Fractions Java

[D Doemer]

Hi Everyone,

I have an example program given by my teacher that is meant to help with our abstract fraction program. I mostly understand what it's supposed to do, but it doesn't compile because there's a couple of issues with it (why he would bother giving and uncompilable program is beyond me, but hey, at least he gave me something for once). I'm going to post it and then point out the issues, and if anyone could tell me how to fix them so I can use it to compare to my own assignment, that would be great.

public class Fraction {

    private int numerator, denominator;

    public Fraction() {
        numerator = denominator = 0;
    }

    public int getNumerator() {
        return numerator;
    }

    public void setNumerator(int num) {
        numerator = num;
    }

    public int getDenominator() {
        return denominator;
    }

    public void setDenominator(int den) {
        denominator = den;
    }

    public Fraction add(Fraction b) {
        if ((denominator == 0) || (b.denominator == 0)) {
            throw new IllegalArgumentException("invalid denominator");
        }
        int common = lcd(denominator, b.denominator);
        Fraction commonA = new Fraction();
        Fraction commonB = new Fraction();
        commonA = convert(common);
        commonB = b.convert(common);
        Fraction sum = new Fraction();
        sum.numerator = commonA.numerator + commonB.numerator;
        sum.denominator = common;
        sum = sum.reduce();
        return sum;
    }

    public Fraction subtract(Fraction b) {
        if ((denominator == 0) || (b.denominator == 0)) {
            throw new IllegalArgumentException("invalid denominator");
        }
        int common = lcd(denominator, b.denominator);
        Fraction commonA = new Fraction();
        Fraction commonB = new Fraction();
        commonA = convert(common);
        commonB = b.convert(common);
        Fraction diff = new Fraction();
        diff.numerator = commonA.numerator - commonB.numerator;
        diff.denominator = common;
        diff = diff.reduce();
        return diff;
    }

    public Fraction multiply(Fraction b) {
        if ((denominator == 0) || (b.denominator == 0)) {
            throw new IllegalArgumentException("invalid denominator");
        }
        Fraction product = new Fraction();
        product.numerator = numerator * b.numerator;
        product.denominator = denominator * b.denominator;
        product = product.reduce();
        return product;
    }

    public Fraction divide(Fraction b) {
        if ((denominator == 0) || (b.numerator == 0)) {
            throw new IllegalArgumentException("invalid denominator");
        }
        Fraction result = new Fraction();
        result.numerator = numerator * b.denominator;
        result.denominator = denominator * b.numerator;
        result = result.reduce();
        return result;
    }

    public void input() {

        System.out.print("Please enter an integer for numerator: ");

        numerator = SavitchIn.readLineInt();

        do {
            System.out.print("Please enter a non-zero integer for denominator: ");
            denominator = SavitchIn.readLineInt();
            // make sure it is non-zero
            if (denominator == 0) {
                System.out.println("Invalid value.  Please try again.");
            }
        } while (denominator == 0);
    }

    public void output() {
        System.out.print(this);
    }

    public String toString() {
        String buffer = numerator + "/" + denominator;
        return buffer;
    }

    private int lcd(int denom1, int denom2) {
        int factor = denom1;
        while ((denom1 % denom2) != 0) {
            denom1 += factor;
        }
        return denom1;
    }

    private int gcd(int denom1, int denom2) {
        int factor = denom2;
        while (denom2 != 0) {
            factor = denom2;
            denom2 = denom1 % denom2;
            denom1 = factor;
        }
        return denom1;
    }

    private Fraction reduce() {
        Fraction result = new Fraction();
        int common = 0;
        int num = Math.abs(numerator);
        int den = Math.abs(denominator);
        if (num > den) {
            common = gcd(num, den);
        } else if (num < den) {
            common = gcd(den, num);
        } else {
            common = num;
        }

        result.numerator = numerator / common;
        result.denominator = denominator / common;
        return result;
    }

    public static void main(String args[]) {
        Fraction f1 = new Fraction();
        Fraction f2 = new Fraction();

        f1.setNumerator(1);
        f1.setDenominator(3);
        f2.setNumerator(1);
        f2.setDenominator(6);

        Fraction result = new Fraction();

        result = f1.add(f2);
        System.out.println(f1 + " + " + f2 + " = " + result);
        result = f2.add(f1);
        System.out.println(f2 + " + " + f1 + " = " + result);
        System.out.println();

        result = f1.subtract(f2);
        System.out.println(f1 + " - " + f2 + " = " + result);
        result = f2.subtract(f1);
        System.out.println(f2 + " - " + f1 + " = " + result);
        System.out.println();

        System.out.println("Fraction 1:");
        f1.input();
        System.out.println();
        System.out.println("Fraction 2:");
        f2.input();
        System.out.println();

        result = f1.multiply(f2);
        f1.output();
        System.out.print(" * ");
        f2.output();
        System.out.print(" = ");
        result.output();
        System.out.println();

        result = f1.divide(f2);
        f1.output();
        System.out.print(" / ");
        f2.output();
        System.out.print(" = ");
        result.output();
        System.out.println();
    }

}

The parts of the code I highlighted in red are the problems. The "Savitch.In" is an issue, and the "common = convert(common)" is also. This is the output I'm getting for that program.

1/3 - 1/6 = 0/1
1/6 - 1/3 = 0/1

Fraction 1:
Please enter an integer for numerator: Please enter a non-zero integer for denominator:
Fraction 2:
Please enter an integer for numerator: Please enter a non-zero integer for denominator:
1/3 * 1/6 = 1/18
1/3 / 1/6 = 2/1
BUILD SUCCESSFUL (total time: 1 second)


Thanks.

[D Doemer]

Whoops, I guess it won't color the text within a code. Sorry.

[Ulf Dittmer]

You can find the source code to the SavitchIn class on the net. It seems to be widely used.

[Campbell Ritchie]

No, the colours do not appear; I have removed them because the idea is that you can copy and paste the code from the code tags (click the view plain link) and execute it yourself. The colour tags would prevent that.

Why have you got a no‑arguments constructor? I would suggest you get rid of that because I don't think you create a Fraction with no numbers in. If yo udo keep it, there are two things I suggest you change:-

1: Don't have two assignments on the same line: poor style.

2: Assign your denominator to 1 otherwise you have the dangerous error of denominator 0 which can cause division by 0.

I think you need some debugging code. Start with this at the end of each constructor:-

System.out.printf("Line 123: Fraction constructor: den = %d num = %d%n", num, den);//test

That will show you whether you are successfully initialising your objects.
The second part of the output, with 1/18 and 2/1, looks correct to me. When your debugging code gets to subtraction, you will need lots of printouts like that to find the error. What formula are you using? I think it should be

d1 × n2 - d2 × n1
      d1 × d2