code: @Override
      public Double getDoubleOrDie(String key) {
          Double value = getDouble(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 used in the configuration file.
  @return the Double or a RuntimeException will be thrown.