1. CompSci 280 S2 2107 Introduction to Software Development
Class Diagrams

2. Today’s Agenda Topics: Reading: Introduction Class Diagrams
Relationships
Associations
Generalization
Aggregation
Class Diagram Examples
How to build a class diagram
Reading:
Software Engineering 10th Edition, Ian Somerville
Chapter 5
Fowler, Martin, UML distilled : a brief guide to the standard object modeling language, Third Edition, Boston : Addison-Wesley, c2004 – Chapter 9
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3. 1.Introduction Structural models
Structural models of software display the organization of a system in terms of the components that make up that system and their relationships.
Structural models may be static models, which show the structure of the system design.
Class diagrams are used when developing an object-oriented system model to show the classes in a system and the associations between these classes.
An object class can be thought of as a general definition of one kind of system object.
An association is a link between classes that indicates that there is some relationship between these classes.
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4. 2.Class diagrams Steps: Identifying Classes Specifying Attributes
Identify classes by examining the problem statement or by performing a “Grammatical Parse” on the use-cases
UML class notation is a rectangle divided into three parts: class name, attributes, and operations.
write <<interface>> on top of interfaces' names
use italics for an abstract class name
Specifying Attributes
should include all fields of the object
Defining operations
Super …
Generalisation
Attribute1: type
Sub1
Operation1()
Operation2(): type
Sub2
Pieces 1
0..*
Associated
Association
Aggregation
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5. 2.Class diagrams Classes
A description of a group of objects all with similar roles in the system
Structural features define what objects of the class know
Behavioral features define what objects of the class can do
Identifying object classes is often a difficult part of object oriented design.
There is no 'magic formula' for object identification. It relies on the skill, experience and domain knowledge of system designers.
Object identification is an iterative process. You are unlikely to get it right first time.
Approaches
Use a grammatical approach based on a natural language description of the system.
Base the identification on tangible things in the application domain.
Use a behavioural approach and identify objects based on what participates in what behaviour.
Use a scenario-based analysis. The objects, attributes and methods in each scenario are identified.
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6. 2.Class diagrams Specifying Attributes
Attributes (fields/instance variables) are the set of data objects that fully define the class within the context of the problem.
i.e. represent the state of an object of the class
To develop attributes for class, can study use-case and select those “things” that reasonably “Belong” to the class.
Static members are underlined. Instance members are not.
Access specifiers appear in front of each member.
Private (-): can be used only by the owning class
Protected (#): can be used by the owing class and descendents of the class
Public (+): can be used by any other class
Package/default (~)
Default value: the value for a newly created object if the attribute isn't specified during creation
#ID: int
#Name: String
#Password: String
Person
<access specifier> <name> : <type> <multiplicity> = <default>
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7. 2.Class diagrams Defining operations
Operations define the behavior of an object.
Operations are descriptions of behavioral or dynamic features of a class
Four broad categories:
Operations that manipulate data in some way, perform a computation, inquire about the state of an object, monitor an object for the occurrence of the controlling event.
To derive a set of operations, study a use-case( or narrative) and select those operations that reasonably belongs.
The parameter list shows each parameter type preceded by a colon.
Access specifiers appear in front of each member.
Return-type: the type of the returned value
<access specifier> <name> ( <parameter list>) : <return type>
-date: String
-returned: boolean …
Rental
+rent(customerID, videoID): boolean
+return(customerID, videoID): boolean
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8. 2.Class diagrams Examples
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9. 3.Relationship: Association
A relationship between instances of the two classes. There is an association between two classes if an instance of one class must know about the other in order to perform its work.
e.g. Passing messages and receiving responses
In a diagram, an association is a solid line connecting two classes.
A navigability arrow on an association shows which direction the association can be traversed or queried.
Multiplicity of Associations
Specifies how many instances of one class may relate to a single association
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10. 3.Association Multiplicity
Multiplicity specifies how many objects an instance of the class at the other end of the association can be linked to.
1..*: 1-to-many
e.g A RectangleList has many rectangles
1-to-1
e.g. each student must carry exactly one ID card
0..1
zero or one
0..* / *
Zero or more 1
exactly one
1..*
One or more
2..4
Specified range
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11. 3.Association Connecting the Classes: Associations
Note:
Connect classes with a line
Specify multiplicities on each side of association: min..max (min and max of connected instances on each side)
Use roles to make the model more self-explanatory
A lecturer teaches an arbitrary number of courses (possibly none)
1..*
0..*
0..* 1
coordinator
participant
A course is taken by one or more students (the participants)
A student can take an arbitrarily number of courses (possibly none)
A course has exactly one coordinator
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12. 3.Association Classes and associations in the MHC-PMS
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13. 4.Relationships: Generalization
Generalization: an inheritance link indicating one class is a superclass (the more general) of the child class/subclass (the more specific element)
i.e. is-a relationship
An inheritance link indicating one class is a superclass of the other.
