Convert Roman numerals to Arabic ones

Roman.java

import java.util.HashMap;
import java.util.Random;

/**
 * Created by shail on 03/04/17.
 */
public class Roman {
    public static int toArabic(String roman) {
        HashMap<Character, Integer> hm = new HashMap();
        hm.put('M', 1000);
        hm.put('D', 500);
        hm.put('C', 100);
        hm.put('L', 50);
        hm.put('X', 10);
        hm.put('V', 5);
        hm.put('I', 1);

        int a[] = new int[roman.length()];
        for(int i = 0; i < roman.length(); i++)
            a[i] = hm.get(roman.charAt(i));

        int ans = 0;
        for(int i = 0; i < a.length-1; i++) {
            if(a[i+1] > a[i])
                ans -= a[i];
            else ans += a[i];
        }
        ans+=a[a.length-1];

        return ans;
    }

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

        int a[] = new int[]{1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1};
        String b[] = new String[]{"M", "CM", "D", "CD", "C", "XC", "L", "XL", "X", "IX", "V", "IV", "I"};

        for(int i = 0; i < a.length; i++) {
            while(arabic >= a[i]) {
                arabic -= a[i];
                sb.append(b[i]);
            }
        }

        return sb.toString();
    }

    public static void main(String args[]) {
        Random r = new Random();
        for(int i = 0; i < 10; i++) {
            int n = r.nextInt(1001);
            String roman = toRoman(n);
            int arabic = toArabic(roman);
            System.out.println(n+" "+roman+" "+arabic);
        }
    }
}