Concerning multiple constructors in Java

Question

I have a question about multiple instances of a constructor in Java.

My assignment is to receive two fractions and then multiply and divide those fractions.

I am unsure as to how to go about having separate values for the instances of the class objects themselves.

Here is the sample code of where I am having the issue:

import java.util.Scanner;

public class TextLab05
{
    static int num1, den1;   // numerator and denominator of the 1st rational number
    static int num2, den2;   // numerator and denominator of the 2nd rational number

    public static void main (String args[])
    {
        enterData();

        Rational r1 = new Rational(num1,den1);
        Rational r2 = new Rational(num2,den2);
    }
}

class Rational
{

    private int firstNum;   // entered numerator
    private int firstDen;   // entered denominator
    private int num;        // reduced numerator
    private int den;        // reduced denominator

    public Rational()
    {

    }

    public Rational(int n, int d)
    {
        n = TextLab05.num1;
        d = TextLab05.den1;
        //Here specifically is where I am having comprehension issues. How can I include num2 and den2 if I only have int n and int d?
    }
}

If this is hard to understand out of context, here is the entire starting code I was given:

import java.util.Scanner;

public class TextLab05
{
    static int num1, den1;   // numerator and denominator of the 1st rational number
    static int num2, den2;   // numerator and denominator of the 2nd rational number

    public static void main (String args[])
    {
        enterData();

        Rational r1 = new Rational(num1,den1);
        Rational r2 = new Rational(num2,den2);
        Rational r3 = new Rational();

        r3.multiply(r1,r2);
        System.out.println("\n\n" + r1.getOriginal() + " * " + r2.getOriginal() + "  =  " + r3.getRational());
        r3.divide(r1,r2);
        System.out.println("\n" + r1.getOriginal() + " / " + r2.getOriginal() + "  =  " + r3.getRational());

        //      100 Point Version Only
        //      r3.add(r1,r2);
        //      System.out.println("\n" + r1.getOriginal() + " + " + r2.getOriginal() + "  =  " + r3.getRational());
        //      r3.subtract(r1,r2);
        //      System.out.println("\n" + r1.getOriginal() + " - " + r2.getOriginal() + "  =  " + r3.getRational());
        System.out.println();
    }

    public static void enterData()
    {
        Scanner input = new Scanner(System.in);
        System.out.print("\nEnter the 1st numerator ----> ");
        num1 = input.nextInt();
        System.out.print("\nEnter the 1st denominator --> ");
        den1 = input.nextInt();
        System.out.print("\nEnter the 2nd numerator ----> ");
        num2 = input.nextInt();
        System.out.print("\nEnter the 2nd denominator --> ");
        den2 = input.nextInt();
    }
}

class Rational
{

    private int firstNum;   // entered numerator
    private int firstDen;   // entered denominator
    private int num;        // reduced numerator
    private int den;        // reduced denominator

    public Rational()
    {

    }

    public Rational(int n, int d)
    {
        n = TextLab05.num1;
        d = TextLab05.den1;
    }

    private int getGCF(int n1,int n2)
    {
        int rem = 0;
        int gcf = 0;
        do
        {
            rem = n1 % n2;
            if (rem == 0)
                gcf = n2;
            else
            {
                n1 = n2;
            n2 = rem;
            }
        }
        while (rem != 0);
        return gcf;
     }

    public int getNum()
    {
        return TextLab05.num1;
    }

    public int getDen()
    {
        return TextLab05.den1;
    }

    public double getDecimal()
    {
        return (double)TextLab05.num1 / TextLab05.den1;
    }

    public String getRational()
    {
        String rational = "" + TextLab05.num1 + "/" + TextLab05.den1;
        return rational;
    }

    public String getOriginal()
    {
        String original = "" + TextLab05.num1 + "/" + TextLab05.den1;
        return original;
    }

    public void reduce()
    {

    }
    public void multiply(Rational r1, Rational r2)
    {

    }
    public void divide(Rational r1, Rational r2)
    {

    }
    public void add(Rational r1, Rational r2)
    {

    }
    public void subtract(Rational r1, Rational r2)
    {

    }
}

Answer 1

The most important thing to do is understand the concept. You are going to store a rational number in your Rational class. When you do this:

Rational r1 = new Rational(num1,den1);

You are making a single instance of a Rational and naming it r1. r1 should now contain a numerator and a denominator (in this case num1 and den1).

Let's say you want to make the number one half, or 1/2. You could do this:

Rational oneHalf = new Rational(1,2);

Realize that new Rational(1,2) is calling the constructor of your Rational class. In your constructor you need to assign num and den to the passed values (in this case 1 and 2). So you would need something like this:

this.num = num1;
this.den = den1;

So if you want to have the ability to multiply one Rational with another Rational you would need a method or function to do that. In your Rational class, create a method called multiply(Rational anotherRational).

That function would do something like this:

this.num = this.num * anotherRational.num;
this.den = this.den * anotherRational.den;

I gave away half the answer, I'll let you do the rest. Don't just copy what you find here, think about what you're doing.

Answer 2

When you call:

Rational r1 = new Rational(num1, den1);
Rational r2 = new Rational(num2, den2);

in the main method of your program you are creating two instances of the Rational class, one named r1 and one named r2. Because you are passing int values to the Rational constructors, the constructor that will be called is the constructor which requires two integer arguments:

public Rational(int n, int d)
{
    ...
} 

The compiler knows this because it matches the name of the Constructor as well as the types of the arguments passed (known as matching the "signature" of the Constructor).

In the code you have provided, the Rational Constructor code doesn't really make sense - this code:

public Rational(int n, int d)
{
   n = TextLab05.num1;
   d = TextLab05.den1;    
}

should probably look something like this:

public Rational(int n, int d)
{
   this.firstNum = n;
   this.firstDen = d;    
}

The values n and d are passed to the constructor, and then in the body of the constructor the instance variables firstNum and firstDen (which are declared in the private part of the Rational class and effectively "belong" to the instance being created) would then be initialised to the values of n and d.

Everywhere within the body of the Rational class you should refer to the member variables firstNum and firstDen, rather than variables that do not "belong" to the class instance.

Answer 3

I am not sure if your implementation of Rational is what you intend, but the constructor is not limited to local varables, it can access any static variables from other classes it can access.

public Rational(int n, int d)
{
    n = TextLab05.num1;
    d = TextLab05.den1;
}

n and d are local variables, num1 and den1 are static variables in class TextLab05.

So you are assigning the local variables with the static values from the other class.

The code doesn't make sense, as you don't do anything with the values after assigning them to local variables that are disposed when the method ends.

Answer 4

I assume the Rational class is supposed to represent a rational number. You say:

//Here specifically is where I am having comprehension issues. How can I include num2 and den2 if I only have int n and int d?

You don't need to store two numerators and two denominators in the Rational class. You just need to creat two Rational objects. One to store num1 and den1, the other to store num2 and den2. You are already doing this:

Rational r1 = new Rational(num1,den1);
Rational r2 = new Rational(num2,den2);

It does not make sense to store two numerators and two denominators in Rational. A rational number only has one of each.

In summary: r1 stores num1 and den1, while r2 stores the other two. When you create a new Rational, n and d refer to the numerator and denominator of that particular instance you are creating.