Fraction class

public class Fraction {
    private int numerator;
    private int denominator;

    public Fraction(int n, int d) {
        this.numerator = n;
        this.denominator = d;
    }

    @Override
    public String toString() {
        String txt = this.numerator + "/" + this.denominator;
        return txt;
    }

    public Fraction multiply(Fraction other) {
        int n = this.numerator * other.numerator;
        int d = this.denominator * other.denominator;
        Fraction fr = new Fraction(n, d);
        fr.reduce();
        return fr;
    }

    public Fraction divide(Fraction other) {
        int n = this.numerator * other.denominator;
        int d = this.denominator * other.numerator;
        Fraction fr = new Fraction(n, d);
        fr.reduce();
        return fr;
    }

    public Fraction sum(Fraction other) {
        int n = this.numerator * other.denominator + this.denominator * other.numerator;
        int d = this.denominator * other.denominator;
        Fraction fr = new Fraction(n, d);
        fr.reduce();
        return fr;
    }

    public Fraction subtract(Fraction other) {
        int n = this.numerator * other.denominator - this.denominator * other.numerator;
        int d = this.denominator * other.denominator;
        Fraction fr = new Fraction(n, d);
        fr.reduce();
        return fr;
    }

    public void reduce() {
        int nod = gcd(this.denominator, this.numerator);
        this.numerator /= nod;
        this.denominator /= nod;
    }

    private int gcd(int a, int b) {
        if (b == 0) {
            return a;
        } else {
            return gcd(b, a % b);
        }
    }

}