what does return this do in this code?

Question

In this code

class Parent {

    void show() {
        System.out.print("parent");
    }

    Parent a() {
        return this;
    }

}

class Child extends Parent {

    void show() {
        System.out.print("child");
    }

    public static void main(String arg[]) {
        Parent a = new Child();
        Parent b = a.a();
        b.show();
    }
}

What does return this; do? b.show() is invoking the child method show. So does this return reference to its child class? If not then how is the show() method of child being called?

Not sure of the downvotes, this is actually not a bad question. — Qix - MONICA WAS MISTREATED, Jul 17 '14 at 15:10
http://stackoverflow.com/questions/19325512/understanding-use-of-this-in-java?rq=1 - pasted the wrong thing before, stupid windows clipboard — , Jul 17 '14 at 15:11
@JarrodRoberson That still isn't necessarily a duplicate. This is asking why dogs bark. The question you link is asking why dogs eat. Sure they're both about dogs, but not duplicates. — Qix - MONICA WAS MISTREATED, Jul 17 '14 at 15:11
Random tip of the day - Classes should start with a capital letter `child` --> `Child`. — takendarkk, Jul 17 '14 at 15:13
I didn't mean you should edit the OP's code. Now you give the impression that they really know what they are doing when in fact they do not. — takendarkk, Jul 17 '14 at 15:13
And what makes them not know what they're doing? It's improving the quality of the post. The question itself is good, but the code might be a little shabby. Why shame posters? — Qix - MONICA WAS MISTREATED, Jul 17 '14 at 15:15
You didn't teach the man to fish, you gave it to him. This is _"what makes them not know what they're doing?"_ — takendarkk, Jul 17 '14 at 15:16
You posted your comment; had you not, I would have. Normally I don't edit code but he 1) wasn't having a problem, and 2) given the code, the problem wouldn't have been with the casing of the types anyway. — Qix - MONICA WAS MISTREATED, Jul 17 '14 at 15:24

score 6 · Accepted Answer · answered Jul 17 '14 at 15:14

First of all, all non private and non static methods in Java are virtual, which means that you will always execute the method of the type of the object reference.

In this case, you have

parent a=new child();

which means the client (the class/method) using a can only execute the methods defined in parent class but the behavior is defined by type child.

After this, when you execute:

parent b=a.a();

a.a() will return this, but a is a child and this is the current object reference, which is a. Value of b is a.

Then you execute

b.show();

Which is invoking child#show, so the program will output "child".

what does return this do in this code?

1 Answers1