Inner_Classes

• Sometimes we can declare a class inside another class such type of classes are called inner classes.

• Sun people introduced inner classes in 1.1 version as part of "EventHandling" to resolve GUI bugs.

• But because of powerful features and benefits of inner classes slowly the programmers starts using in regular coding also.

• Without existing one type of object if there is no chance of existing another type of object then we should go for inner classes.

Example 1:

• Without existing University object there is no chance of existing Department object hence we have to define Department class inside University class.

Example 2:

• Without existing Bank object there is no chance of existing Account object hence we have to define Account class inside Bank class.

Example 3:

• Without existing Map object there is no chance of existing Entry object hence Entry interface is define inside Map interface.

Map is a collection of key-value pairs, each key-value pair is called an Entry.

Note : 

• Without existing Outer class Object there is no chance of existing Inner class Object.

• The relationship between outer class and inner class is not IS-A relationship and it is Has-A relationship.

• Based on the purpose and position of declaration all inner classes are divided into 4 types.

They are:

1. Normal or Regular inner classes 
2. Method Local inner classes
3. Anonymous inner classes
4. Static nested classes.

1.Normal (or) Regular inner class

• If we are declaring any named class inside another class directly without static modifier such type of inner classes are called normal or regular inner classes.

Example:

class Outer {
   class Inner {
   }
} 

Output:

Example:

class Outer {
   class Inner {
   }
   public static void main(String[] args) {
      System.out.println("outer class main method");
   }
 }

Output:

• Inside inner class we can't declare static members. Hence it is not possible to declare main() method and we can't invoke inner class directly from the command prompt.

Example:

class Outer {
   class Inner {
      public static void main(String[] args) {
         System.out.println("inner class main method");
      } 
   }
}

Output:

      E:\scjp>javac Outer.java
      Outer.java:5: inner classes cannot have static declarations public static void              main(String[] args)

Accessing inner class code from a static area of outer class:

Example:

class Outer {
   class Inner {
   public void methodOne(){ 
      System.out.println("inner class method");
   } 
   
   public static void main(String[] args) {
   } 
}

Accessing inner class code from instance area of outer class:

Example:

class Outer {
  class Inner {
    public void methodOne() {
      System.out.println("inner class method");
    }
   }

   public void methodTwo() {
      Inner i=new Inner(); 
      i.methodOne();
   }

   public static void main(String[] args) {
      Outer o=new Outer(); 
      o.methodTwo();
   } 
}

Output:

   E:\scjp>javac Outer.java E:\scjp>java Outer Inner class method

   Accessing inner class code from outside of outer class:

Example:

class Outer {
   class Inner {
      public void methodOne() {
         System.out.println("inner class method");
      }
   }
}
class Test {
   System.out.println("inner class method");
   public static void main(String[] args) {
      new Outer().new Inner().methodOne();
   } 
}

Output:

   Inner class method

• From inner class we can access all members of outer class (both static and non- static, private and non private methods and variables) directly.

Example :

class Outer {
   int x=10; 
   staticint y=20; 
   
   class Inner{
      public void methodOne() {
         System.out.println(x);//10 
         System.out.println(y);//20
      } 
   }
   public static void main(String[] args) {
      new Outer().new Inner().methodOne();
   } 
}

• Within the inner class "this" always refers current inner class object. To refer current outer class object we have to use "outer class name.this".

Example: 

class Outer {
   int x=10; 
   class Inner {
      int x=100;
      public void methodOne() {
         int x=1000; 
         System.out.println(x);//1000 
         System.out.println(this.x);//100 
         System.out.println(Outer.this.x);//10
      public static void main(String[] args) {
         new Outer().new Inner().methodOne();
      }
   }
}        

The applicable modifiers for outer classes are:

1. public 
2. default 
3. final
4. abstract 
5. strictfp

But for the inner classes in addition to this the following modifiers also allowed. 

Nesting of Inner classes :

We can declare an inner class inside another inner class

class A {
   class B {
      class C {
         public void m1() {
            System.out.println("C class method");
         }
      } 
   }             
   public static void main(String[] args) {
      A a = new A();
      A.B b = a.newB(); 
      A.B.C c = b.newC();
      c.m1(); 
   }
}          

2. Method local inner classes

• Sometimes we can declare a class inside a method such type of inner classes are called method local inner classes.

• The main objective of method local inner class is to define method specific repeatedly required functionality.

• Method Local inner classes are best suitable to meet nested method requirement.

• We can access method local inner class only within the method where we declared it. That is from outside of the method we can't access. As the scope of method local inner classes is very less, this type of inner classes are most rarely used type of inner classes.

