code: private Set<String> getPersistedSet(@Nullable final Set<String> defaultValue) {
          if (!shouldPersist()) {
              return defaultValue;
          }
  
          return getPreferenceManager().getSharedPreferences().getStringSet(getKey(), defaultValue);
      }
docs: Loads and returns the currently persisted set, which belongs to the preference's key, from
  the shared preferences.
  
  @param defaultValue
  The default value, which should be returned, if no set with the preference's key is
  currently persisted, as an instance of the type {@link Set}
  @return The currently persisted set or the given default value as an instance of the type
  {@link Set}