is no different than any other program element: it is known only within its enclosing scope.
As explained, an inner class has access to all of the members of its enclosing class, but the reverse is not true. Members of the inner class are known only within the scope of the inner class and may not be used by the outer class.
For example,
// This program will not compile.
class Outer {
int outer_x = 100;
void test() {
Inner inner = new Inner();
inner.display();
}
// this is an inner class
class Inner {
int y = 10; // y is local to Inner
void display() {
System.out.println("display: outer_x = " + outer_x);
}
}
void showy() {
System.out.println(y); // error, y not known here!
}
}
class InnerClassDemo {
public static void main(String args[]) {
Outer outer = new Outer();
outer.test();
}
}
Here, y is declared as an instance variable of Inner. Thus it is not known outside of that class and it cannot be used by showy( ).
Although we have been focusing on nested classes declared within an outer class scope, it is possible to define inner classes within any block scope.
For example, you can define a nested class within the block defined by a method or even within the body of a for loop, as this next program shows.
// Define an inner class within a for loop.
class Outer {
int outer_x = 100;
void test() {
for(int i=0; i<10; i++) {
class Inner {
void display() {
System.out.println("display: outer_x = " + outer_x);
}
}
Inner inner = new Inner();
inner.display();
}
}
}
class InnerClassDemo {
public static void main(String args[]) {
Outer outer = new Outer();
outer.test();
}
}
The output from this version of the program is shown here.
display: outer_x = 100
display: outer_x = 100
display: outer_x = 100
display: outer_x = 100
display: outer_x = 100
display: outer_x = 100
display: outer_x = 100
display: outer_x = 100
display: outer_x = 100
display: outer_x = 100
While nested classes are not used in most day-to-day programming, they are particularly helpful when handling events in an applet. We will return to the topic of nested classes in Post 20. There you will see how inner classes can be used to simplify the code needed to handle certain types of events. You will also learn about anonymous inner classes, which are inner classes that don't have a name. One final point: Nested classes were not allowed by the original 1.0 specification for Java. They were added by Java 1.1.
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