String index out bound exception

Question

Hi everyone I'm trying to write a program to convert from roman numerals to Arabic numerals. However, I keep having issues with StringIndexOutOfBoundsException: String index out of range: -1. below is my code

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;
    }

Answer 1

int next = map.get(romanNumeral.charAt(i-1));

"i-1" is -1 when you walk through the loop for the first time (i=0)

Answer 2

Your problem is in your first "for loop" iteration when "i=0"

In the following line you properly get a character "C" which has an index "0"

int current = map.get(romanNumeral.charAt(i));

Then on the next line "i" becomes -1 when code executes the statement (i-1)

int next = map.get(romanNumeral.charAt(i-1));

and thus you try to get a character with index "-1"

int next = map.get(romanNumeral.charAt(-1));

and it follows you to the String index out bound exception

Because string characters indexing starts with zero

Answer 3

As both mentioned you are starting at i = 0, then trying an operation of (i-1),

Fix this by starting your loop at i = 1, or only doing this line (and the rest of the code):

int next = map.get(romanNumeral.charAt(i-1));

if i is greater than or equal to 1.