Question

I’m studying a chapter in java related to Inheritance, and i have a few questions.

I’ have basic understanding how inheritance works ( overriding methods, information hiding, how to use private fields from superclass in a subclass etc ), but i have just one problem and i hope you might help me.

When superclass have non default constructor(s) – without parameters, that means that in a subclass i have to create new constructor (it can be default – without parameters ), but in a first statement must be superclass constructor call.

Ok, so far so good. I understand so far. In subclass you must call superclass constructor, matching any of constructors parameters.

But lets check following code: (Superclass)

public class Vehicle {

    private int numOfWheels;
    private double avgGallonsPerMile;   

    public Vehicle(int numOfWheels, double avgGallonsPerMile) {

        this.numOfWheels = numOfWheels;
        this.avgGallonsPerMile = avgGallonsPerMile;
    }
}

And another Subclass code:

public class Car extends Vehicle{

    public Car(double avgGallonsPerMile) {
        super(What should i write here?, avgGallonsPerMile);

        //force numOfWheels to 4;

    }   
}

Here is the exercise for subclass:

Each subclass
contains a constructor that accepts the miles-per-gallon value as an argument and
forces the number of wheels to the appropriate value—2 for a MotorCycle and 4 for
a Car.

In subclass constructor i don’t need numOfWheels field, because i will force it to 4 ( for car ) and 2(for motorbike) anyway.

But stil i need that data for superclass anyway. Where to get that data? What should as first parameter in call to superclass constructor.

But still this isn’t the lonely case. I got lots of exercises that i don’t need certain data in subclass constructor as parameters, BUT still i need them in superclass constructor call.

What should i do in such cases ?

I really hope you understood me, what i want to tell. It’s kinda difficult.

Answer 1

If its anyway the same 4 for cars and 2 for motorcycles than make if fix!

super(4, avgGallonsPerMile);

or the better way – declare a constant:

private static final int NUM_OF_WHEELS = 4;
..
super(Car.NUM_OF_WHEELS, avgGallonsPerMile);