How to loop this program??

ok guys i need a little help here, i have made this program that inputs a fraction and give the output in its simplest forms, the only problem im having is that i nee to edit this coding to that if the user inserts a char, or 0 in the denominator, so the program should give an error message and return to the start until the user wants to exit the program...

here is my coding,

Code:

---

public class Fraction {
        private int numer = 0;
        private int denom = 0;

        public Fraction(int numer, int denom) {
                if (denom == 0)
                throw new NumberFormatException("denominator is zero");
                this.numer = numer;
                this.denom = denom;
                this.reduce();
        }

        public int getNumerator() {
                return this.numer;
        }

        public int getDenominator() {
                return this.denom;
        }

        public void reduce()
        {
                int        d;

                //while (true)
                do{
                        d = this.gcd(this.numer, this.denom);
                        if (d == 1)
                                return;
                        this.numer /= d;
                        this.denom /= d;
                }while(true);
        }

        private int gcd(int a, int b) {
                int        t;

                while (b != 0) {
                        t = a;
                        a = b;
                        b = t%a;
                }
                return (a);
        }

        public String output() {
                return ( this.numer + "/" + this.denom );
        }
       
        public double doubleValue() {
                return (1.0*this.numer/this.denom);
        }
        }

---

Can you post the code where use the constructor?

Code:

---

                    public static void main(String[] args) {
                Scanner in=new Scanner(System.in);
               
                Fraction        f1;
                int n,d;
               
                System.out.print("Enter Numerator: ");
                        n=in.nextInt();
                System.out.print("Enter Denominator: ");
                        d=in.nextInt();
               
                        f1 = new Fraction(n, d);
                System.out.println("Fraction Output: " + f1.output());
               
        }
}

---

Using in.nextInt(); is inadvisable, you should read the next line and parse it to an integer:
Integer.parseInt(in.nextLine());
This will throw an exception if the line in not an integer.
Catch that exception and restart that part of the program; use a for( ;; ) or while loop.

If an exception was not thrown, just check if the denominator is 0.

you can use a while loop and a try catch in your main

Code:

---

    public static void main(String[] args) {
        Fraction    f1;
        int n,d;
        while(true){
            try {
                Scanner in=new Scanner(System.in);
                System.out.print("Enter Numerator: ");
                n=in.nextInt();
                System.out.print("Enter Denominator: ");
                d=in.nextInt();
                break;
            } catch ( InputMismatchException ex ){
                System.err.println("Enter a valid integer");
            }
        }
        f1 = new Fraction(n, d);
        System.out.println("Fraction Output: " + f1.output());
    }

---