1. www.monash.edu.au IMS1805 Systems Analysis Topic 3: Doing Analysis (continued from previous weeks)

2. www.monash.edu.au 2 Agenda Aim: To introduce modelling concepts which developed as part of O-O perspectives on systems analysis To give a basic introduction to: Use case diagrams Object class diagrams Sequence diagrams

3. www.monash.edu.au 3 1. Use Case Modelling Initially developed for O-O modelling, but is not inherently O-O in nature; is now used quite widely as part of many analytical approaches Used to identify actors (stakeholders) in a system, and how they interact with system processes Consist of: Use case diagrams Use case documentation See example given in class

4. www.monash.edu.au 4 Key elements of use case models: Actors Actors - people who use or interact with the system (like ‘stakeholders’ in soft systems terminology) ‘Actors’ can also extend to include non- human things which interface with the system - other systems, devices, etc Some possible variations in types of actors: Primary vs secondary Direct vs indirect Abstract vs concrete Actor definitions

5. www.monash.edu.au 5 Key elements of use case models: Use cases Use case: Describes some aspect of what a system does (compare with functions/ processes/actions) Logical, not physical (ie what is done, not how it is done) Also records which actors interact with the function/activity described by the use case Represent diagrammatically as ellipses; textually as table descriptions

6. www.monash.edu.au 6 Key elements of use case models: Associations Association: shows which actors/objects are involved in the function/activity described by the use case Triggers: events which cause an actor to become involved in a use case Can also show relationships between activities/processes between use cases ‘Extend’ relationship ‘Include’ relationship ‘Depends on’ relationship

7. www.monash.edu.au 7 How does a use case help systems analysis? Similar in general intent/purpose to a DFD Includes better descriptive detail about each process and how it is triggered, etc Includes better information about who is involved with system processes; helps highlight stakeholders in each process (use case)

8. www.monash.edu.au 8 2. Class Diagrams Developed as an essential element of object modelling Used to show the object classes and the relationships they have to each other in a system Can be thought of as the O-O equivalent of an entity-relationship diagram, but also include attribute and behaviour information See example shown in class

9. www.monash.edu.au 9 Key elements of a class diagram: object classes Class = a grouping of objects defined by certain specified characteristics [Note distinction between classes and objects (eg ‘Unit’ = class; ‘IMS1805’ = object; ‘Lecturer’ = class; ‘Martin Atchison’ = object)] Class notation: Name (similar rules as for entities in E-R) Attributes (similar rules as for entity attributes in E-R) Operations (behaviours the object can perform)

10. www.monash.edu.au 10 Key elements of a class diagram: Class relationships Inheritance: One object class is a particular type of another object class The lower-level class inherits the properties of its ‘parent’ Association: One object class uses another object class Usually 2-way; relationship is loose, in that each object class can exist without the other Aggregation: A special close form of association in which one object class contains or is made up of another object class Usually 1-way; relationship is ‘tight’ in that one object class would not exist without the other

11. www.monash.edu.au 11 Key elements of a class diagram: Class documentation Used to record the defining characteristics of an object class and some features of inter-object relationships Similar to the definitions of entities and their relationships in E-R modelling For example: Attribute visibility Relationship multiplicities

12. www.monash.edu.au 12 How does a class diagram help systems analysis? Similar strengths to a data model such as an E-R diagram Enables an analyst to capture and record everything about every object class in a system, and define inter-object relationships Direct connection to creation of class templates for coding in O-O languages Use full class diagram features when modelling at implementation end; use only basic entity class features when modelling at business analyst end

13. www.monash.edu.au 13 3. Sequence Diagrams Developed initially as a part of use case diagrams to show a ‘scenario’ - a single path of processing in a use case Then developed in their own right to show a system function as a sequence of actor or object behaviours triggered by messages coming from other actors and objects Focus on actor/object behaviours See example shown in class

14. www.monash.edu.au 14 Key elements of a sequence diagram: Actors/objects As for previous diagrams. Actors and objects are system components which have certain specified attributes and behaviours A sequence diagram aims to show behavioural interactions between actors and objects

15. www.monash.edu.au 15 Key elements of a class diagram: Operations and messages Every system function can be defined as a series of events in which actors/objects perform some of the operations which are defined by their behaviours Each operation in a system function is triggered by a message received from some other actor/object; in turn it initiates a message to another actor/object Sequence diagrams identify the operations and messages which are involved in a given system function

16. www.monash.edu.au 16 Key elements of a class diagram: Timelines A system function requires its component operations to be performed in a given sequence The sequence is driven by the exchange of messages between operations The timeline in a sequence diagram shows the order in which these messages are passed (Note contrast with DFD, which shows no information about sequence of processing)

17. www.monash.edu.au 17 Other possible elements of sequence diagrams Focus of control (execution occurrence) Lifelines Object creation and destruction

18. www.monash.edu.au 18 How does a sequence diagram help systems analysis? Helps analyst to see the a system function as a narrative - storyboard showing what happens and what causes it to happen Links operations to the actors/objects which perform them and the messages they generate

19. www.monash.edu.au 19 Cross-model analysis Note the consistent use of model elements across the different diagram types (contrast with data vs process models) Cross-checking for compatibility between models Further cross-connections with other UML model types

20. www.monash.edu.au 20 Summary Use case models, object class models and sequence models each offer different perspectives of systems Linked with O-O thinking, but not entirely O-O based Integrated through UML