Java Anonymous Class

In Java, a class can contain another class known as nested class. It's possible to create a nested class without giving any name.

A nested class that doesn't have any name is known as an anonymous class.

An anonymous class must be defined inside another class. Hence, it is also known as an anonymous inner class. Its syntax is:

class outerClass {

    // defining anonymous class
    object1 = new Type(parameterList) {
         // body of the anonymous class
    };
}

Anonymous classes usually extend subclasses or implement interfaces.

Here, Type can be

1. a superclass that an anonymous class extends
2. an interface that an anonymous class implements

The above code creates an object, object1, of an anonymous class at runtime.

Note: Anonymous classes are defined inside an expression. So, the semicolon is used at the end of anonymous classes to indicate the end of the expression.

Example 1: Anonymous Class Extending a Class

class Polygon {
   public void display() {
      System.out.println("Inside the Polygon class");
   }
}

class AnonymousDemo {
   public void createClass() {

      // creation of anonymous class extending class Polygon
      Polygon p1 = new Polygon() {
         public void display() {
            System.out.println("Inside an anonymous class.");
         }
      };
      p1.display();
   }
}

class Main {
   public static void main(String[] args) {
       AnonymousDemo an = new AnonymousDemo();
       an.createClass();
   }
}

Output

Inside an anonymous class.

In the above example, we have created a class Polygon. It has a single method display().

We then created an anonymous class that extends the class Polygon and overrides the display() method.

When we run the program, an object p1 of the anonymous class is created. The object then calls the display() method of the anonymous class.

Example 2: Anonymous Class Implementing an Interface

interface Polygon {
   public void display();
}

class AnonymousDemo {
   public void createClass() {

      // anonymous class implementing interface
      Polygon p1 = new Polygon() {
         public void display() {
            System.out.println("Inside an anonymous class.");
         }
      };
      p1.display();
   }
}

class Main {
   public static void main(String[] args) {
      AnonymousDemo an = new AnonymousDemo();
      an.createClass();
   }
}

Output

Inside an anonymous class.

In the above example, we have created an anonymous class that implements the Polygon interface.

Advantages of Anonymous Classes

In anonymous classes, objects are created whenever they are required. That is, objects are created to perform some specific tasks. For example,

Object = new Example() {
   public void display() {
      System.out.println("Anonymous class overrides the method display().");
   }
};

Here, an object of the anonymous class is created dynamically when we need to override the display() method.

Anonymous classes also help us to make our code concise.