You want to convert numbers into their Roman numerals representation and vice versa. The solution here is to tackle the problem as a unary problem where the Roman numerals represented as a single element, the “I” character. We start by representing the number as a repeated sequence of the “I” characters. And then replace the characters according to next bigger symbol in roman numeral.

To convert from the Roman numerals to numbers we reverse the process. By the end of the process we will get a sequence of repeated “I” characters. The length of the final string returned by this process is the result of the roman numeral conversion to number.

In the code snippet below we create two methods. The toRoman(int number) method for converting number to roman numerals and the toNumber(String roman) method for converting from roman numerals to number. Both of this method utilize the String.replace() method for calculating the conversion result.

Let’s see the code in action.

package org.kodejava.lang;

public class RomanNumber {
    public static void main(String[] args) {
        for (int n = 1; n <= 4999; n++) {
            String roman = RomanNumber.toRoman(n);
            int number = RomanNumber.toNumber(roman);

            System.out.println(number + " = " + roman);
        }
    }

    private static String toRoman(int number) {
        return String.valueOf(new char[number]).replace('\0', 'I')
                .replace("IIIII", "V")
                .replace("IIII", "IV")
                .replace("VV", "X")
                .replace("VIV", "IX")
                .replace("XXXXX", "L")
                .replace("XXXX", "XL")
                .replace("LL", "C")
                .replace("LXL", "XC")
                .replace("CCCCC", "D")
                .replace("CCCC", "CD")
                .replace("DD", "M")
                .replace("DCD", "CM");
    }

    private static Integer toNumber(String roman) {
        return roman.replace("CM", "DCD")
                .replace("M", "DD")
                .replace("CD", "CCCC")
                .replace("D", "CCCCC")
                .replace("XC", "LXL")
                .replace("C", "LL")
                .replace("XL", "XXXX")
                .replace("L", "XXXXX")
                .replace("IX", "VIV")
                .replace("X", "VV")
                .replace("IV", "IIII")
                .replace("V", "IIIII").length();
    }
}

The 10 randoms result of the conversion listed below:

18 = XVIII
208 = CCVIII
843 = DCCCXLIII
1995 = MCMXCV
2000 = MM
2017 = MMXVII
2562 = MMDLXII
3276 = MMMCCLXXVI
4067 = MMMMLXVII
4994 = MMMMCMXCIV