code: public List getCustomTrackingParameter(String key){
          if (key == null){
              throw new NullPointerException("Key cannot be null.");
          }
          List l = customTrackingParameters.get(key);
          if (l == null){
              return new ArrayList(0);
          }
          return new ArrayList(l);
      }
docs: Gets the list of objects currently stored at the specified custom tracking
  parameter.  An empty list will be returned if there are no objects set at
  that key.
  @param key the key of the parameter whose list of objects to get.  Cannot be null
  @return the list of objects currently stored at the specified key