Java: Roman Numeral Tool

I was looking for a tool to check if a given String is a valid roman numeral or not. Remember old Math? Like IV = 4, or V = 5 or X1 = 11 and so on. I got lucky and chanced upon this link. I did a little modification to cater the code to my needs. This class has 2 public methods namely getRomanNumeral and isRomanNumeral.

getRomanNumeral takes in an int parameter and converts it to its roman numeral equivalent. If the value is less than 1 or greater than 3,999, it throws a NumberFormatException. isRomanNumeral meanwhile, takes in a String parameter and checks if the given data is a valid roman numeral or not. It’s a great helper class to handle roman numeral issues.

/*
	An object of type RomanNumeral is an integer between 1 and 3999.  It can
	be constructed either from an integer or from a string that represents
	a Roman numeral in this range.  The function toString() will return a
	standardized Roman numeral representation of the number.  The function
	toInt() will return the number as a value of type int.
*/
public class RomanNumeralTool {
	private static int num;	// The number represented by this Roman numeral.
	/* The following arrays are used by the toString() function to construct
	  the standard Roman numeral representation of the number.  For each i,
	  the number numbers[i] is represented by the corresponding string, letters[i].
	*/
	private static int[]  numbers = { 1000,  900,  500,  400,  100, 90, 50, 40, 10,  9,  5,  4,  1 };
	private static String[] letters = {"M",  "CM",  "D",  "CD", "C",  "XC", "L",  "XL",  "X",  "IX", "V",  "IV", "I" };
	public static String getRomanNumeral(int arabic) {
		// Constructor.  Creates the Roman number with the int value specified
		// by the parameter.  Throws a NumberFormatException if arabic is
		// not in the range 1 to 3999 inclusive.
		if (arabic < 1) throw new NumberFormatException("Value of RomanNumeral must be positive.");
		if (arabic > 3999) throw new NumberFormatException("Value of RomanNumeral must be 3999 or less.");
		num = arabic;
		return convertIntToRomanNumeral();
	}
	public static boolean isRomanNumeral(String roman) {
		boolean isRomanNumeral = true;
		// Constructor.  Creates the Roman number with the given representation.
		// For example, RomanNumeral("xvii") is 17.  If the parameter is not a
		// legal Roman numeral, a NumberFormatException is thrown.  Both upper and
		// lower case letters are allowed.
		if (roman.length() == 0) {
			isRomanNumeral = false;
			//throw new NumberFormatException("An empty string does not define a Roman numeral.");
		}
		roman = roman.toUpperCase();  // Convert to upper case letters.
		int i = 0;  // A position in the string, roman;
		int arabic = 0; // Arabic numeral equivalent of the part of the string that has
						// been converted so far.
		while (i < roman.length()) {
			char letter = roman.charAt(i);    // Letter at current position in string.
			int number = letterToNumber(letter);  // Numerical equivalent of letter.
			if (number < 0) {
				isRomanNumeral = false;
				//throw new NumberFormatException("Illegal character \"" + letter + "\" in roman numeral.");
			}
			i++;  // Move on to next position in the string
			if (i == roman.length()) {
				// There is no letter in the string following the one we have just processed.
				// So just add the number corresponding to the single letter to arabic.
				arabic += number;
			 } else {
				// Look at the next letter in the string.  If it has a larger Roman numeral
				// equivalent than number, then the two letters are counted together as
				// a Roman numeral with value (nextNumber - number).
				int nextNumber = letterToNumber(roman.charAt(i));
				if (nextNumber > number) {
					// Combine the two letters to get one value, and move on to next position in string.
					arabic += (nextNumber - number);
					i++;
				} else {
					// Don't combine the letters.  Just add the value of the one letter onto the number.
					arabic += number;
				}
			}
		}
		if (arabic > 3999) {
			//throw new NumberFormatException("Roman numeral must have value 3999 or less.");
			isRomanNumeral = false;
		}
		num = arabic;
		return isRomanNumeral;
	}
	private static int letterToNumber(char letter) {
		// Find the integer value of letter considered as a Roman numeral.  Return
		// -1 if letter is not a legal Roman numeral.  The letter must be upper case.
		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: return -1;
		}
	}
	private static String convertIntToRomanNumeral() {
		// Return the standard representation of this Roman numeral.
		String roman = "";  // The roman numeral.
		int N = num;    // N represents the part of num that still has
							// to be converted to Roman numeral representation.
		for (int i=0; i= numbers[i]) {
				roman += letters[i];
				N -= numbers[i];
			}
		}
		return roman;
	}
	private static int toInt() {
		// Return the value of this Roman numeral as an int.
		return num;
	}
 }