Calling a constructor from a parent class in a derived class

Question

I'm trying to create a parent class with a constructor that takes a single int as a parameter. I also need to derive a child class that creates two instances of the parent class using a constructor that takes two ints. I know I use the "super" keyword to use the constructor from the parent class, but how can I use the second int in the child constructor to call the same parent constructor? I can't use "super" twice, so is there any other way to use the second parameter? I know that I could just call the child constructor twice from the main method, but I specifically need a child constructor that takes two parameters and creates two objects of the parent class. Thanks.

public class Int 
{   
    public int numberOne;

    public Int(int numberHere)
    {
        numberOne = numberHere;
    }

    //methods...
}   



public class Rational extends Int
{
    Int numerNum;
    Int denomNum;

    public Rational(int oneHere, int twoHere)
    {
        super(oneHere);
    }

    //methods...
}

Answer 1

You're trying to use inheritance when you need to be using composition.

Use this as a starting point instead:

public class Rational {

    private Int numerator;
    private Int denominator;

    public Rational(int num, int den) {
        numerator = new Int(num);
        denominator = new Int(den);
    }
}

As a general rule of thumb, inheritance is best used for "Is-a" relationships. Ask yourself, "Is a rational number an integer?" The answer is clearly no.

Answer 2

By inheritance, you model a "is-a" relationship. By making the Rational be a subclass of Integer, you are basically saying that the Rational IS A Integer (is it?).

Moreover, the constructor of an object or of it's superclass is used to initialize that object. You cannot use the constructor to initialize multiple objects.

What you really want to do is to model a "has-a" relationship. A Rational has 2 integers: a denominator and a numerator. Which you can model by having 2 Int members.

Answer 3

The problem is that you are abusing inheritance to model something that is not an “is a” relationship. A rational number “is not an integer”. It's also not two integers – besides the fact that in Java you can only derive from exactly one class. On the contrary, a rational number “has two” integers: the numerator and the denominator. There are signs in your code that you already recognized this, but not to the full extent. In short, remove the inheritance relationship and simply assign the two members.

public final class Int {

    private final int value;

    public Int(final int val) {
        this.value = val;
    }

}

public final class Rational {

    private final Int numerator;
    private final Int denominator;

    public Rational(final int numer, final int denom) {
        this.numerator = new Int(numer);
        this.denominator = new Int(demom);
    }

}

Actually, instead of new'ing up the Int objects, I'd prefer them to be passed in to the constructor.

public final class Rational {

    private final Int numerator;
    private final Int denominator;

    public Rational(final Int numer, final Int denom) {
        this.numerator = numer;
        this.denominator = demom;
    }

}