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Chapter 12

Inner Classes

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Table of Contents

1. Inner Classes .................................................................................................... 4

1.1 Inner Classes .............................................................................................. 4

1.2 Member Classes ........................................................................................ 5

1.3 Local Classes .............................................................................................. 5

1.4 Anonymous Classes ................................................................................... 8

1.5 Instance Initializers .................................................................................... 8

1.6 Static Nested Classes ................................................................................ 10

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Chapter 12

Inner Classes

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12Documentation Convention

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12Chapter 12: Inner Classes

In the following chapter we are going to deal with different types of classes namely inner

classes, local classes, anonymous classes etc. The nested classes are also discussed here.

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121. Inner Classes

1.1 Inner Classes

The class file whose name is present inside another class is called inner class. The scope of

this class is present inside its parent class. Thus they are invisible to other classes in the

same package. It is a non-static nested class. It is fully under the scope of its enclosing class.

The following program shows how an inner class is used.

Example program

//Program for inner class

class Outerone {

int outerone_z = 500;

void test() {

Innerone innerone = new Innerone();

innerone.display();

}

// this is the inner class

class Innerone {

void display() {

System.out.println("display: outerone_z = " +

outerone_z); }}}

class InnerClassDemo {

public static void main(String args[]) {

Outerone outerone = new Outerone();

outerone.test();

}

}

Output

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12In the above program an inner class named as innerone is defined within the scope of

class outerone. Therefore, any code in class innerone can directly access the variable

outerone. The display() method is defined inside innerone. This method displays outerone_x

in the output stream. An error message is displayed by the compiler if any code outside of

class outerone tries to instantiate the inner class.

Rules of inner classes

- Inner classes cannot have static members, only static final variables.- Interfaces are never inner- Static classes are not inner classes.- The static members may be inherited by inner classes that are not compile time

constants even though they may not declare them.

1.2 Member Classes

A member class is a class defined inside the body of a class. They can also use instances of

the same class. An object of an inner class must be internally linked to an object of the

enclosing class. Thus a member class is a true inner class.

Why we use member classes?

The main objective of using member classes is to access the other members of the class.

The methods of member class have direct access to all the members enclosing the members

of the enclosing classes. Hence it is mainly useful at instances where there is no reason for

an object of a member class to exist in the absence of an object of the enclosing class and

where the methods of the object of the member class need to access to members of the

object of the enclosing class.

1.3 Local Classes

A local class is a class defined within a block of Java code.

A local class is visible only within that particular block. The local classes are defined within

an enclosing class though they are not member classes. Hence they share many features of

member class.

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12The features of local classes are:

They are only visible and usable within the block of code in which they are defined. They can use any final local variables or method parameters that are visible from the

scope in which they are defined.

Example program

interface IDrawable {

void draw();

}

class Shape implements IDrawable {

public void draw() { System.out.println("how to draw

a Shape."); }

}

class Painter { // Top-level

Class

public Shape createCircle(final double radius) { //

(4) Non-static Method

class Circle extends Shape { // Non-static

local class

public void draw() {

System.out.println("how to draw a Circle

of radius: " + radius);

} }

return new Circle(); // Object of

non-static local class

}

public static IDrawable createMap() { // Static

Method

class Map implements IDrawable { // Static

local class

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12 public void draw() {

System.out.println("Drawing a Map."); }

} return new Map(); //

Object of static local class

}}

public class Local {

public static void main(String args[]) {

IDrawable[] drawables = { //

new Painter().createCircle(5),// Object of

non-static local class

Painter.createMap(), // Object of

static local class

new Painter().createMap() // Object of

static local class

};

for (int i = 0; i < drawables.length; i++) //

drawables[i].draw();

System.out.println("Local Class Names are:");

System.out.println(drawables[0].getClass()); //

System.out.println(drawables[1].getClass()); //

}

}

Output

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1.4 Anonymous Classes

An anonymous class is a kind of local class that has no name. Instead of using a local class

and then instantiating it we can use an anonymous class to combine these two steps. It

combines the syntax for class definition with the syntax for object instantiation. While a

local class definition is a Java statement, an anonymous class definition (and instantiation) is

a Java expression, so it can appear as part of a larger expression, such as method invocation.

Interfaces cannot be defined anonymously.

The following are the cases in which an anonymous class can be used instead of local class:

- The class has a very short body.- Only one instance of the class is needed.- The class is used right after it is defined.- The name of the class does not make your code any easier to understand.

Syntax to declare anonymous classes:

new() {< class

declarations>}

1.5 Instance Initializers

The anonymous classes cannot define constructors, since they do not have names. Hence a

new feature was added known as instance initialize.

Example Program

public class DemoInitializer

{

// This is an instance variable.

public int[] a1;

// This is an instance initializer.

//It is a code that runs for every new instance, after

the superclass constructor

// and before the class constructor, if any. It can

serve the same

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12// function as a constructor with no arguments.

{

a1 = new int[10];

for(int i = 0; i < 10; i++) a1[i] = i;

}

// The line below contains another instance initializer.

The instance

// initializers for an object are run in the order in

which they appear

// in the class definition.

int[] a2 = new int[10]; { for(int i=0; i

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121.6 Static Nested Classes

A class within another class is said as nested classes. The scope of nested class is enclosed

within the enclosing class. The nested class is of two types

- Static- Non-static

A static nested class is one in which the static modifier is applied. As it is static it can access

the members of its enclosing class through an object. It cannot refer to its member class

directly.

Static nested classes are accessed using the enclosing name

OuterClass.StaticNestedClass

To create an object to the nested class

OuterClass.StaticNestedClass nestedObject = new OuterClass.StaticNestedClass();

Example Program

public class Outer{

int x = 300;

class Inner{

int x = 500;

public void check(){

System.out.println("Value of x is: "+

Outer.this.x );

}

}

public void check(){

new Inner().check();

}

public static void main(String args[]){

new Outer().check(); }

}

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12Output

Advantage

It does not need an object of the containing class to work. Hence it reduces the number of

objects that our application creates at runtime.

Exercise

1. Write a program using local classes.2. Write a program using anonymous classes3. Why do we use instance initializers

Summary

From the above chapter we are familiar with

Inner classes Local classes Static nested classes Anonymous classes