The statechart diagrams and activity diagram are related in a sense that statechart diagram refers on object undergoing a transition process and an activity diagram refers on the flow of activities added in a single transition process.
In domain modeling it is imperative that the diagram give which object (or class) is responsible for each activity. Activity diagrams may be broken into object swimlanes that determine which object is responsible for which activity. The swimlanes are very useful because they collaborate the activity diagram's depiction of logic with the communication diagram's depiction of responsibility. A single transition takes off each activity, connecting to the next activity. A transition may fork or join.