Java Program to Convert Number to Word

public class NumberToWord {
    static void convert(char[] num) {
        int len = num.length;
        if (len == 0) {
            System.out.println("Empty string");
            return;
        }
        if (len > 4) {
            System.out.println("Length more than 4 is not supported");
            return;
        }

        String[] single_digits = new String[]{"zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine"};
        String[] two_digits = new String[]{"", "ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen"};
        String[] tens_multiple = new String[]{"", "", "twenty", "thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety"};
        String[] tens_power = new String[]{"hundred", "thousand"};

        System.out.print(String.valueOf(num) + ": ");

        if (len == 1) {
            System.out.println(single_digits[num[0] - '0']);
            return;
        }

        int x = 0;
        while (x < num.length) {
            if (len >= 3) {
                if (num[x] - '0' != 0) {
                    System.out.print(single_digits[num[x] - '0'] + " ");
                    System.out.print(tens_power[len - 3] + " ");
                }
                --len;
            } else {
                if (num[x] - '0' == 1) {
                    int sum = num[x] - '0' + num[x + 1] - '0';
                    System.out.println(two_digits[sum]);
                    return;
                } else if (num[x] - '0' == 2 && num[x + 1] - '0' == 0) {
                    System.out.println("twenty");
                    return;
                } else {
                    int i = (num[x] - '0');
                    if (i > 0)
                        System.out.print(tens_multiple[i] + " ");
                    else
                        System.out.print("");
                    ++x;
                    if (num[x] - '0' != 0)
                        System.out.println(single_digits[num[x] - '0']);
                }
            }
            ++x;
        }
    }

    public static void main(String[] args) {
        convert("1121".toCharArray());
    }
}

1121: one thousand one hundred twenty one