Inner classes in Java

A class that is defined within another class (or interface) is known as nested class. In other words, nested class is a class contained within another class or interface.

Java provides four types of nested classes-

• static nested class
• inner class
• local inner class
• anonymous inner class

A class defined within a method, constructor, or initialization block is called local class, whereas a non-local class defined inside a class.

Static Nested Class

• A nested class declared with static keyword is called static nested class.
• A static nested class is also known as static inner class.
• You can apply private, protected, default or public access modifier to a static nested class.
• An static inner class can have constructors, static and non static members.
• A static nested class is allowed to access static members of its outer class.
• On compiling the code having static nested class, the compiler produces two .class files- Outer.class and Outer$StaticNested.class.
• There are two ways of creating objects of static nested class-
  1. Within outer class- Inner referenceVariable = new Inner();
  2. Outside outer class- Outer.Inner referenceVariable = new Outer.Inner();

Syntax

public class Outer
{
	access-modifier static class Inner
	{
		//members of static inner class
	}
}

Example of static nested class

class Outer
{	
	static class Inner
	{
		public void display()
		{
			System.out.println("I am a inner class.");
		}
	}
	public void show()
	{
		Inner i = new Inner();
		i.display();
	}
}
public class Test
{
	public static void main(String args[])
	{
		Outer o = new Outer();
		o.show();
		Outer.Inner i = new Outer.Inner();
		i.display();		
	}
}

Output

I am a inner class.
I am a inner class.

Inner Class

• You can apply private, protected, default or public access modifier to a inner class.
• It is also known as member class.
• An inner class can have constructors, variables, and methods.
• It cannot define a static method and nonfinal static variables. Only final static variables can be defined in it.
• On compiling the code having inner class, the compiler produces two .class files- Outer.class and Outer$Inner.class.
• There are two ways of creating objects of inner class-
  1. Within outer class- Inner referenceVariable = new Inner();
  2. Outside outer class- Outer.Inner referenceVariable = new Outer().new Inner();

Syntax

public class Outer
{
	access-modifier class Inner
	{
		//members of inner class
	}
}

Example of inner class

class Outer
{	
	class Inner
	{
		public void display()
		{
			System.out.println("I am a inner class.");
		}
	}
	public void show()
	{
		Inner i = new Inner();
		i.display();
	}
}
public class Test
{
	public static void main(String args[])
	{
		Outer o = new Outer();
		o.show();
		Outer.Inner i = new Outer().new Inner();
		i.display();		
	}
}

Output

I am a inner class.
I am a inner class.

Local Inner Class

• The local inner class is defined within static or instance method of a class.
• You cannot apply private, protected, default or public access modifier to a local inner class.
• It class can have constructors, variables, and methods.
• You can only create an object of local inner class inside a method, and object creation must appear after the declaration of the local inner class.
• It can access all the variables and methods of its outer class. It can only access final local variables of the method in which it is defined.

Syntax

public class Outer
{
	access-modifier return-type methodName(parameters)
	{
		class Inner
		{
			//members of inner class
		}	
	}
}

Example of local inner class

class Outer
{		
	public void show()
	{
		class Inner
		{
			public void display()
			{
				System.out.println("I am a inner class.");
			}
		}
		Inner i = new Inner();
		i.display();	
	}	
}
public class Test
{
	public static void main(String args[])
	{
		Outer o = new Outer();
		o.show();				
	}
}

Output

I am a inner class.

Anonymous Inner Class

• A class that has no name is called anonymous inner class. It is created in two ways-
  1. Combine object creation with inheritance.
  2. Combine object creation with implementing an interface.
• It can override zero or more methods of the inherited class, but it must implement all the methods of the interface.
• An instance of the anonymous class can be used in three ways-
  1. You can pass it as a method parameter.
  2. You can return it from a method.
  3. You can assign it to any variable(instance variable, static variable, or local variable).
• If you define an anonymous inner class in a method, then it can only access final variables of that method.
• An anonymous inner class is mostly used in Android programming.

Syntax

new ClassName(){
	//Override method
}

Example of Anonymous Inner Class

abstract class Shape
{		
	abstract public void show();	
}
public class Test
{
	public static void main(String args[])
	{
		Shape s = new Shape(){
			public void show(){
				System.out.println("I am an anonymous inner class.");
			}		
		};
		s.show();
	}
}

Output

I am an anonymouse inner class.