Java Inner Class and Extend

Question

public class Home {

    public void getHomeDetails(){
        Rooms r = new Rooms();
        r.getRooms();
    }

    public class Rooms{
        public void getRooms(){
            System.out.println("20");
        }
    }
}



public class BigHome extends Home{

    public class Inner extends Home.Rooms{
        public void getRooms(){
            System.out.println("100");
        }
    }
}



public class Main {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        BigHome bigHome = new BigHome();
        bigHome.getHomeDetails(); // output:20 (Expecting 100)
    }

}

I have a class Home and its inner class Room which prints 20. Another class named BigHome extends Home and its inner class. But when I initiate BigHome in Main Class. I get output as 20 (I was expecting output as 100)

How do I call function getRoom of the BigHome Class ?

Answer 1

BigHome does not have a bigHome.getHomeDetails(); method, so it uses the method in the Home class which uses home's rooms. What you can do is setup constructors for the 2 classes, and in these constructors, then setup which Room class you want to use. Like:

public class Home
{
    Rooms r;

    public Home()
    {
        r = new Rooms();
    }

    public void getHomeDetails()
    {
        r.getRooms();
    }

    public class Rooms
    {

        public void getRooms()
        {
            System.out.println("20");
        }
    }
}

class BigHome extends Home
{
    public BigHome()
    {
        r = new Inner();
    }

    public class Inner extends Home.Rooms
    {

        public void getRooms()
        {
            System.out.println("100");
        }
    }
}

Now Home and BigHome are both in charge of specifying which Rooms they want to use.

Another approach, which takes away from the whole concept of inheritance, is to override the getHomeDetails method in the BigHome class to set which rooms BigHome is using.

public void getHomeDetails()
{
    Inner.Rooms r = new Inner.Rooms();
    r.getRooms();
}

I would not advise on this though.

Answer 2

You could do this by moving the construction of a new Rooms instance into another method in Home , which you would then override in BigHome to return a new Inner instance instead:

public class Home {
    public Rooms createRooms() {
        return new Rooms();
    }

    public void getHomeDetails(){
        Rooms r = createRooms();
        r.getRooms();
    }

    ...
}

public class BigHome extends Home{
    @Override
    public Rooms createRooms() {
        return new Inner();
    }

    ...
}

Answer 3

You define the method getHomeDetails() as

public void getHomeDetails(){
    Rooms r = new Rooms();
    r.getRooms();
}

and you never override this in any subclass. As such it never changes that you call getRooms() on a Room object and it will therefor always print 20.

That BigHome contains another class which also implements a getRooms() method is irrelevant. That this new class is also called Rooms doesn't mean it's the same class. When you call getHomeDetails() only the first Room object nested within Home will be instantiated and getRooms() will be called on this object and not any other.

Answer 4

BigHome does not have a getRoom method, there is a getRooms in Rooms and Inner .

If you create an instance of BigHome.Inner and call .getRooms() , it will call BigHome.Inner.getRooms . If you create an instance of Home.Rooms and call .getRooms() , it will call Home.Rooms.getRooms .

The problem you are facing is that getHomeDetails is defined in the parent class, Home and not anywhere else. It creates an instance of Home.Rooms because that is what the code says to do and thus the 20.

Try over-writing getHomeDetails in Inner :

public void getHomeDetails(){
    Inner.Rooms r = new Inner.Rooms();
    r.getRooms();
}

That should print "100" instead of "20".