code: @SuppressWarnings("unchecked")
    public <T> T getAttribute(String key) {
      return (T) attrs.get(key);
    }
docs: Retrieve a specific attribute of this resource.
  
  @param key a string key associated with the value to be retrieved.
  @param <T> the type of the attribute to be retrieved.
  @return the actual value of the attribute, or null, if there the key was not found.