code: protected final <T> String getString(final IKey<T> key) {
          final Object value = config.get(key.key());
  
          return String.valueOf(value != null ? value : key.getDefaultValue());
      }
docs: returns the value as string according to given key. If no value is set, the
  default value will be returned.
  
  @param key The key to read.
  @return the value as string.