code: public ComponentList<T> getComponents(final String propertyValue) {
          ComponentList<T> components = index.get(propertyValue);
          if (components == null) {
              components = EMPTY_LIST;
          }
          return components;
      }
docs: Returns a list of components containing a property with the
  specified value.
  @param propertyValue the value of the property contained in the
  returned components
  @return a component list