NumbersToWords

import java.text.DecimalFormat;

public class EnglishNumberToWords {

  private static final String[] tensNames = {
    "",
    " Ten",
    " Twenty",
    " Thirty",
    " Forty",
    " Fifty",
    " Sixty",
    " Seventy",
    " Eighty",
    " Ninety"
  };

  private static final String[] numNames = {
    "",
    " One",
    " Two",
    " Three",
    " Four",
    " Five",
    " Six",
    " Seven",
    " Eight",
    " Nine",
    " Ten",
    " Eleven",
    " Twelve",
    " Thirteen",
    " Fourteen",
    " Fifteen",
    " Sixteen",
    " Seventeen",
    " Eighteen",
    " Nineteen"
  };

  private static String convertLessThanOneThousand(int number) {
    String soFar;

    if (number % 100 < 20){
      soFar = numNames[number % 100];
      number /= 100;
    }
    else {
      soFar = numNames[number % 10];
      number /= 10;

      soFar = tensNames[number % 10] + soFar;
      number /= 10;
    }
    if (number == 0) return soFar;
    return numNames[number] + " Hundred" + soFar;
  }


  public static String convert(long number) {
    // 0 to 999 999 999 999
    if (number == 0) { return "Zero"; }

    String snumber = Long.toString(number);

    // pad with "0"
    String mask = "000000000000";
    DecimalFormat df = new DecimalFormat(mask);
    snumber = df.format(number);

    // XXXnnnnnnnnn
    int billions = Integer.parseInt(snumber.substring(0,3));
    // nnnXXXnnnnnn
    int millions  = Integer.parseInt(snumber.substring(3,6));
    // nnnnnnXXXnnn
    int hundredThousands = Integer.parseInt(snumber.substring(6,9));
    // nnnnnnnnnXXX
    int thousands = Integer.parseInt(snumber.substring(9,12));

    String tradBillions;
    switch (billions) {
    case 0:
      tradBillions = "";
      break;
    case 1 :
      tradBillions = convertLessThanOneThousand(billions)
      + " Billion ";
      break;
    default :
      tradBillions = convertLessThanOneThousand(billions)
      + " Billion ";
    }
    String result =  tradBillions;

    String tradMillions;
    switch (millions) {
    case 0:
      tradMillions = "";
      break;
    case 1 :
      tradMillions = convertLessThanOneThousand(millions)
      + " Million ";
      break;
    default :
      tradMillions = convertLessThanOneThousand(millions)
      + " Million ";
    }
    result =  result + tradMillions;

    String tradHundredThousands;
    switch (hundredThousands) {
    case 0:
      tradHundredThousands = "";
      break;
    case 1 :
      tradHundredThousands = "One Thousand ";
      break;
    default :
      tradHundredThousands = convertLessThanOneThousand(hundredThousands)
      + " Thousand ";
    }
    result =  result + tradHundredThousands;

    String tradThousand;
    tradThousand = convertLessThanOneThousand(thousands);
    result =  result + tradThousand;

    // remove extra spaces!
    return result.replaceAll("^\\s+", "").replaceAll("\\b\\s{2,}\\b", " ");
  }

  /**
   * testing
   * @param args
   */
  public static void main(String[] args) {
    System.out.println("*** " + EnglishNumberToWords.convert(0));
    System.out.println("*** " + EnglishNumberToWords.convert(1));
    System.out.println("*** " + EnglishNumberToWords.convert(16));
    System.out.println("*** " + EnglishNumberToWords.convert(100));
    System.out.println("*** " + EnglishNumberToWords.convert(118));
    System.out.println("*** " + EnglishNumberToWords.convert(200));
    System.out.println("*** " + EnglishNumberToWords.convert(219));
    System.out.println("*** " + EnglishNumberToWords.convert(800));
    System.out.println("*** " + EnglishNumberToWords.convert(801));
    System.out.println("*** " + EnglishNumberToWords.convert(1316));
    System.out.println("*** " + EnglishNumberToWords.convert(1000000));
    System.out.println("*** " + EnglishNumberToWords.convert(2000000));
    System.out.println("*** " + EnglishNumberToWords.convert(3000200));
    System.out.println("*** " + EnglishNumberToWords.convert(700000));
    System.out.println("*** " + EnglishNumberToWords.convert(9000000));
    System.out.println("*** " + EnglishNumberToWords.convert(9001000));
    System.out.println("*** " + EnglishNumberToWords.convert(123456789));
    System.out.println("*** " + EnglishNumberToWords.convert(2147483647));
    System.out.println("*** " + EnglishNumberToWords.convert(3000000010L));

    /*
     *** zero
     *** one
     *** sixteen
     *** one hundred
     *** one hundred eighteen
     *** two hundred
     *** two hundred nineteen
     *** eight hundred
     *** eight hundred one
     *** one thousand three hundred sixteen
     *** one million
     *** two millions
     *** three millions two hundred
     *** seven hundred thousand
     *** nine millions
     *** nine millions one thousand
     *** one hundred twenty three millions four hundred
     **      fifty six thousand seven hundred eighty nine
     *** two billion one hundred forty seven millions
     **      four hundred eighty three thousand six hundred forty seven
     *** three billion ten
     **/
  }
}