Anonymous classes

 It is an inner class without a name and for which only a single object is created. An anonymous inner class can be useful when making an instance of an object with certain “extras” such as overloading methods of a class or interface, without having to actually subclass a class.

package com.company.bean;

public class A {
    public void show(){
        System.out.println("class A");
    }
}

package com.company.bean;

public class B extends A{
    public void show(){
        System.out.println("class B");
    }
}
package com.company;

import com.company.bean.A;

public class Main {

    public static void main(String[] args) {
        A obj = new A(){
            public void show(){
                System.out.println("mutant");
            }
        };
        obj.show();
        System.out.println(obj.getClass().getName());
    }
}
Outcome:
mutant
com.company.Main$1

*It is also possible for Interface and Abstract clesses.
Example:
package com.company.bean;

public interface Test {
    int a = 29;
    void getAge();
}
package com.company.bean;

public abstract class Car {

    public void speed(){
        System.out.println("Speed");
    }
}
package com.company;

import com.company.bean.Car;
import com.company.bean.Test;

public class Main {

    public static void main(String[] args) {
        Test t = new Test() {
            @Override
            public void getAge() {
                System.out.println("Age is " + a);
            }
        };
        t.getAge();
        System.out.println(t.getClass().getName());

        Car c = new Car() {
            @Override
            public void speed() {
                super.speed();
            }
        };
        c.speed();
        System.out.println(c.getClass().getName());
    }
}
Outcome:
Age is 29
com.company.Main$1
Speed
com.company.Main$2

As we see everytime anonymous class is being created automatically. I suggest You to go into 
out->production: here we can easily find anonymous class, and if we examine this class we can 
detect inside it: Inner class is written!