Fraction calculator

package kr_2;

import java.util.Scanner;

public class FractionCalc {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        while (true) {
            String exp = sc.nextLine();
            if (exp.equalsIgnoreCase("end")) {
                break;
            }
            char op = exp.charAt(exp.indexOf(" ") + 1);
            String[] drobi = exp.split(" ([+]|[-]|[*]|[/]) ");
            Fraction d1 = new Fraction(drobi[0]);
            Fraction d2 = new Fraction(drobi[1]);
            Fraction d3 = null;
            if (op == '+') {
                d3 = d1.sum(d2);
            } else if (op == '-') {
                d3 = d1.subtract(d2);
            } else if (op == '*') {
                d3 = d1.multiply(d2);
            }
            if (op == '/') {
                d3 = d1.divide(d2);
            }
            System.out.println(d3);
        }
    }
}