DigitMapper.java

public class DigitMapper {

  public DigitMapper() {

  }

  public String mapToWord(int digit) throws DigitRangeException {
    if (digit < 0 || digit > 9) throw new DigitRangeException();

    switch (digit) {
      case 0:
        return "Zero";
      case 1:
        return "One";
      // case 2,3,...9
      default:
        return "";
    }
  }
}