How to use Java to convert numbers to words.

Hi, I'm trying to write a java code to convert numbers to words, but I somewhat can't find a way to finish it. The application needs two dialog box. The first one ask the user to enter a 4 digit number. The second one shows the result (ex. 123 is equal to one hundred twenty three). Below is what I have so far. I have put the part that I don't how to finish in the notation. Could you guys please give me a hand, or referr me to some similar example? Thank you very much.


import javax.swing.JOptionPane

public class NumberConverterApp {
    public static void main (String[] args){

        String choice="";
        while (!(choice.equalsIgnoreCase("X"))){
              //[how to get and parse input?]
               String[] units = {"one, "two", "three, "four", "five", "six", "seven", "eight", "nine");
               String[] teens{"ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen"};
               String[] tens ={"twenty", "thirty", "fourty", "fifty", "sixty", "seventy", "eighty", "ninety"};
               String word = "";
               if (num.length == 4){
                     //[how to convert to words?]
               }
               choice = JOptionPane.showInputDialog(numbers + " is equal to \n"
                          + word + "\n Press enter to continue or 'x' to exit.");
          }//end while
          System.exit(0);
       }
   }