code: public final void setId(String id) {
          requireNonNull(id);
          if (MODEL.isLoggable(FINE)) {
              MODEL.fine("setting id to " + id); //$NON-NLS-1$
          }
          this.id = id;
      }
docs: Assigns a new ID to the entry. IDs do not have to be unique. If several
  entries share the same ID it means that they are representing the same
  "real world" entry. An entry spanning multiple days will be shown via
  several entries in the month view. Clicking on one of them will select
  all of them as they all represent the same thing.
  
  @param id the new ID of the entry