code: public boolean addActor(String objectType, String displayName, String url, String[] description)
      {
          Map<String, Object> container = new HashMap<String, Object>();
      
          if (objectType != null)
          {
              container.put("objectType", objectType);
          }
          else
          {
              return false;
          }
          if (displayName != null)
          {
              container.put("displayName", displayName);
          }
          if (url != null)
          {
              container.put("url", url);
          }
          if (description != null && description.length > 0)
          {
              container.put("description", description);
          }
          
          return addChild("actor", container, null);
      }
docs: Add an actor object to this activity
  
  @param objectType The type of actor (required)
  @param displayName Name of the actor
  @param url URL of a page representing the actor
  @param description Array of descriptiosn of this actor
  @return True if added, false if not (due to missing required fields)