Introducing Nested and Inner Classes - Java

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 does not exist independently 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. A nested class that is declared directly within its enclosing class scope is a member of its enclosing class. It is also possible to declare a nested class that is local to a block.

There are two types of nested classes: static and non-static. A static nested class is one that has the static modifier applied. Because it is static, it must access the non-static members of its enclosing class through an object. That is, it cannot refer to non-static 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.

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;

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 an instance of Inner can be created only in the context of class Outer. The Java compiler generates an error message otherwise. In general, an inner class instance is often created by code within its enclosing scope, as the example does.

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 inner classes declared as members 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 applicable to all situations, they are particularly helpful when handling events. We will return to the topic of nested classes in Chapter 24. 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.