Why nested abstract class in java

Question

I am wondering what does it mean to have a nested abstract class ? for example,

   abstract class A{

          abstract class B{

          }
   }
         


        

Answer 1

abstract classes are used to provide a partial implementation of a class for inheritance. it allows you to define the scheme of a class without providing the full definiton, so that it can be specified in a child class. it works somewhat like a Interface in that you can perform any operation specified in the abstract class upon an instance of any classes derived from it. Nested abstracted classes are designed to be inherited by other inner classes (even anonymous ones I think) but not by classes defined outside the outermost class.

public class HelloEveryone{
    abstract class Hello{
        void SayHello(){
            System.out.println("Hello!");
        }

        abstract void SayHelloAlt();
    }

    public class HelloWorld extends Hello{
        public void SayHelloAlt(){
            System.out.println("HelloWorld!");
        } 
    }

    public class HelloUniverse extends Hello{
        public void SayHelloAlt(){
            System.out.println("HelloUniverse!");
        } 
    }

    void Go(){
        ArrayList hellos = new ArrayList();
        hellos.add(new HelloWorld());
        hellos.add(new HelloUniverse());


        for (Hello h : hellos){
            h.SayHello();
            h.SayHelloAlt();
        }
    }   

}

static void main(){
    HelloEveryone hello = new HelloEveryone();
    hello.Go();
}