Example: 

class Test {
   public void methodOne() {
      class Inner {
         public void sum(inti,int j) {
            System.out.println("The sum:"+(i+j));
         }
      }
      Inner i=new Inner(); 
      i.sum(10,20); 
      ;;;;;;;;;;;;; 
      i.sum(100,200); 
      ;;;;;;;;;;;;;;; 
      i.sum(1000,2000); 
      ;;;;;;;;;;;;;;;;;
   }
   public static void main(String[] args) {
      new Test().methodOne();
   } 
}

Output:
   The sum: 30 
   The sum: 300 
   The sum: 3000

• If we are declaring inner class inside instance method then we can access both static and non static members of outer class directly.

• But if we are declaring inner class inside static method then we can access only static members of outer class directly and we can't access instance members directly.    

Example:

class Test {
   int x=10;
   static int y=20;
   public void methodOne() {
      class Inner {
         public void methodTwo() {
            System.out.println(x);//10 
            System.out.println(y);//20
         } 
      }
      Inner i=new Inner(); 
      i.methodTwo();
   }
   
   public static void main(String[] args) {
      new Test().methodOne();
   }
}

• If we declare methodOne() method as static then we will get compile time error saying "non-static variable x cannot be referenced from a static context".

• From method local inner class we can't access local variables of the method in which we declared it. But if that local variable is declared as final then we won't get any compile time error.

Example: 

class Test {
   int x=10;
   public void methodOne() {
      int y=20; 
      class Inner {
         public void methodTwo() {
            System.out.println(x);//10 
            System.out.println(y); //C.E: local variable y is accessed from within inner class; needs to be declared //final.
         } 
      }
      Inner i=new Inner(); 
      i.methodTwo();
   }
   public static void main(String[] args) {
      new Test().methodOne();
   } 
}

If we declared y as final then we won't get any compile time error.

Consider the following declaration.

class Test {
   int i=10;
   static int j=20;
   public void methodOne() {
      int k=30;
      finalint l=40; 
      class Inner {
         public void methodTwo() {
            System.out.println(i); 
            System.out.println(j); //-->line 1 
            System.out.println(k); 
            System.out.println(l);
         } 
      }
      Inner i=new Inner(); 
      i.methodTwo();
   }
   public static void main(String[] args) {
      new Test().methodOne();
   } 
}

• At line 1 which of the following variables we can access ? i,j and l

• If we declare methodTwo() method as static then which variables we can access at line 1 ?

• we will get compile time error because we cannot declare static members inside Inner classes

• If we declare methodTwo() as static then we will get compile time error because we can't declare static members inside inner classes.

The only applicable modifiers for method local inner classes are: 

1. final
2. abstract
3. strictfp

By mistake, if we are declaring any other modifier we will get compile time error.

3. Anonymous inner classes

• Sometimes we can declare inner class without name such type of inner classes are called anonymous inner classes.

• The main objective of anonymous inner classes is "just for instant use".

• There are 3 types of anonymous inner classes

•  Anonymous inner class that extends a class.
•  Anonymous inner class that implements an interface.
•  Anonymous inner class that defined inside method arguments.

• Anonymous inner class that extends a class:

class PopCorn {
   public void taste() {
      System.out.println("spicy");
   } 
}
class Test {
 public static void main(String[] args) {
   PopCorn p=new PopCorn() {
      public void taste() {
         System.out.println("salty");
      } 
   };
   
   p.taste();//salty
   PopCorn p1=new PopCorn() 
   p1.taste();//spicy
 }
}

Analysis:

• 1. PopCorn p=new PopCorn(); We are just creating a PopCorn object.
• 2. PopCorn p=new PopCorn()
• 3. {
• 4. };
• 5. We are creating child class without name for the PopCorn class and for that child class we are creating an object with Parent PopCorn reference. PopCorn p=new PopCorn()
• 6.PopCorn p=new PopCorn()
• 7. {
• 8.    public void taste()
• 9.    {
• 10.      System.out.println("salty");
• 11.   }
• 12.};
• 13.

• We are creating child class for PopCorn without name.
• We are overriding taste() method.
• 3We are creating object for that child class with parent reference.

Note: 

• Inside Anonymous inner classes we can take or declare new methods but outside of anonymous inner classes we can't call these methods directly because we are depending on parent reference.[parent reference can be used to hold child class object but by using that reference we can't call child specific methods]. These methods just for internal purpose only.

