EnglishNumberToWords.java

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 EnglishNumberToWords() {}

  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;
    if (number%100==0)
    return numNames[number] + "hundred" + soFar;
    else
    return numNames[number] + "hundredand" + 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 = "onethousand";
      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) {
	  int sum = 0;
	  for (int k = 1; k<=1000; k++){
		  	sum+=EnglishNumberToWords.convert(k).length();
		  	System.out.println(EnglishNumberToWords.convert(k));
		  	System.out.println(EnglishNumberToWords.convert(k).length());
		  	
	  }
	  System.out.println(sum-27);
	  System.out.println(EnglishNumberToWords.convert(115).length());
 

    /*
     *** 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
     **/
  }
}