code: public Set<String> getAttributesFor(String activity) throws CompatibilityException {
                  validateActivity(activity);
                  if (activityDataUsage.get(activity) == null) {
                          return new HashSet<>(); //No input data elements for this activity
                  }
                  return Collections.unmodifiableSet(activityDataUsage.get(activity).keySet());
          }
docs: Returns all attributes of the given activity.<br>
  If the given attribute has no attributes, the returned set contains
  no elements. The given activity has to be known by the context, i.e.
  be contained in the activity list.
  
  @param activity Activity for which the attributes are requested.
  @return All attributes of the given activity.
  @throws ParameterException
  @throws CompatibilityException
  @throws IllegalArgumentException IllegalArgumentException If the
  given activity is not known.
  @see #getActivities()