ReverseT9.java

public class ReverseT9 {

    public static void main(String x) {
        ReverseT9 phone = new ReverseT9();
        System.out.println(phone.reverse_t9(x));
    }

    char getCharacter(char key, int index) {
        char[][] characterMapping = new char[][] {
                {'+'},
                {' '},
                {'A', 'B', 'C'},
                {'D', 'E', 'F'},
                {'G', 'H', 'I'},
                {'J', 'K', 'L'},
                {'M', 'N', 'O'},
                {'P', 'Q', 'R', 'S'},
                {'T', 'U', 'V'},
                {'W', 'X', 'Y', 'Z'},
            };
        char[] selected = characterMapping[Integer.parseInt(String.valueOf(key))];
        return selected[index % selected.length];
    }

    String reverse_t9(String keys) {
        int len = keys.length(), count = 0;
        char currentChar = ' ', key;
        String result = "";
        for (int i=0; i<len; i++) {
            key = keys.charAt(i);
            if (currentChar != key) {
                if (currentChar != ' ') {
                    result += String.valueOf(getCharacter(currentChar, count));
                }
                count = 0;
                currentChar = key;
            } else {
                count++;
            }
        }
        result += String.valueOf(getCharacter(currentChar, count));
        return result;
    }

}