Example 1:

classPopCorn {
   public void taste() {
           System.out.println("spicy");
   } 
 }
class Test {
   public static void main(String[] args) {  
      PopCorn p=new PopCorn() {
         public void taste() {
            methodOne();//valid call(internal purpose) 
            System.out.println("salty");
         }
         public void methodOne() {
            System.out.println("child specific method");
         } 
      };
      //p.methodOne();//here we can not call(outside inner class)
       p.taste();//salty
      
      PopCorn p1=new PopCorn();
      p1.taste();//spicy
  }
}

Output:
   Child specific method Salty
   Spicy

Example 2:

class Test {
  public static void main(String[] args) {
      Thread t=new Thread() {
         public void run() {
            for(int i=0;i<10;i++) {
                System.out.println("child thread");
            } 
         }
      };
      t.start();
      for(int i=0;i<10;i++) {
         System.out.println("main thread");
      }
  } 
}

Anonymous Inner Class that implements an interface: 

Example:

classInnerClassesDemo {
   public static void main(String[] args) {
      Runnable r=new Runnable() 
      //here we are not creating for Runnable interface, we are creating implements class object.
       {
        public void run() {
         for(int i=0;i<10;i++) {
            System.out.println("Child thread");
         }
       }; 
      Thread t=new Thread(r); t.start();
      for(int i=0;i<10;i++) {
         System.out.println("Main thread");
      }
   }
}

Anonymous Inner Class that defines inside method arguments:

Example:

class Test {
   public static void main(String[] args) {
      new Thread( new Runnable() {
         public void run() {
            for(int i=0;i<10;i++) {
               System.out.println("child thread");
          }
         } 
      }
   }).start();
   for(int i=0;i<10;i++) {
      System.out.println("main thread");
   } 
  }
} 

Output:
This output belongs to example 2, anonymous inner class that implements an interface example and anonymous inner class that define inside method arguments example.

Main thread 
Main thread 
Main thread 
Main thread 
Main thread 
Main thread 
Main thread 
Main thread 
Main thread 
Main thread 
Child thread 
Child thread 
Child thread 
Child thread 
Child thread 
Child thread 
Child thread 
Child thread 
Child thread 
Child thread

Difference between general class and anonymous inner classes:

General Class

• A general class can extends only one class at a time

• A general class can implement any no. Of interfaces at a time.

• A general class can extends a class and can implement an interface simultaneously.      

• In normal Java class we can write constructor because we know name of the class.

Anonymous Inner Class

• Of cource anonymous inner class also can extends only one class at a time.

• But anonymous inner class can implement only one interface at a time.

• But anonymous inner class can extends a class or can implements an interface but not both simultaneously.

• But in anonymous inner class we can't write constructor because anonymous inner class not having any name.

     

Explain the application areas of anonymous inner classes ?

• anonymous inner classes are best suitable to define call back functions in GUI components.

import java.awt.*; importjava.awt.event.*;
public class AnonymousInnerClassDemo {
   public static void main(String args[]) {
      Frame f=new Frame();
      f.addWindowListener(new WindowAdaptor(){
         public void windowClosing(WindowEvent e) {
             System.exit(0);
         }
      });
   
      f.add(new Label("Anonymous Inner class Demo !!!")); 
      f.setSize(500,500);
      f.setVisible(true);
   }
}

Without Anonumous Inner class :

class GUI extends JFrame implements ActionListener { 
   JButton b1,b2,b3,b4;
   public void actionPerformed(ActionEvent e) {
     if(e.getSource()==b1) {
       //perform b1 specific functionality
     }
     else if(e.getSource==b2){
       //perform b2 specific functionality 
     }
      
   } 
}            

With Anonumous Inner class :

class GUI extends JFrame { 
   JButton b1,b2,b3,b4 ;
   b1.addActionListener(new ActionListener() { 
      public void actionPerformed(ActionEvent e) {
         //perform b1 specific functionality 
      }
  });
}

Static nested classes 

• Sometimes we can declare inner classes with static modifier such type of inner classes are called static nested classes.

• In the case of normal or regular inner classes without existing outer class object there is no chance of existing inner class object. i.e., inner class object is always strongly associated with outer class object.

• But in the case of static nested class without existing outer class object there may be a chance of existing static nested class object. i.e., static nested class object is not strongly associated with outer class object.

Example:

class Test {
   static class Nested {
   
   public void methodOne() {
      System.out.println("nested class method");
   } 
   public static void main(String[] args) {
      Test.Nested t=new Test.Nested(); 
      t.methodOne();
   } 
}

• Inside static nested classes we can declare static members including main() method also. Hence it is possible to invoke static nested class directly from the command prompt.

