CODEWITHSUNDEEP
javainheritancepolymorphismthis
argamanza
argamanza

Reputation: 1142

Returning `this` in a super method of inherited class

let's say i have class A and class B which extends A, here are the classes:

A:

public class A {
    public int x;
    public static int y;
    public A(int x, int y) {
        this.x = x;
        this.y = y;
    }
    public int getX() { return x; }
    public static int getY() { return y; }
    public A get1() { return this; }
    public A get2() { return new B(x, y); }
}

B:

public class B extends A {
    public int x;
    public B(int x, int y) {
        super(x, y);
        this.x = x*2;
        this.y = y*2;
    }
    public int getX() { return x; }
        public static int getY() { return y*3; }
    public A get1() {
        x++;
        return super.get1();
    }
    public A get2() { return get1(); }
}

Here is the main function:

public static void main(String[] args) {
    A a1 = new A(5, 10);
    A a2 = a1.get2();
    A a3 = a2.get2();

    System.out.println("a1.x=" + a1.x);
    System.out.println("a1.y=" + a1.y);
    System.out.println("a2.x=" + a2.x);
    System.out.println("a2.getX()=" + a2.getX());
    System.out.println("a2.getY()=" + a2.getY());
    System.out.println("((B)a2).getY()=" + ((B)a2).getY());
    System.out.println("((B)a2).x=" + ((B)a2).x);
    System.out.println("a3 is A: " + (a3.getClass() == A.class));
    System.out.println("a3 is B: " + (a3 instanceof B));
    System.out.println("a3==a2: " + (a3 == a2));
}

My problem is with the a2 and a3 objects, a3 is basically a2.get2(), after following the method it will reach the A get1() method that returns this.

Since the method is found in class A i was sure it will only return the reference to A part of object a2 and not a reference to the whole object,

So when i try this line: a3.getClass() == A.class i will get True.

When i debugged a3.getClass() is "class B".

Can someone explain to me what the return this line actually does when it's inside a father class?

Thanks!

Upvotes: 2

Views: 119

Answers (2)

Chetan Kinger
Chetan Kinger

Reputation: 15212

Let's trace the statements step by step :

  1. a1 is a reference to an instance of type A.
  2. a1.get2() calls the get2() method in A which returns a reference to an instance of type B so a2 refers to an instance of type B.
  3. a2.get2() calls the get2() method in B. Remember that a2 is an instance of type B so this refers to B.
  4. get2() method in B calls the get1() method in B. this still refers to B.
  5. get1() method in B calls super.get1(). This is where it can get a bit confusing. Even though you call the get1 method from the parent class, this still refers to B at runtime.
  6. Therefore, super.get1() returns B. get1() in B returns B. get2() in B returns B. Therefore, a3 refers to an instance of type B.

From the java docs for Object#getClass

public final Class getClass()

Returns the runtime class of this Object

The getClass method returns the runtime class of an object so that's what you get when you call getClass on an a reference to an instance of type B. If getClass was not designed to return the actual instance type, it would have always returned Object which would make the method pointless.

Upvotes: 2

arcy
arcy

Reputation: 13123

The keyword this refers to the current object instance. There is no "A part of the B object", i.e., there is no reference to a superclass from within a subclass. Objects that are inherited are not divided into their different parts; you instantiate one object, and it is referred to by this from within instance methods, regardless of where those instance methods are declared.

So you have a B object, and this within a method declared in A. If the method is called, directly or indirectly, from a B, then it is going to refer to that B object.

Upvotes: 1

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