State about the Activity Diagram

Activity diagrams show flow from one activity to other. It is an ongoing non atomic execution within a state machine. Activity finally results in some action that is made up of executable atomic computations which result in a change in state of system, or return of a value. It consists of activity states, action states, objects and transition states. The activity diagram for encryption of a message send via e-mail is shown as:

Figure: Activity diagram for E-mail encryption