How to loop this program??

[7auz]

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,

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);
	}
	}

[Hibernate]

Can you post the code where use the constructor?

[7auz]

                     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());
		
	}
}

[Hibernate]

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.

[JavaHater]

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

    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());
    }