Phone keypad implementation

I was wondering, beyond the standard improvements that this needs, is there a more "clever" way of solving this problem?

The problem is to translate letters into digits without changing the format of the original string. The conversion is based on the international standard letter/number mapping on phones (i.e. where ABC is 2, DEF is 3, ..., PQRS is 7, TUV is 8, and WXYZ is 9).

public class Keypad {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        System.out.print("Enter a string: ");
        String s = input.nextLine();

        System.out.println(getNumbers(s));
    }

    public static String getNumbers(String s) {
        String result = new String();

        //Read and append s onto result
        for (int i = 0; i < s.length(); i++) {
            if (Character.isLetter(s.charAt(i))) {
            result += getNumber(Character.toUpperCase(s.charAt(i)));
            }
            else {
                result += s.charAt(i);
            }
        }
        return result;
    }

    public static int getNumber(char upperCaseLetter) {
        int number = ((upperCaseLetter - 'A') / 3) + 2;
        if (number < 7) {
            return number;
        }
        else if (upperCaseLetter - 'A' < 20) {
            return 7;
        }
        else if (upperCaseLetter - 'A' < 23) {
            return 8;
        }
        else {
            return 9;
        }
    }
}

A much simpler solution would be to use a lookup table. A mapping implemented using a lookup table would be easier to understand, with no magic numbers.

Furthermore, phone numbers are more numeric strings than numbers, so I'd make it a char-to-char lookup.

Repeated string concatenation is bad for performance. To compose a string, use a StringBuilder.

public class Keypad {
    private static final char[] DIGITS = (
                 // ABC     DEF
                   "222" + "333" +
        // GHI      JKL     MNO
          "444"  + "555" + "666" +
        // PQRS     TUV     WXYZ
          "7777" + "888" + "9999").toCharArray();

    public static String getNumbers(CharSequence s) {
        StringBuilder result = new StringBuilder(s.length());
        for (int i = 0; i < s.length(); i++) {
            char c = Character.toUpperCase(s.charAt(i));
            if ('A' <= c && c <= 'Z') {
                result.append(DIGITS[c - 'A']);
            }
        }
        return result.toString();
    }
}

StackExchange Code Review Q#90079, answer score: 4