NumberToWords.java

private static final String[] LESS_THAN_20 = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
private static final String[] TENS = {"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
private static final String[] THOUSANDS = {"", "Thousand", "Million", "Billion"};

public String numberToWords(int n) {
    if(n == 0) return "Zero"

    StringBuilder sb = new StringBuilder();
    for(int i = 0; n > 0; n /= 1000, i++) {
        int modThousand = n % 1000;
        if(modThousand != 0) {
            sb.insert(0, " ");
            sb.insert(0, THOUSANDS[i]);
            sb.insert(0, " ");
            sb.insert(0, numberLessThanThousandToString(modThousand));
        } 
    }

    return sb.toString().trim();
}

private static String numberLessThanThousandToString(int n) {
    if(n == 0)
        return "";

    if(n < 20)
        return LESS_THAN_20[n];

    StringBuilder sb = new StringBuilder();

    if(n < 100) {
        sb.append(TENS[n/10]);
        sb.append(" ");
        sb.append(numberLessThanThousandToString(n%10));
    }
    else {
        sb.append(LESS_THAN_20[n/100]);
        sb.append(" Hundred ");
        sb.append(numberLessThanThousandToString(n%100));
    }

    return sb.toString().trim();
}