Example:

class Test {
   static class Nested {
      public static void main(String[] args) {
         System.out.println("nested class main method");
      } 
   }
   public static void main(String[] args) {
      System.out.println("outer class main method");
   }
}

Output:

   E:\SCJP>javac Test.java E:\SCJP>java Test
   Outer class main method E:\SCJP>java Test$Nested Nested class main method
 

• From the normal inner class we can access both static and non static members of outer class but from static nested class we can access only static members of outer class.

Example:

class Test {
   int x=10;
   static int y=20; 
   static class Nested {
      public void methodOne() {
         System.out.println(x);//C.E:non-static variable x cannot be referenced from a static
         System.out.println(y);
      }    
   }
}

Compression between normal or regular class and static nested class 

Normal /regular inner class 

• Without existing outer class object there is no chance of existing inner class object. That is inner class object is always associated with outer class object.

• Inside normal or regular inner class we can't declare static members.

• Inside normal inner class we can't declare main() method and hence we can't invoke regular inner class directly from the command prompt.

• From the normal or regular inner class we can access both static and non static members of outer class directly.

Static nested class

• Without existing outer class object there may be a chance of existing static nested class object. That is static nested class object is not associated with outer class object.

• Inside static nested class we can declare static members.

• 3) Inside static nested class we can declare main() method and hence we can invoke static nested class directly from the command prompt.

• 4) From static nested class we can access only static members of outer class directly.

     

Various possible combinations of nested class &interfaces : 

1. class inside a class :

• We can declare a class inside another class
• Without existing one type of object, if there is no chance of existing another type of object, then we should go for clas inside a class.

   class University { 
      class Department {
      } 
   }

•    Without existing University object, there is no chance of existing Department object. i.e., Department object is always associated with University

2. interface inside a class :

•    We can declare interface inside a class.

   class X { 
      interface Y {
      } 
   } 

• Inside class if we required multiple implements of an interface and these implementations of relevant to a perticular class, then we should declare interface inside a class.

  classVehicleType { 
   interface Vehicle {
      publicintgetNoOfWheels();
   }
   
   class Bus implements Vehicle { 
      publicintgetNoOfWheels() {
         return 6; 
      }
   }
   class Auto implements Vehicle { 
      publicintgetNoOfWheels() {
         return 3; 
      }
   } 
}

3. interface inside a interface :

We can declare an interface inside another interface.

 Interface Map {
   interface Entry {
      public Object getKey();
      public Object getValue();
      public Object getValue(Object new );
   } 
}

• Nested interfaces are always public,static whether we are declaring or not. Hence we can implements inner inteface directly with out implementing outer interface.

 interface Outer {
   public void methodOne(); 
   interface Inner {
      public void methodTwo();
   } 
}
class Test implements Outer.Inner {
   public void methodTwo() { 
      System.out.println("Inner interface method");
   }
   public static void main(String args[]) {
      Test t=new Test(); 
      t.methodTwo();
   } 
}

• Whenever we are implementing Outer interface , it is not required to implement Inner interfaces.

 class Test implements Outer {
   public void methodOne() { 
      System.out.println("Outer interface method ");
   }
   
   public static void main(String args[]) {
      Test t=new Test(); 
      t.methodOne();
   }
}

   i.e., Both Outer and Inner interfaces we can implement independently.

4. class inside a interface :

• We can declare a class inside interface. If a class functionality is closely associated with the use interface then it is highly recommended to declare class inside interface

Example:

 interfaceEmailServer {
   public void sendEmail(EmailDetails e);
   
   classEmailDetails { 
      String from; String to;
      String subject;
   }  
}

• In the above example Emaildetails functionality is required for EmailService and we are not using anywareelse . Hence we can declare EmailDetails class inside EmailServiceinterface .

• We can also declare a class inside interface to provide default implementation for that interface.

Example :

 interface Vehicle { 
   publicintgetNoOfWheels();
   classDefaultVehicle implements Vehicle { 
      publicintgetNoOfWheels() {
         return 3;
      }
    }
   }
  class Bus implements Vehicle { 
   publicintgetNoOfWheels() {
       return 6;
   } 
}

class Test {
 public static void main(String args[]) {
   Bus b=new Bus(); 
   System.out.println(b.getNoOfWheels());
   
   Vehicle.DefaultVehicle d=new Vehicle.DefaultVehicle(); 
   System.out.println(d.getNoOfWheels());
 } 
}

• In the above example DefaultVehicle in the default implementation of Vehicle interface where as Bus customized implementation of Vehicle interface.

• The class which is declared inside interface is always static ,hence we can create object directly without having outer interface type object.

Conclusions :

• We can declare anything inside any thing with respect to classes and interfaces.

• Nesting interfaces are always public, static whether we are declaring or not.

• class which is declared inside interface is always public,static whether we are