Inner Class

Without existing one type of oject if there is no chance of existing another type of object then go for inner classses
Example university and department
University consists of several depts and hence dept is inner class of university
Example 2 : engine is inner class of car
Example 3:
Map is a group of key value pairs and each key value pair is called an Entry
With out existing map object there is no chance of existing entry object hence interface Entry is defined
inside Map interface

interface Map{
interface Entry{
}
}

Note :
1)With out existing outer class object there is no chance of existing inner class object
2)Relation b/w outer and inner class is has-A relation

Types

1)Normal or regular inner classes

2)Method local inner classes

3)Anonymous inner clases

4)Static nested class

Normal or regular inner classes

If we are declaring any named class directly inside a class with out static modifier such type of inner class is normal or regular inner class

When wee compile outer.java following files are generated

outer$inner.class and outer.class

Class outer{

class inner

{

}

psvm()

{

S.O.p("hello");

}

}

If we run outer class we get hello as output

If we run inner class java outer$inner no such method error main

Note:

staic memebers not allowed in inner class including main method

lass outer{

class inner

{

psvm()

{

S.O.p("hello");

}

}

}

Above we get compile time error as staic memebers not allowed in inner class including main method

Accessing inner class code from static area of outer class

Class outer{

class inner

{

public void m1()

{

S.O.p("hello");

}

}

psvm()

{

//To call m1 we need inner class obj and before that create outer class object

outer o = new outer();

outer.inner i = o.new inner();

i.m1();

}

}

accessing inner class code from instance area of outer class 

Class outer{

class inner

{

public void m1()

{

S.O.p("hello");

}

}

public void m2()

{

S.O.p("hello m2");

//calling m1;

inner i = new inner();

i.m1();

}

psvm()

{

//call m2

outer o = new outer();

o.m2;

}

}

Accessing inner class method From outside outer class

class outer{

class inner

{

m1()

{

S.O.P("hello inner");

}

}

}

Class Test{

psvm()

{

// want to access m1

outer o = new outer();

outer.inner i = o.new inner();

i.m1();

}

}

Note

1)From normal inner class we can access both static or non static members of outer class

2)With in inner clas this always refers current inner class object if we want to refer current outer class object

we have to use Outer Class name ".this"

3)Mofifiers for outer class

public , default , final , abstract ,strictfp

4)Modifiers for inner class

public , default , final , abstract ,strictfp ,private , protected , static

Nesting of inner classes

Inside inner class we can declare another inner class i,e nesting of inner classes is possible

Example

Class A

{

 Class B

 {

   Class C

   {

public void m1()

{

S.O.P("hello");

}

   }

 }

}

Class Test{

P S V M()

{

A a = new A();

A.B b = a.new B();

A.B.C c = b.new C();

c.m1();

}

Method local inner classes

Main purpose of Method local inner class is to define method specific repeatedilty required functionality

Method local inner classes are best suitable to meet nested method requirements

Scope of method local inner classes is with in method only

These are rarely used

Note

1)Nested methods not alloewed in java

2)We can eclare method local inner class inside both instance and static methods

3)If we declare method local inner class in the instance method of outer class then we can access both instnce and static variables of outer class from the method local inner class

4)If we declare method local inner class in the static method of outer class then we can access only static variables of outer class from the method local inner class

5)From Method local inner class we can't access local variables of method in which we declared inner class , if local variable declared as final then we can access

Example

Class Test{

int i = 10;

static int j = 30;

public void m1()

{

int k =10;

final int m = 40;

Class inner{

public void m2()

{

Line 1;

}

}

}

}

From lne 1 which variables we can access

Except k we can access i , j and m variables

If we declare m2 as static then it will be a ompile time error since we cannot declare static in inner classes

Note 

Only applicable modifiers for method local classes are final , abstract , strict fp

}


Anonymous inner classes

Sometimes we can declare inner class with out name these are anonymous inner class
Only one time use ,i.e instant use classes are anonymous inner class
Main purpose is one time usage
3 types are there

1)Anonymous inner class that extends a class
2)1)Anonymous inner class that implements an interface
3)1)Anonymous inner class that is defined inside arguments


1)Anonymous inner class that extends a class

only one time requirement or  then use this
Eample
Class PopCorn
{

public void taste()
{
S.O.P("salty");
}
}

Class Test{

P S V M()
{
PopCorn p = new PopCorn()
{

public void taste()
{
S.O.P("spicy");
}
};

p.taste();//spicy

PopCorn p1 = new PopCorn();
p1.taste();//salty

PopCorn p2 = new PopCorn()
{
public void taste()
{

S.O.P("Sweet");
}

};

p2.taste();//Sweet

}



}


Note:
1)PopCorn p = new PopCorn(); // we are creating popcorn object
2)PopCorn p = new PopCorn()
{


};//we are declaring a class that extends popcorn witout name(anonymous inner class)
For that child class we are creating an object with parent reference
3)2)PopCorn p = new PopCorn()
{

public void taste()
{
S.O.P("gg");
}

};

//we are declaring a class that extends popcorn with out name (anonymous inner class)
In that child class we are overriding taste method
For that child class we are creating an object with parent reference

Normal java class Vs Anonymous inner class

1)Both can extend only one class
2)Anonymous inner class can implement only one interface
3)Anonymous inner class can extend a class and implement interface but not both simultaneously
4)constructor cannot be written in anonymous inner class since these classes do not have any name
Note ;

Some times we can declare inner class with static modifier such type of inner classes are called static nested classes.Thesse ra enot strogly associated with outer classes
To access static nested class method outer class object is not required
If we want to create nested class obj outside of outer class then we can create as follows
OuterClassname.Nested n = new OuterClassname.Nested();
In static nested classes We can declare static methods including main method and hence we an invoke static nested class directly from command prompt
From static nested class we cannot access instance members of outer class


Normal inner class vs static nested class
1)Neste class not closely associated with outer class , normal opposite
2)We cannot declare static members in normal inner class opp in nested
3)Can't declare main methid in normal inner classes , nested opp
4)From normal inner class we can access both static and non static members of outer class but in nested we can access only static members