GreatestCommonDivisor.java

import java.util.Scanner;   // for user input

public class GreatestCommonDivisor {

    public static void main(String[] args) {

        /**
        *   Program:    GreatestCommonDivisor.java
        *   Purpose:    Find greatest common divisor of two numbers
        *   Creator:    Chris Clarke
        *   Created:    07.11.2012
        */

        int a = 0, b = 0;   // the 2 numbers input
        int c;          // the answer

        a = getNumber("Enter your first positive integer: ");
        b = getNumber("Enter your second positive integer: ");

        c = getGreatestCommonDivisor( a, b);

        System.out.println("The greatest common divisor of "+a+" and "+b+" is "+c);
    } // end main

    public static int getNumber(String prompt) {
        int n = 0;
        Scanner scan = new Scanner(System.in);
        System.out.print(prompt);
        try {
            n = scan.nextInt();
        } catch (Exception e) {
            System.out.println("That\'s not an integer!");
            System.exit(1);
        }

        if (n <= 0) {
            System.out.println("That\'s not positive!");
            System.exit(1);
        }
        return n;

    } // end getNumber

    public static int getGreatestCommonDivisor(int a, int b) {
        int gcd = 0;
        int limit = a >= b? a: b;   // set limit to a or b, whichever is the greater

        for (int i = 1; i <= limit; i++) {
            if (a % i == 0 && b % i == 0) {
                gcd = i;
            }
        }

        return gcd;

    } // end getGreatestCommonDivisor
} // end class