Polymorphic call to method from constructor?

[Nidhi Sar]

Please take a look at the following code:

  class Parent {
	public Parent() {
		doStuff();
	}
	public void doStuff() {
		System.out.println("Parent's doStuff()");
	}
}

public class Child extends Parent {
	public static void main(String[] args) {
		new Child();
	}
	public void doStuff() {
		System.out.println("Child's doStuff()");
	}
}
  

I would expect the output to be Parent's doStuff(), but actually it is Child's doStuff().

Why is it so? I understand the concept of polymorphic call to child's method if invoked on a child instance, but here it is just call from a constructor to a method of the same class. So why is it going to the child's method?

Thanks,
Nidhi

[W. Joe Smith]

Nidhi Sar wrote:Please take a look at the following code:

  class Parent {
	public Parent() {
		doStuff();
	}
	public void doStuff() {
		System.out.println("Parent's doStuff()");
	}
}

public class Child extends Parent {
	public static void main(String[] args) {
		new Child();
	}
	public void doStuff() {
		System.out.println("Child's doStuff()");
	}
}
  

I would expect the output to be Parent's doStuff(), but actually it is Child's doStuff().

Why is it so? I understand the concept of polymorphic call to child's method if invoked on a child instance, but here it is just call from a constructor to a method of the same class. So why is it going to the child's method?

Thanks,
Nidhi

Because, even though it is the super class constructor, it is still a Child instance. Java will find the most specific method it can, and run that one. Since you have a Child instance, which has its own doStuff() method, Java will use that one.

[Nidhi Sar]

Thanks so much for your answer!

That makes sense.

So the call to doStuff() in the constructor actually is this.doStuff() and inside the Parent's constructor this refers to the instance of Child's class upcast to the Parent class! Is that correct?

[Rob Spoor]

Yes. Keep in mind that fields of the Child instance are not yet initialized. That might cause problems if you expect them to be initialized already.

[Nidhi Sar]

Thanks guys