Klasa ułamek w Javie

import java.util.Scanner;

public class Ulamek{
    public static void main(String[] args){

        Scanner input = new Scanner(System.in);
        System.out.println("Podaj licznik i mianownik pierwszego ulamka: ");
        Fraction ulamek1 = new Fraction(input.nextInt(), input.nextInt());
        System.out.println("Podaj licznik i mianownik drugiego ulamka: ");
        Fraction ulamek2 = new Fraction(input.nextInt(), input.nextInt());
        System.out.println("Jaka operacje chcesz wykonac? ( +, -)");
        char c = input.next().charAt(0);
        System.out.print(ulamek1);
        System.out.print(" " + c + " ");
        System.out.println(ulamek2);
        switch(c){
            case '-':
            System.out.println(ulamek1.sub(ulamek2));
            break;
            case '+':
            System.out.println(ulamek1.sum(ulamek2));
            break;
        }
    }
}

class Fraction{
    private int numerator;
    private int denominator;

    public Fraction(int numerator, int denominator){
        if(denominator < 0){
           this.numerator = -numerator;
           this.denominator = -denominator;
        }else{
              this.numerator = numerator;
              this.denominator = denominator;
        }
    }

    public Fraction sum(Fraction other){
        int nww =  nww(other.denominator, this.denominator);
        int newNumerator = nww/this.denominator*this.numerator + nww/other.denominator*numerator;
        return new Fraction(newNumerator, nww).reduce();
    }

    public Fraction sub(Fraction other){
        int nww = nww(this.denominator, other.denominator);
        int newNumerator = nww/this.denominator*this.numerator - nww/other.denominator*other.numerator;
        return new Fraction(newNumerator, nww).reduce();
    }

    public Fraction reduce(){
        int nwd = nwd(this.numerator, this.denominator);
        return new Fraction(this.numerator/nwd, this.denominator/nwd);
    }

    private int nwd(int a, int b){
        if(a < 0){
            a = -a;
        }
        if(b < 0){
            b = -b;
        }
       while(b != 0){
          int temp = b;
          b = a % b;
          a = temp;
       }
       return a;
    }

    private int nww(int a, int b){
        return a*b/nwd(a, b);
    }

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

}

    public Fraction(int numerator, int denominator)
    {
        if(denominator<0)
        {
           numerator=-numerator;
           denominator=-denominator;
        }
        int _nwd=nwd(numerator,denominator);
        this.numerator=numerator/_nwd;
        this.denominator=denominator/_nwd;
    }
    
    public static Fraction sum(Fraction a,Fraction b)
    {
        return new Fraction(a.numerator*b.denominator+b.numerator*a.denominator,a.denominator*b.denominator);
    }

    public static Fraction sub(Fraction a,Fraction b)
    {
        return new Fraction(a.numerator*b.denominator-b.numerator*a.denominator,a.denominator*b.denominator);
    }
    
    private int nwd(int a,int b)
    {
        a=Math.abs(a);
        b=Math.abs(b);
        while(b!=0)
        {
          int temp=b;
          b=a%b;
          a=temp;
       }
       return a;
    }