1. UML for design: Class Diagrams ENGR 110 #13 2016

2. UML What is the Unified Modeling Language?
A diagram language for specifying/describing many different aspects of a system.
Helps in the process of design and implementation.
Targeted to software systems, but many aspects are useful for other kinds of engineered systems also.
Includes diagrams for Finite State Machines
More expressive (and therefore complicated) than the diagrams we used.
We will see five parts of UML:
Object diagrams
Class diagrams
Use cases diagrams
Sequence diagrams
Activity diagrams
For designing Object Oriented programmes
For specifying the required behaviour of a system.
All relevant beyond software systems.
Part of the requirements analysis phase

3. A modelling language What is the value of these diagrams?
Why is it a “language”?
Why is it important to draw the diagrams this way?
A language is shared – makes communication easier
A language helps you think EW Go EW
Stopping NS Go
carEW
→ NSamb
→ setTimer(3)
carNS
→ EWamb
→ setTimer(3)
timer
→ EWred
→ NSgrn NS
Stopping
timer
→ NSred
→ EWgrn

4. A modelling language What makes a Language: Simple view: Vocabulary
Grammar Rules
Deeper view:
“syntax” – rules about how to say/write/draw statements.
most obvious aspect; least important aspect.
“semantics” – how to interpret statements.
critical for being precise and for communicating
“ontology” – identifies important set of concepts, ideas, relationships
makes distinctions that help you think

5. Language of FSM Diagrams
Ontology:
there are states and transitions  distinction
there are conditions for taking a transition
there are actions
Syntax:
labelled circles, labelled arrows
Semantics:
how to interpret them EW Go EW
Stopping NS Go
carEW
→ NSamb
→ setTimer(3)
carNS
→ EWamb
→ setTimer(3)
timer
→ EWred
→ NSgrn NS
Stopping
timer
→ NSred
→ EWgrn

6. Designing programs
Not a good idea to jump into coding a program without thinking first about the design of the program.
COMP 102/112 didn’t talk much about design!
we gave you the design of the programs – you just had to turn the design into code
why?
What does the design of a software system look like?
How the system is broken down into components
What the components do.
COMP102 programs: classes, methods

7. Modularity Complexity Modularity
Central problem in most engineering tasks:
Complexity
Key technique to handle it:
Modularity
Break the task up into chunks
chunks should be as independent as possible
solve/design/build each chunk separately
assemble them together
Used in all branches of engineering

8. OO Design and Class Diagrams
Key principle in Object Oriented design for software systems:
Primary Modularity = objects and classes
Given a task, need to identify
the different objects and classes that will make up the system
what is in the objects and what they do
how they are related and how they interact

9. OO Design and Class Diagrams
How?
UML provides a language for this design step:
object diagrams
class diagrams  most common and most important UML diagram for design
Ontological distinctions:
objects
attributes of objects
associations/relationships between objects
behaviour of objects
classes of objects
Start by identifying objects.
Work out what we need to know about them.

10. The Frog game
The FrogGame class is a simple game that lets a player try to hop their frogs to the other side of a three-lane road without being hit by the cars driving along the road.  Frogs start just below the road and have to hop to the far side (just past lane 4). Cars drive along the road at different speeds and with different sized gaps. The player has only one frog at a time, and can make it hop forward or back (one lane at a time). If the player gets a frog to the far side of the road, they gain 10 points, the frog hops off into the field, and the player gets another frog. When a frog is hit by a car, it dies, and the player gets a new frog if they have any lives left. The game keeps track of how many lives the player has left (initially 5) and the current score. The game ends when the player has run out of lives.

11. Object Diagram for FrogGame
name, type,
What are the objects in the world:
frog: Frog
car1: Car
car3: Car
car2: Car
nearSide: Lane
lane3: Lane
lane3: Lane
lane2: Lane
lane1: Lane
lane2: Lane
farSide: Lane

12. Object Diagram: attributes
name and type,
attributes and values
What are their attributes?
frog: Frog
lane = nearSide
status = alive
car1: Car
lane = 3
direction = east
position = 350
car3: Car
lane = 1
direction = east
position = 511
car2: Car
lane = 2
direction = west
position = 230
nearSide: Lane
lane3: Lane
lane1: Lane
lane2: Lane
farSide: Lane

13. Object Diagram for FrogGame
What are the associations between objects:
frog: Frog
status = alive
car1: Car
direction = east
position = 350
car2: Car
direction = west
position = 230
car3: Car
direction = east
position = 511
lane
lane
lane
lane
nearSide: Lane
lane3: Lane
lane1: Lane
lane2: Lane
farSide: Lane

14. Object diagrams What is the ontology of Object diagrams? Objects:
entities in the world – frogs, buildings, books, people
events – enrolment, return, transaction
intangible entities – courses, companies, images
roles – client, manager, member, surgeon
entities that can do stuff – searcher, sorter,
components – windows, buttons ….
Attributes of objects
information/values describing the objects (other than other objects)
Associations (relationships) between objects

15. Object Diagram
What happens when the problem gets bigger? Gets very messy!!!
car9: Car
lane: 1
direction: east
position: 20
frog1: Frog
lane: 0
status: alive
car2: Car
lane: 2
direction: west
position: 230
car1: Car
lane: 3
direction: east
position: 350
frog2: Frog
lane: 3
status: dead
car8: Car
lane: 2
direction: west
position: 430
car4: Car
lane: 2
direction: west
position: 280
car5: Car
lane:1
direction: east
position: 260
car6: Car
lane: 3
direction: east
position: 30
car3: Car
lane: 1
direction: east
position: 511
car10: Car
lane: 2
direction: west
position: 120
car7: Car
lane: 3
direction: east
position: 300
frog3: Frog
lane: 2
status: dead
lane3: Lane
lane1: Lane
lane2: Lane

16. Class diagrams More useful for real design
Abstract from individual objects to classes of objects.
What is the ontology of Class diagrams?
Classes:
categories of objects all of the same type, with the same behaviour
Attributes of the objects in the classes
Associations (relationships) between objects in the classes
Actions/behaviour of objects in the classes
Frog
status: boolean
lane

17. Class Diagrams
Describe the categories of the objects, rather than the objects themselves:
Notes
Associations can have additional information on them
Behaviour specifies the actions that can be done on the objects
direction: string
position: integer
colour: Color
speed: float
Car
class name,
attributes
incl associations
behaviour
Frog
status: boolean
lane
lane
Lane

18. Designing Class Diagrams
How do you work out what classes there should be?
No mechanical algorithm: it is a design issue
Starting point: the nouns/noun phrases in the specification
eg, the initial text description, the use cases, etc.
note: this is not enough:
Some nouns don’t need to be classes
Some classes aren’t explicitly mentioned in the specification.

19. Group Project sign-up system
System should allow students to sign up for group projects for their courses.
Each course will have one or more group projects, and a number of projects per student.
Each project will have a task, a lab place, a day, a maximum size, and a leader.
A course administrator needs to be able to set up the projects for their course.
Students should be able to specify the projects that they would like to do, and a preference order.
When sufficient students have entered their preferences, the system should show which projects students are likely to be assigned to, along with the likelihood.
The course administrator can “commit” to an allocation, at which point, the system will permanently allocate signed up students to projects.
Students should be able to withdraw from projects, and enter new preferences, but they won’t be actually assigned until the next time the administrator “commits”.
Administrators should be able to get lists of students in each project of their course.

20. What are the objects and classes?
Next lecture: techniques, and more details.