Thread: Beginner's question about inheritance

Hello all, I have just began to read the book "Thinking in Java" by Bruce Eckel, and I've got a question about inheritance.

It is said in book that

the derived class may have additional or modified methods comparing to the base class;
the derived class may have additional data comparing to the base class.

I am just wondering, is it possible that the derived class has "modified data" comparing to the base class? Because the asymmetry between the "method" and "data" at this point makes me uncomfortable.

Thanks for your attention!

This is a tricky question.
If you look at this example Code:

public class Vehicle {
 
	protected int speed;
 
	public int getSpeed() {
		return speed;
	}
 
}

public class Car extends Vehicle {
 
	protected int speed;
 
	public Car(int speed) {
		this.speed = speed;
	}
 
}

Then you might see, that the car and the vehicle classes both have an attribute called "speed".
But the speed of the car is not a modification of the speed of the vehicle.
A sub-class (in this example the car) can only add new attributes, but neither remove, nor change existing ones from the super class.

Objects of the Car class have 2 attributes which are both called speed.

If you create a car from that example like this:

		Car c = new Car(150);
		System.out.println("Speed of car: "+c.getSpeed());

The printed speed will be 0.

That is because the car only changes its own speed attribute. The speed of the underlying vehicle class was not changed in the constructor.



What the programmer of that example really should have written is:

public class Car extends Vehicle {
 
	public Car(int speed) {
		super.speed = speed;
	}
 
}

Because this way the value of the speed attribute of the super class was modified.
And now the Car only has 1 attribute called speed, the one returned by the getSpeed() method.

Originally Posted by waltforme

To my understanding, the first two sections of your code are actually adding a new attribute (although with the same name) to the derived class. Am I right?

Yes exactly. The new class Car will have 2 different attributes which are both called speed.

Thanks Cornix!