Problem with a string method

Question

Hello, I am working on a program that converts Roman numerals to Arabic numbers and the opposite. I have the Arabic to Roman part down, yet Roman to Arabic part is causing troubles for me.
I know that there are many solutions out there, yet I would like just solutions within my code that would fix the problem it has.
Instead of the whole code, here's the method that changes Roman to Arabic.
     //method to convert Roman numerals to Arabic numbers
     public static int toArabic (String x)
          int arabica=0;
          char chars;
          for (int y=0; y<=(x.length()-1); y++)
               chars=x.charAt(y);
               switch (chars)
                    case 'C':
                         if( x.length() == 1)
                              arabica+=100;
                              y++;
                         else if (x.charAt(y+1)=='M')
                              arabica+=900;
                              y++;
                         else if (x.charAt(y+1)=='D')
                              arabica+=400;
                              y++;
                         else
                              arabica+=100;
                         break;
                    case 'X':
                         if(x.length() == 1)
                              arabica+=10;
                              y++;
                         else if (x.charAt(y+1)=='C')
                              arabica+=90;
                              y++;
                         else if (x.charAt(y+1)=='L')
                              arabica+=40;
                              y++;
                         else
                              arabica+=10;
                         break;
                    case 'I':
                         if(x.length() == 1)
                              arabica+=1;
                              y++;
                         else if (x.charAt(y+1)=='X')
                              arabica+=9;
                              y++;
                         else if (x.charAt(y+1)=='V')
                              arabica+=4;
                              y++;
                         else
                              arabica++;
                         break;
                    case 'M':
                         arabica+=1000;
                         break;
                    case 'D':
                         arabica+=500;
                         break;
                    case 'L':
                         arabica+=50;
                         break;
                    case 'V':
                         arabica+=5;
                         break;
There's a problem with this, however, is that whenever I put something in like XX, CC, XXX, or II, the program says
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 2
at java.lang.String.charAt(String.java:687)
at RomanNumerals.toArabic(RomanNumerals.java:172)
at RomanNumerals.main(RomanNumerals.java:33)
I think this problem is caused by the if-else and else-if statements in my method for cases C, X, and I, as this problem doesn't come up when I do similar things to cases without the if statements. Could you perhaps find out what is causing this problem? I've been working on it for days after finishing the rest and I can't figure it out.
Thanks

Answer 1

import java.io.*;
public class RomanNumerals{
          public static void main (String [] args)
          DataInput keyboard=new DataInputStream (System.in);
          String input;
          try
               //options
               System.out.println("1. Roman numerals to Arabic numbers");
               System.out.println("2. Arabic numbers to Roman numerals");
               System.out.println("3. Exit");
               System.out.print("Enter your option: ");
               input=keyboard.readLine();
               int choice=Integer.parseInt(input);
               switch (choice)
                    //Roman numerals to Arabic numbers
                    case 1:
                         String romanInput, ro;
                         int answer1;
                         System.out.print("Enter a Roman numeral: ");
                         romanInput=keyboard.readLine();
                         ro=romanInput.toUpperCase();
                         answer1=toArabic(ro); //line 33 where the error occurs
                         System.out.println("The Arabic number is: "+answer1);
                         break;
                    //Arabic numbers to Roman numerals
                    case 2:
                         String arabicInput, answer;
                         System.out.print("Enter an Arabic number: ");
                         arabicInput=keyboard.readLine();
                         int arabic=Integer.parseInt(arabicInput);
                         answer=toRomans(arabic);
                         System.out.println("The Roman numeral is: "+answer);
                         break;
                    case 3:
                         break;
                    default:
                         System.out.println("Invalid option.");
          catch(IOException e)
               System.out.println("Error");
               //method to convert Arabic numbers to Roman numerals
     public static String toRomans (int N)
          String roman="";
          while (N>=1000)
               roman+="M";
               N-=1000;
          while (N>=900)
               roman+="CM";
               N-=900;
          while (N>=500)
               roman+="D";
               N-=500;
          while (N>=400)
               roman+="CD";
               N-=400;
          while (N>=100)
               roman+="C";
               N-=100;
          while (N>=90)
               roman+="XC";
               N-=90;
          while (N>=50)
               roman+="L";
               N-=50;
          while (N>=40)
               roman+="XL";
               N-=40;
          while (N>=10)
               roman+="X";
               N-=10;
          while (N>=9)
               roman+="IX";
               N-=9;
          while (N>=5)
               roman+="V";
               N-=5;
          while (N>=4)
               roman+="IV";
               N-=4;
          while (N>=1)
               roman+="I";
               N-=1;
          return(roman);
     //method to convert Roman numerals to Arabic numbers
     public static int toArabic (String x)
          int arabica=0;
          char chars;
          for (int y=0; y<=(x.length()-1); y++)
               chars=x.charAt(y);
               switch (chars)
                    case 'C':
                         if( x.length() == 1)
                              arabica+=100;
                              y++;
                         else if (x.charAt(y+1)=='M')
                              arabica+=900;
                              y++;
                         else if (x.charAt(y+1)=='D')
                              arabica+=400;
                              y++;
                         else
                              arabica+=100;
                         break;
                    case 'X':
                         if(x.length() == 1)
                              arabica+=10;
                              y++;
                         else if (x.charAt(y+1)=='C')   //the line 172 where error occurs
                              arabica+=90;
                              y++;
                         else if (x.charAt(y+1)=='L')
                              arabica+=40;
                              y++;
                         else
                              arabica+=10;
                         break;
                    case 'I':
                         if(x.length() == 1)
                              arabica+=1;
                              y++;
                         else if (x.charAt(y+1)=='X')
                              arabica+=9;
                              y++;
                         else if (x.charAt(y+1)=='V')
                              arabica+=4;
                              y++;
                         else
                              arabica++;
                         break;
                    case 'M':
                         arabica+=1000;
                         break;
                    case 'D':
                         arabica+=500;
                         break;
                    case 'L':
                         arabica+=50;
                         break;
                    case 'V':
                         arabica+=5;
                         break;
          return(arabica);
     }When I put in XX as the input, this is what the error comes out as:
Enter a Roman numeral: XX
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 2
    at java.lang.String.charAt(String.java:687)
    at RomanNumerals.toArabic(RomanNumerals.java:172)
    at RomanNumerals.main(RomanNumerals.java:33)