Number to String

package demo;
// convert number in range [0 ... 999] to string

public class HelloJava {
    public static void main(String[] args) {
        int number = 511;

        int length = String.valueOf(number).length();

        String numberStr = convertNumberToString(number, length);

        System.out.println(numberStr);
    }

    public static String convertNumberToString(int number, int length) {
        String str = null;

        if (length == 1) {
            str = readSingleDigit(number);
        } else if (length == 2) {
            str = readDoubleDigit(number);
        } else {
            str = readTripleDigit(number);
        }

        return str;
    }

    public static String readSingleDigit(int num) {
        String str = null;

        switch (num) {
        case 0:
            str = "zero";
            break;
        case 1:
            str = "one";
            break;
        case 2:
            str = "two";
            break;
        case 3:
            str = "three";
            break;
        case 4:
            str = "four";
            break;
        case 5:
            str = "five";
            break;
        case 6:
            str = "six";
            break;
        case 7:
            str = "seven";
            break;
        case 8:
            str = "eight";
            break;
        case 9:
            str = "nine";
            break;
        }

        return str;
    }

    public static String readDoubleDigit(int num) {
        String str = null;

        if (num < 20) {
            switch (num) {
            case 10:
                str = "ten";
                break;
            case 11:
                str = "eleven";
                break;
            case 12:
                str = "twelve";
                break;
            case 13:
                str = "thirteen";
                break;
            case 14:
                str = "fourteen";
                break;
            case 15:
                str = "fifteen";
                break;
            case 16:
                str = "sixteen";
                break;
            case 17:
                str = "seventeen";
                break;
            case 18:
                str = "eighteen";
                break;
            case 19:
                str = "nineteen";
                break;
            }
        } else {
            switch (num / 10) {
            case 2:
                str = "twenty";
                break;
            case 3:
                str = "thirty";
                break;
            case 4:
                str = "fourty";
                break;
            case 5:
                str = "fifty";
                break;
            case 6:
                str = "sixty";
                break;
            case 7:
                str = "seventy";
                break;
            case 8:
                str = "eighty";
                break;
            case 9:
                str = "ninety";
                break;
            }

            int lastDigit = num % 10;

            if (lastDigit != 0) {
                str += " " + readSingleDigit(lastDigit);
            }
        }

        return str;
    }

    public static String readTripleDigit(int num) {
        String str = null;

        int firstDigit = num / 100;

        str = readSingleDigit(firstDigit);
        str += " hundred";

        int lastDigits = num % 100;
        if (lastDigits != 0) {
            str += " and ";

            if (lastDigits < 10) {
                str += readSingleDigit(lastDigits);
            } else {
                str += readDoubleDigit(lastDigits);
            }
        }

        return str;
    }
}