Inheritance and polymorphism in Java

Question

I have 2 classes: Triangle and RightAngledTr. RightAngledTr inherits from Triangle. The code is as follows:

The Triangle class:

class Triangle {
    public void draw() { 
        System.out.println(“Base::draw\n“);
    }

    public void computeCentroid(Triangle t) {
        System.out.println Base::centroid\n“);
    }
}

The RightAngledTr class:

class RightAngledTr extends Triangle {
    public void draw() { 
        System.out.println(“RightAngle::draw\n“);
    }

    public void computeCentroid(RightAngled t) {
        System.out.println(RtAngle::centroid\n“);
    }
}

In my Driver program I have the following lines of code:

Triangle tr= new RightAngledTr ();
RightAngledTr rtr= new RightAngledTr ();
tr.computeCentroid(rtr);
tr.draw();
rtr.computeCentroid(tr);

The output I expected was:

Base::centroid
Base::draw
Base::centroid

However, the output I got was:

Base::centroid
RightAngle::draw
Base::centroid

My question is, why does tr.computeCentroid(rtr) call the Triangle class' method when tr.draw() calls the RightAngleTr class' method?

It doesn't make sense. Liskov substitution principle doesn't seem to apply.

"Overloaded methods use early binding whereas overridden methods use late binding". See stackoverflow.com/questions/10915828/… — nsane
– nsane, Commented Oct 25, 2015 at 20:27

ndc85430 · Accepted Answer · 2015-10-25 09:17:30Z

3

The signatures of the computeCentroid() method are different. You are therefore not overriding, but overloading.

answered Oct 25, 2015 at 9:17

ndc85430

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4 Comments

Rahman Over a year ago

He is talking about draw() method i think

chrylis -cautiouslyoptimistic- Over a year ago

And this is why you always use @Override.

Akshay Damle Over a year ago

But tr is an object of type Triangle, so why is the draw() method of the Triangle class not called when I do tr.draw() ?

Ravindra babu Over a year ago

Have a look at my answer.

Anatoly Shamov · Accepted Answer · 2017-09-19 13:50:11Z

That's how java works. The draw () method is overridden.

Have a look at oracle documentation article for understanding the concepts.

An instance method in a subclass with the same signature (name, plus the number and the type of its parameters) and return type as an instance method in the superclass overrides the superclass's method.

Triangle (parent) holds the reference of RightAngledTr (child) and draw() method is invoked on child objects. It's called overriding feature.

Childs are type of parents. Parent holding child instance invokes child's overridden method. If method is not present in child, parent method will be invoked.

Overriding is redefining base class (parent) behaviour with same signature. If you change the signature of base class method in child class, it's called overloading. You have done the same for other method : computeCentroid().

But do not expect the same for same variables declared in both parent and child.

Variables are not overridden. Only methods are overridden.

Daniel Eder · Accepted Answer · 2015-10-25 09:23:46Z

1

You are calling the method draw on the specialised object. Even when you wrap it inside the base class you are calling the overridden draw method.

answered Oct 25, 2015 at 9:23

Daniel Eder

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