Roman number conversion, following the rules: Symbols are placed from left to right in order of value, starting with the largest. However, in a few specific cases,[2] to avoid four characters being repeated in succession (such as IIII or XXXX) these can be reduced using subtractive notation as follows:[3][4] the numeral I can be placed before V …

RomanConversion.java

public class RomanConversion {

    public static void main(String[] args){
        System.out.println(toRoman(1904));
    }

    public static String toRoman(int x){
        StringBuilder sb = new StringBuilder();

        String number = x + "";

        //Thousands
        int i = number.length()-3;
        if(i > 0){
            int thousands = Integer.parseInt(number.substring(0, i));
            for(; thousands > 0; thousands--){
                sb.append("M");
            }
        }

        int l = number.length();
        //Hundreds
        if(number.length() >= 3) {
            int hundreds = Integer.parseInt(number.substring(l-3, l-2));
            switch (hundreds) {
                case 1:
                    sb.append("C");
                    break;
                case 2:
                    sb.append("CC");
                    break;
                case 3:
                    sb.append("CCC");
                    break;
                case 4:
                    sb.append("CD");
                    break;
                case 5:
                    sb.append("D");
                    break;
                case 6:
                    sb.append("DC");
                    break;
                case 7:
                    sb.append("DCC");
                    break;
                case 8:
                    sb.append("DCCC");
                    break;
                case 9:
                    sb.append("CM");
                    break;
            }
        }

        //Tens
        if(number.length() >= 2) {
            int tens = Integer.parseInt(number.substring(l-2, l-1));
            switch (tens) {
                case 1:
                    sb.append("X");
                    break;
                case 2:
                    sb.append("XX");
                    break;
                case 3:
                    sb.append("XXX");
                    break;
                case 4:
                    sb.append("XL");
                    break;
                case 5:
                    sb.append("L");
                    break;
                case 6:
                    sb.append("LX");
                    break;
                case 7:
                    sb.append("LXX");
                    break;
                case 8:
                    sb.append("LXXX");
                    break;
                case 9:
                    sb.append("XC");
                    break;
            }
        }

        //Units
        if(number.length() >= 1) {
            int units = Integer.parseInt(number.substring(l-1, l));
            switch (units) {
                case 1:
                    sb.append("I");
                    break;
                case 2:
                    sb.append("II");
                    break;
                case 3:
                    sb.append("III");
                    break;
                case 4:
                    sb.append("IV");
                    break;
                case 5:
                    sb.append("V");
                    break;
                case 6:
                    sb.append("VI");
                    break;
                case 7:
                    sb.append("VIII");
                    break;
                case 8:
                    sb.append("VIIII");
                    break;
                case 9:
                    sb.append("IX");
                    break;
            }
        }

        return sb.toString();

    }
}