roman numerals converter

RomanNumber.java

import java.util.HashMap;
import java.util.Map;

public class RomanNumber {

    static final Map<Character, Integer> ROMAN_NUMERALS;

    static {
        HashMap<Character, Integer> m = new HashMap<>();
        m.put('I', 1);
        m.put('V', 5);
        m.put('X', 10);
        m.put('L', 50);
        m.put('C', 100);
        m.put('D', 500);
        m.put('M', 1000);
        ROMAN_NUMERALS = m;
    }

    static int romanToDecimal(String roman) {
        int[] left = new int[1];
        return new StringBuilder(roman).reverse().chars()
                .map(i -> ROMAN_NUMERALS.get((char) i))
                .map((right) -> {
                    int result = right < left[0] ? -right : right;
                    left[0] = right;
                    return result;
                }).sum();
    }

    public static void main(String[] args) {
        String roman = "XLII";
        int decimal = romanToDecimal(roman);
        System.out.printf("roman: %s  decimal: %d%n", roman, decimal);
    }
}