code: @Override
      public String getOrDie(String key) {
          String value = get(key);
          if (value == null) {
              throw new IllegalArgumentException(String.format(ERROR_KEYNOTFOUND, key));
          } else {
              return value;
          }
      }
docs: The "die" method forces this key to be set. Otherwise a runtime exception
  will be thrown.
  
  @param key the key
  @return the String or a IllegalArgumentException will be thrown.