大家好，我正在写一个从罗马数字到阿拉伯数字的转换程序。但是，stringindexoutofboundsexception始终存在问题：字符串索引超出范围：-1。下面是我的代码

package com.company;

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

public class Main {

    public static void main(String[] args) {

        System.out.println(romanToArabic("CIV"));
    }
    public static int romanToArabic(String romanNumeral){
        Map <Character, Integer> map = new HashMap<>();
        map.put('I', 1);
        map.put('V', 5);
        map.put('X', 10);
        map.put('L', 50);
        map.put('C', 100);
        map.put('D', 500);
        map.put('M', 1000);

        int result = 0;
        for (int i=0; i < romanNumeral.length(); i++){
            int current = map.get(romanNumeral.charAt(i));
            int next = map.get(romanNumeral.charAt(i-1));
            if (i>0 && current > next){
                result += current - 2*next;
            }

            result += current;
        }
        return result;
    }