Representing dynamic behavior in OOA

Dynamic behaviour of a system means how objects interact, how their state changes over time, and how events and flows of control happen when the system runs. In OOA it is represented with UML behavioural diagrams, each showing a different aspect.

1. Use case diagram and descriptions

The functional behaviour from the user's point of view: actors and their goals, with a step by step scenario of each use case.

2. System sequence diagram (SSD) and sequence diagram

The events between actor and system (SSD), or the messages among objects in time order.

 Customer         :ATM
    | insertCard()   |
    |--------------->|
    | enterPIN(pin)  |
    |--------------->|
    |<-- accepted ---|

3. Collaboration (communication) diagram

The same interaction, drawn by the links among objects, with numbered messages.

 :Customer --1:enterPIN()--> :ATM --2:verify()--> :Bank

4. State machine diagram

The life cycle of one object, its states, events and transitions.

 (*)-->[Idle]--insertCard-->[Verify]--ok-->[Active]
          ^                                   |
          +---------- ejectCard --------------+

5. Activity diagram

The flow of work with decisions, loops and concurrency (fork, join), also for business processes with swimlanes.

6. Operation contracts

Define the pre-conditions and post-conditions (state changes) of system operations.

Example: Library book issue - the use case describes the issue steps; the SSD shows issueBook(memberId, bookId); the state diagram shows Book going from Available to Issued to Overdue and back; the activity diagram shows the checks (member valid? book free?). Together these models describe how the system behaves in time.