Anonymous Classes

Anonymous classes are just like local classes, except these don't have name.

Syntax of Anonymous Classes

The syntax of an anonymous class expression is like the invocation of a constructor, except there is a class definition contained in a block of code.

Example   

Uniform zpschool = new Uniform(){
 String color;
 void setColor(String color){
  this.color = color;
 }

 String getColor(){
  return color;
 }
}; 

The anonymous class expression consists of the following:

       The new operator

     The name of an interface to implement or a class to extend.

     Parentheses that contain the arguments to a constructor

Creating Anonymous Classes using Abstract class

class AnonymousEx{
 abstract class Uniform{
  abstract void setColor(String color);
  abstract String getColor();
 }

 Uniform zpschool = new Uniform(){
  String color;
  
  void setColor(String color){
   this.color = color;
  }
  
  String getColor(){
   return color;
  }
 };

 public static void main(String args[]){
  AnonymousEx obj = new AnonymousEx();
  obj.zpschool.setColor("Blue");
  System.out.println(obj.zpschool.getColor());
 }
}

Output

Blue

 

Creating Anonymous class using Interface

class AnonymousEx{
 interface Uniform{
  void setColor(String color);
  String getColor();
 }

 Uniform zpschool = new Uniform(){
  String color;
  
  public void setColor(String color){
   this.color = color;
  }

  public String getColor(){
   return color;
  }
 };

 public static void main(String args[]){
  AnonymousEx obj = new AnonymousEx();
  obj.zpschool.setColor("Blue");
  System.out.println(obj.zpschool.getColor());
 }
}

Output

Blue

1. Can I declare an interface inside Class ?

Yes

2. Can I declare abstract class inside class ?

Yes

3. Can I create an object for an Interface ?

Yes, Using Anonymous classes

4. Can I create an object for abstract class ?

Yes, Using Anonymous classes

5. An anonymous class cannot access local variables in its enclosing scope that are not declared as final

class AnonymousEx{
 String name="HI";

 interface Uniform{
  void setColor(String color);
   String getColor();
 }

 public void print(){
  int val = 10;
   
  class myClass implements Uniform{
   String color;

   public void setColor(String s){
    color = s;
   }

   public String getColor(){
    System.out.println(val);
    return color;
   }
  }
 }
}

When you try to compile compiler throws the below error

        

AnonymousEx.java:18: error: local variable val is accessed from within inner class; needs to be          declared final
System.out.println(val);
^
1 error

To make the program compiles fine, define the local variable as final

     final int val = 10;

  

6. You cannot declare static initializers or member interfaces in an anonymous class.

class AnonymousEx{
 String name="HI";
 
 interface Uniform{
  void setColor(String color);
  String getColor();
 }

 public void print(){
  class myClass implements Uniform{
   static String color;
   public void setColor(String s){
    color = s;
   }

   public String getColor(){
    return color;
   }
  }
 }
}

When you tries to compile the above program, compiler throws the below error

AnonymousEx.java:12: error: Illegal static declaration in inner class myClass
static String color;
^
modifier 'static' is only allowed in constant variable declarations
1 error

7. An anonymous class can have static members provided that they are constant variables

8. Can I declare extra methods in Anonymous classes ?

Yes. It is valid

Example

class AnonymousEx{
 interface Uniform{
  void setColor(String color);
  String getColor();
 }

 Uniform zpschool = new Uniform(){
  String color;

  public void setColor(String color){
   this.color = color;
  }

  public String getColor(){
   return color;
  } 

  void print(){
   System.out.println("I am Extra method");
  }
 };
}

 

As you see in the above program, interface Uniform has 2 abstract methods, setColor, getColor.But the Anonymous class has extra method print().

9. Is below program compile ?

class AnonymousEx{
 abstract class Uniform{
  void setColor(String color);
  String getColor();
   }
}

No, Compiler throws the below error.

AnonymousEx.java:4: error: missing method body, or declare abstract
void setColor(String color);
^
AnonymousEx.java:5: error: missing method body, or declare abstract
String getColor();
^
2 errors

To solve the above error, make the methods in the abstract class as abstract

class AnonymousEx{
 abstract class Uniform{
  abstract void setColor(String color);
  abstract String getColor();
 }
}

         

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