Inheritance and the “this” keyword

Question

Suppose that we have next situation:

Parent class A:

class A{  
    public A(){}
    public doSomething(){  
        System.out.println(this.getClass());
    }
}

with a child class B:

class B extends A{  
    public B(){}
    public void doSomething(){
        super.doSomething();
        System.out.println(this.getClass());
    }
}

and Main class:

class Main{  
    public static void main(String[] args){
        A ab=new B();
        ab.doSomething();
    }
}

When I execute this code result is

B  
B

Why does this, referenced in superclass A, returns B as a class when the reference is of type A?

Answer 1

It doesn't matter what the reference is, it's the instantiated object's class that counts. The object that you're creating is of type B, therefore this.getClass() is always going to return B.

Answer 2

Despite the fact that you are calling the doSomething() method of A, the this during that call is a B, and so the getClass() method is called on B not on A. Basically the this will always be a B whether you are using a method from superclass A, a method from B, or from A's superclass Object (the parent class of all Java classes).

Answer 3

this doesn't do anything for you in this situtaion. Calling this.getClass() is no different than just calling getClass();

So, A calls getClass(), which will return B if you are dealing with an instance of B that extends A.

Answer 4

Think of it in terms of runtime vs static types:

Animal a = new Cat();

The static type (in this case written on the left hand side) of variable a is Animal (you couldn't pass a into a method that required a Cat without a downcast) but the runtime type of the object pointed to by a is Cat.

a.getClass() exposes the runtime type (if it helps think of it as the most specific subtype).

Interestingly in Java overloaded methods are resolved at compile-time (without looking at the runtime type). So given then following two methods:

foo(Cat c);
foo(Animal animal)

Calling foo(a) would call the latter. To 'fix' this the visitor pattern can be used to dispatch based on the runtime type (double dispatch).

Answer 5

The output of program is correct.

When in Main class ab.doSomething(); line get executed doSomething() method of class B will be called then super.doSomething(); line will call doSomething() method of class A. As this keyword indicates reference of current object(ab is the object of class B see A ab=new B(); as constructor is of class B),i.e.System.out.println(this.getClass()); line in class A will print B only.

Again control will come back to System.out.println(this.getClass());in class B, So again B will get print.

In birdeye view only object of class B has created. That is why we are getting B B in output.