Can someone explain to me how this program works?

Question

I am having trouble understanding how to program below works and I would appreciate someone explaining to me how exactly it runs.

public static void main(String[] args) {
    //Enter two number whose GCD needs to be calculated. 
    Scanner scanner = new Scanner(System.in);

    // Title of what program will do
    System.out.println("GCD Finder");
    System.out.println("");

    // Here user is instructed to enter the numbers
    System.out.println("Please enter first number: "); 
    int number1 = scanner.nextInt(); 
    System.out.println("Please enter second number: "); 
    int number2 = scanner.nextInt(); 

    // The numbers are then calculated using findGCD.
    System.out.println("GCD of two numbers " + number1 +" and " + number2 +" is : " + findGCD(number1,number2)); 
    } 


private static int findGCD(int number1, int number2) { 
    //base case 
    if(number2 == 0){ 
        return number1; 
        } 
    // Returns the two numbers 
    return findGCD(number2, number1%number2); 
    } 

This part below is specifically what I am having trouble understanding. Please do not hesitate to explain in detail, I want to understand it fully. Thank you for your time.

private static int findGCD(int number1, int number2) { 
    //base case 
    if(number2 == 0){ 
        return number1; 
        } 
    // Returns the two numbers 
    return findGCD(number2, number1%number2); 
    } 

Answer 1

So the part you are in particular struggling with:

// A function which returns the greatest common divisor.
private static int findGCD(int number1, int number2) { 
    //base case 
    if(number2 == 0){ 
        return number1; 
        } 
    // Returns the two numbers 
    return findGCD(number2, number1%number2); 
} 

This is a recursive function, which is i imagine what is causing you difficulty. To better understand recursive functions in general perhaps read this: http://www.python-course.eu/recursive_functions.php Don't worry that it uses python as an example.

In this case in particular the function will return number 1 when number 2 is 0. if number two isnt zero then it will call the function again with number2 being the remainder of number1 / number2, otherwise known as the modulus % https://en.wikipedia.org/wiki/Modulo_operation.

You could write the function in a while loop aswell:

//PSEUDOCODE WATCH YOURSELF

private static int findGCD(int n1, int n2) {
    int result = 0;
    while(true) {
        if(n2 == 0) {
            return result;
        }
        result = n1;
        n1 = n2;
        n2 = result-n2;
    }
}

Hope that helps a little? Or at least doesn't make anything more confusing.