Roman Numeral to Integer: what is wrong with my code

Question

So I'm trying to make a converter from roman numerals to integers. What I have so far is this:

public int toNumber(String n){
   int number = 0;
   int first = 1;
   int last = 0;      
   String substring = n.substring (last, first);

   while(substring.equals("I")){
            number = number+1;
            last = last +1;
            first=first +1;
        }


   while(substring.equals("V")){
            number = number+5;
            last = last +1;
            first=first +1;
        }
        return number;

}

Obviously I only have I and V in right now, but when I make a tester class to try this out, it returns nothing and keeps letting me put in in a new line.

For the record, here is my tester class

import java.util.Scanner;
public class Tester{
    public static void main (String[] args){
        RomanNumeralConverter quantity = new RomanNumeralConverter();
       Scanner user_input = new Scanner(System.in);

       //this is for roman to number  
          System.out.println("input roman numeral");
          String j  = user_input.nextLine(); 
          int g = quantity.toNumber(j);
          System.out.println(g);

    }
}

I'm almost entirely certain that it's a logic problem, but I have no idea what and I feel like I've tried everything I can think of

Answer 1

Your problem is a loop like this:

while(substring.equals("I")) {
    number = number+1;
    last = last +1;
    first=first +1;
}

If the program enters that loop, it will stuck there, because you're not changing the value of substring . Therefore substring.equals("I") will always return true and the only way to stop that loop is terminating the application.

A better way to convert the entered String could be this:

public int toNumber(String n) {
    int number = 0;

    for (char chr : n.toUpperCase().toCharArray()) {
        switch(chr) {
            case 'I': number += 1;
                      break;
            case 'V': number += 5;
                      break;
            default: break;
        }
    }

    return number;
}

It converts the provided String to uppercase ( viii -> VIII ) to avoid checking both cases of every char and converts it into a char array. This array will be used in a foreach loop that takes every single entry of it and tests it in a switch block. Every supported roman numeral has his own case there to increment number accordingly. Currently it supports I and V like in your code. Btw, number += 1 is just a short version of number = number + 1 .

After that loop, it returns the converted integer value.