Telephone.java

public class Telephone {
    private final String number;
    private final Map<String, String[]> digitMap = new HashMap<>();

    public Telephone(String number) {
        this.number = number;

        digitMap.put("2", new String[]{"A", "B", "C"});
        digitMap.put("3", new String[]{"D", "E", "F"});
        digitMap.put("4", new String[]{"G", "H", "I"});
        digitMap.put("5", new String[]{"J", "K", "L"});
        digitMap.put("6", new String[]{"M", "N", "O"});
        digitMap.put("7", new String[]{"P", "Q", "R", "S"});
        digitMap.put("8", new String[]{"T", "U", "V"});
        digitMap.put("9", new String[]{"W", "X", "Y", "Z"});
    }

    public List<String> getPossibilities() {
        List<String> phoneWords = new ArrayList<>();
        recursePhoneWords(this.number, 0, "", phoneWords);
        return phoneWords;
    }

    public void recursePhoneWords(String number, int position, String currentWord, List<String> currentPhoneWords) {
        if (currentWord.length() == number.length()) {
            currentPhoneWords.add(currentWord);
            return;
        }

        String currChar = String.valueOf(number.charAt(position));
        Integer currInt = asIntOrNull(currChar);
        boolean isMappable = digitMap.containsKey(currChar);

        if (currInt == null || !isMappable) {
            // it is either not an integer, or it is not mappable
            // we just keep whatever it is and move along the number
            recursePhoneWords(number, position + 1, currentWord + currChar, currentPhoneWords);
            return;
        }

        String[] possibleLetters = digitMap.get(currChar);
        recursePhoneWords(number, position + 1, currentWord + possibleLetters[0], currentPhoneWords);
        recursePhoneWords(number, position + 1, currentWord + possibleLetters[1], currentPhoneWords);
        recursePhoneWords(number, position + 1, currentWord + possibleLetters[2], currentPhoneWords);
        if (possibleLetters.length > 3) {
            recursePhoneWords(number, position + 1, currentWord + possibleLetters[3], currentPhoneWords);
        }
    }

    private Integer asIntOrNull(String input) {
        try {
            return Integer.parseInt(input);
        } catch (NumberFormatException ignore) {
            return null;
        }
    }
}