/* T01n17.java -- Showing what will happen if you try to change a formal
 * parameter.  Java lets you do it (it treats formal parameters as local
 * variables), but the change has no effect on the formal parameter's
 * corresponding actual parameter.  In other words, the actual parameter 
 * is copied into the formal parameter when the method is invoked, and 
 * that's the end of their association for this invocation.
 */

import java.util.*;

public class T01n17
{
    public static void main (String [] args)
    {
        int itemsBought;   // Items purchased by a customer at a store
        String rawInput;   // Input from the user, as a String, not an int
        Scanner keyboard = new Scanner (System.in);   // source of input

        do {
            System.out.print("\nHow many items were purchased? : ");
            rawInput = keyboard.nextLine();
        } while (!validInteger(rawInput));

        itemsBought = Integer.parseInt(rawInput);  //

        farewell(itemsBought);

        System.out.println("\t\tWe tried to change the formal arguments in\n"
                         + "\t\tboth methods.  Back in main(), 'rawInput' now"
                         + " references \"" + rawInput + "\"\n\t\twhile "
                         + "'itemsBought' is now [" + itemsBought + "]."
                         + "  Nothing changed!\n");
    }

            /*  validInteger(string) -- Checks to see if the user's input
             *  contains only digits.  Returns 'true' if so, 'false' o.w.
             */

    private static boolean validInteger (String userInput)
    {
        boolean valid = true;    // Our default assumption

        for (int i=0; i<userInput.length(); i++) {
            if (!Character.isDigit(userInput.charAt(i))) {
                valid = false;
                break;     // bumps us out of a loop we needn't finish
            }
        }

                // Including output in a testing method like this one
                // is NOT good programming style.  What if someone wanted
                // to use it in another program?  Would this output suit
                // their needs?  Probably not.  This really belongs in the
                // do-while loop in main().

        if (!valid) {

            System.out.println("I'm sorry; \"" + userInput + "\" isn't a"
                             + " valid integer.\nPlease try again, using"
                             + " only digits.\n");

        } else {  // Let's see if prepending 0's to userInput changes anything!

            userInput = "000" + userInput;
    
            System.out.println("\n\t\t'userInput' now references the string \""
                             + userInput + "\".\n");
        }

        return valid;
    }


            /*  farewell(int) -- Displays a 'thanks' message to the 
             *  customer.
             */

    private static void farewell (int purchases)
    {
        System.out.println("You bought " + purchases + " items "
                         + "at Kwik-E-Mart.  Thank you; come again!");

        purchases = -1;      // And let's also change purchases

        System.out.println("\n\t\t'purchases' now holds ["
                         + purchases + "].\n");
    }
}