Roman2Decimal.java

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

public class Roman2Decimal {

    /**
    *   Program:    Roman2Decimal.java
    *   Purpose:    Convert Roman numerals to decimal numbers
    *   Creator:    Chris Clarke
    *   Created:    01.11.2012
    */

    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        String roman;

        if (args.length > 0)
            roman = args[0];
        else {
            System.out.print("Enter Roman numerals for conversion: ");
            roman = scan.nextLine();
        }

        System.out.println("In decimal: " + convertNumeralToNumber( roman));

    } // end main

    private static int convertNumeralToNumber (String numeral) {
    // read the string from left to right

        int number = 0;
        int first = 0, second = 0;
        numeral = numeral.toUpperCase();

        // while the string isn't empty
        while (!numeral.equals("")) {
            // first numeral
            first = getNumber( numeral.substring(0, 1));
            if (numeral.length() > 1)
                // second numeral
                second = getNumber( numeral.substring(1, 2));
            else
                second = 0;

            // if first numeral is less than second numeral
            if (first < second) {
                // subtract first from second, add result to number
                number += (second - first);
                // remove first two numerals from string
                numeral = numeral.substring(2, numeral.length());
            } else {
                // add value of first numeral
                number += first;
                // remove first numeral from string
                numeral = numeral.substring(1, numeral.length());
            }
        } // end while

        return number;

    } // end convertNumeralToNumber

    public static int getNumber(String letter) {
        switch (letter) {
            case "I": return 1;
            case "V": return 5;
            case "X": return 10;
            case "L": return 50;
            case "C": return 100;
            case "D": return 500;
            case "M": return 1000;
            default : System.out.println("Error: wrong Roman numeral!");
                  System.exit(1);
        } //end switch

        return -1;

    } // end getNumber

} // end class