A generalization has a triangle pointing to the superclass.
Checking Generalizations
If class A is a generalization of a class B, then “Every B is an A”
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14. 4.Generalization Reusing Classes
“Is-a”-relation (inheritance)
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15. 4.Generalization A generalization hierarchy
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16. 4.Generalization Generalization
In modeling systems, it is often useful to examine the classes in a system to see if there is scope for generalization. If changes are proposed, then you do not have to look at all classes in the system to see if they are affected by the change.
In object-oriented languages, such as Java, generalization is implemented using the class inheritance mechanisms built into the language.
In a generalization, the attributes and operations associated with higher-level classes are also associated with the lower-level classes.
The lower-level classes are subclasses inherit the attributes and operations from their superclasses. These lower-level classes then add more specific attributes and operations.
Advantages:
Treating similar objects similarly
Reuse of commonalities of classes
Better maintenance
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17. 5.Relationships: Aggregation
An aggregation model shows how classes that are collections are composed of other classes.
Aggregation models are similar to the part-of relationship in semantic data models. (i.e. has-a relationship)
An aggregation has a diamond end pointing to the part containing the whole.
Normal Aggregation:
Represented by a line with a hollow diamond
Composition (strong aggregation), a Part cannot exist once Whole is destroyed
Represented by a line with a filled black diamond
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18. 5.Aggregation Example Composition Vs Aggregation
If we remove the MovieTheater, we also move the box office (composition), but movies may still exist (aggregation)
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19. 6.Class Diagram Example A customer can have any number of orders
-date: String
-status: String
Order
+calcTotal()
+calcTotalWeight
-address: String
-phone: String
Customer 1
0..*
An order has a collection of OrderDetails
An order belongs to one customer 1
Aggregation
1..*
-quantity: int
-taxStatus: String
OrderDetail
+calcSubTotal()
+calcWeight()
-shippingWeight: double
-description: String …
Item
+getWeight()
+getPriceForQuantity 1
0..*
An OrderDetail can be queried about its Item, but not the other way around.
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20. 6.Class Diagram Example
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21. 7.How to build a class diagram
Design is driven by criterion of completeness either of data or responsibility
Data Driven Design identifies all the data and see it is covered by some collection of objects of the classes of the system
Responsibility Driven Design identifies all the responsibilities of the system and see they are covered by a collection of objects of the classes of the system
Noun identification
Identify noun phrases: look at the use cases and identify a noun phrase. Do this systematically and do not eliminate possibilities
Eliminate inappropriate candidates: those which are redundant, vague, outside system scope, an attribute of the system, etc.
Validate the model…
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22. Exercise: ATM Machine Identifying Classes
So far: we know basic requirements for system.
Now want to specify basics of how system works:
identify the data & functions for a system
group these into appropriate objects
determine object inter-relationships: both “data” relationships & “behavioural” relationships
In full system development, also develop set of “non-functional requirements”
Steps:
work from data/functions in English requirements or use case definitions
group them into objects (often takes several goes!!!)
Common approach: nouns = attributes/objects, verbs = functions...
I find it easiest to work from data/functions in English requirements or use case definitions
group them into objects (often takes several goes!!!)
find focusing on data aspects of objects (“entities”) then adding functions works best
Common approach: nouns = attributes/objects, verbs = functions...
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23. Step 1: Identify the data & functions for a system from use cases:
Grouped closely-related attributes -> entities
Grouped functions by data they act on
Complete the following:
Tips:
DON’T get carried away with objects for user/device interfaces, databases, data structures, etc. - these are DESIGN, not analysis
We want to capture the “essence” of the system
i.e. important data/functions
Customer
Account
ATM
Transaction
Bank
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24. Step 2: Determine inter-object relationships:
Draw UML Class Diagrams for system structure
Tips: Now: how are the objects related???
Three main ways:
1. generalisation e.g. staff -> person
2. association e.g. customer -> account
3. aggregation e.g. account -> transaction
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25. Step 3: Multiplicity Association #1 (“data”):
customer has 1 or 2 accounts
account has bunch of transactions
each transaction is made from 1 account
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