Creating and Deleting Participants

Sequence diagrams show some extra notation for creating and deleting participants (Figure 4.3). To create a participant, you draw the message arrow directly into the participant box. A message name is optional here if you are using a constructor, but I usually mark it with "new" in any case. If the participant immediately does something once it's created, such as the query command, you start an activation right after the participant box.

Deletion of a participant is indicated by big X. A message arrow going into the X indicates one participant explicitly deleting another; an X at the end of a lifeline shows a participant deleting itself.

In a garbage-collected environment, you don't delete objects directly, but it's still worth using the X to indicate when an object is no longer needed and is ready to be collected. It's also appropriate for close operations, indicating that the object isn't usable any more.