Utils.java

public class Utils { 
   public static String mapDigitToWord(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 "";
    }
  }      
}