System Sequence Diagrams

Recap When to create SSD? How to identify classes/instances? Use case descriptions UML notations for SSD

Contents Interaction diagrams UML notation Examples

Interaction Diagrams Show how objects interact with one another UML supports two types of interaction diagrams  Sequence diagrams  Collaboration diagrams

Sequence Diagrams AKA Interaction Diagrams – Semantically equivalent to Collaboration Diagrams Dynamic Model relating use cases and class diagrams Illustrates how objects interacts with each other Shows time ordering of interactions Generally a set of messages between collaborating objects Ordering of objects not significant

Sequence Diagrams Show only one flow of control Can model simple sequential flow, branching, iteration, recursion and concurrency May need multiple diagrams – Primary – Variant – Exceptions

UML sequence diagrams sequence diagram: an "interaction diagram" that models a single scenario executing in the system – perhaps 2nd most used UML diagram (behind class diagram) relation of UML diagrams to other exercises: – CRC cards -> class diagram – use cases -> sequence diagrams 7

Key parts of a sequence diag. participant: an object or entity that acts in the sequence diagram – sequence diagram starts with an unattached "found message" arrow message: communication between participant objects the axes in a sequence diagram: – horizontal: which object/participant is acting – vertical: time (down -> forward in time) 8

Sequence Diagram (Basic) Object : Class or Actor Lifeline message name X Object Destruction/ Termination > Focus of Control/ Activation

Sequence Diagram (Basic) Student aClass: Class Register :Scheduler adjustRoom checkRooms

Sequence Diagrams (Advanced) Seq# [Guard] *[Iteration] Return-List := Operation-Name (Argument-List) recursion *[Iteration Condition] Conditional Lifeline {transient}

Sequence Diagram(make a phone call) CallerPhoneRecipient Picks up Dial tone Dial Ring notificationRing Picks up Hello

Sequence Diagrams – Object Life Spans Creation  Create message  Object life starts at that point Activation  Symbolized by rectangular stripes  Place on the lifeline where object is activated.  Rectangle also denotes when object is deactivated. Deletion  Placing an ‘X’ on lifeline  Object’s life ends at that point Activation bar A B Create X Deletion Return Lifeline

Sequence diag. from use case 14

Lifetime of objects creation: arrow with 'new' written above it – notice that an object created after the start of the scenario appears lower than the others deletion: an X at bottom of object's lifeline – Java doesn't explicitly delete objects; they fall out of scope and are garbage-collected 15

Representing objects squares with object type, optionally preceded by object name and colon – write object's name if it clarifies the diagram – object's "life line" represented by dashed vert. line 16

Messages between objects message (method call) indicated by horizontal arrow to other object – write message name and arguments above arrow 17

Messages between objects – dashed arrow back indicates return – different arrowheads for normal / concurrent (asynchronous) methods

Example

Summary SSD revision Loops Conditions in SSD