Курсова Работа 2 - 4.5 Калкулатор за обикновени дроби

import java.util.Scanner;

public class program {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int n = Integer.parseInt(sc.nextLine());
        int d = Integer.parseInt(sc.nextLine());

        fraction drob = new fraction(n, d);
        System.out.println("Drob1 = " + drob);

        fraction drob2 = new fraction(7, 5);
        System.out.println("Drob2 = " + drob2);

        fraction drob3 = drob.multiply(drob2);
        System.out.println("Drob3 = " + drob3);

        fraction drob4 = drob.sum(drob3);
        System.out.println("Drob4 = " + drob4);

        fraction drob5 = drob.substract(drob4);
        System.out.println("Drob5 = " + drob5);
    }

}

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

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

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

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

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

    public fraction substract(fraction other) {
        int n = this.numerator * other.denominator - other.numerator * this.denominator;
        int d = this.denominator + other.denominator;
        fraction fr = new fraction(n, d);
        return fr;
    }

    public int gcd(int a, int b) {
        for (int i = Math.min(a, b); i >= 1; i--) {
            if (a % 1 == 0 && b % 1 == 0) {
                return i;
            }
        }
        return 1;
    }
}