Basic Class Diagrams in software engineering

Generalizable
Element
Namespace
Model
Element
name
visibility
isSpecification
Classifier
isRoot
Constraint
Body
Coming up: Introduction to
Objects
Objects, Classes, and
Basic Class Diagrams
CS/SWE 421
Introduction to Software Engineering
Dan Fleck
(Slides adapted from Dr. Stephen Clyde with permission)

Coming up: Introduction to Objects
Introduction to Objects
 Objects are the fundamental building blocks
of object-oriented systems
 What is an object?
– It represents any “thing”
– It has a boundary in space and time
– It is an abstraction
– It encapsulates state and/or behavior
– It has identity

Coming up: Exercise - Object Hunt
Introduction to Objects
 What aren’t objects?
– Events (sometimes)
– Relationships between objects (most of the
time)
– Behavior (most of the time)
– Constraints (most of the time)

Coming up: Introduction to Classes
Exercise - Object Hunt
 Part 1 - List examples of objects in the
Third National Bank Elevator System
 Part 2 - List some concepts from this
system that would not typically be modeled
as objects

Coming up: Three Perspectives
Introduction to Classes
 Classes are abstractions that allow us to deal with whole collections of objects that share some
commonalties
 Examples of classes in a Student Records Management System
Student
Course
Class
Section
Instructor
Class
Grade Major
Department
College
Semester

Coming up: Classes from different Perspectives
Three Perspectives
 Objects and classes, as well as all other
modeling components in UML, can be
interpreted from different perspectives:
 Three common perspectives:
– Analysis - description of the problem
domain
– Specification - logical description of
software system
– Implementation - description of software
components and their deployment

Coming up: Class Syntax
Classes from different Perspectives
 Meaning from three perspectives
– Analysis: sets of objects
– Specifications: interfaces to encapsulated
software representations of objects
– Implementations: abstract data types
Student
Analysis Specification
Student
{Joe, Sue, Mary,
Frank, Tim, …}
Interface Student
{…}
Implementation
class Student
{…}
Student

Coming up: Class Names
Class Syntax
 A box divided into
compartments
– Name
– Attributes
– Operations
– Responsibilities
– Used-defined
compartments
Student
major: String
gpa: Real
standing: String
add(Class Section)
drop(Class Section)
-- The set of students
known to the registration
system
-- An exception occurs if
gpa falls below 2.0

Coming up: Class Name Syntax
Class Names
 The name should be a noun or noun
phrase
 The name should be singular and
description of each object in the class
 The name should be meaningful from a
problem-domain perspective
– “Student” is better than “Student Data” or
“S-record” or any other implementation
driven name
 Avoid jargon in the names
 Try to make the name descriptive of the
class’s common properties

Coming up: Return to Objects – Object Syntax
Exercise – Class Identification
 Identify meaningful classes in the Elevator
System

Coming up: Object Name Syntax
Return to Objects – Object Syntax
 Object syntax is
similar to class syntax,
except
– the name identifies
specific or generic
object
– the name includes the
class that the object
belongs to
 Remember, individual
objects are instances
of classes
joe: Student
major: String = “CS”
gpa: Real = 4.0
standing: String = “”
add(Class Section)
drop(Class Section)

Coming up: Attributes from an Analysis Perspective
Attributes
 Attributes represent characteristics or
properties of objects
 They are place holders or slots that hold
values
 The values they hold are other objects
 The name of an attribute communicates its
meaning
 An attribute can be defined for individual
objects or classes of objects
– If defined for a class, then every object in
the class has that attribute (place holder)

Coming up: Attributes from a Specification Perspective
Attributes from an Analysis
Perspective
 An attribute relates an object to some other
object
 It has the same semantics as an association
joe: Student
name: String = “Joe Jones”
joe: Student Joe Jones : String
Is basically the
same as ...
name
1

Coming up: Attributes from an Implementation Perspective
Attributes from a Specification
Perspective
 An attribute represents an obligation to
provide or manage a certain (single) piece
of information
Student
major: String
gpa: Real
standing: String
 For example, each
Student object must
be able to
encapsulate a
major, a GPA, and
a standing

Coming up: Exercise – Attributes
Attribute Syntax
visibility: public “+”, protected “#”, or private “-”
name: capitalize first letter of each word that
makes up the name, except for the first
multiplicity: number, range, or sequence of
number or ranges.
type: build-in type or any user-defined class
initial-value: any constant and user-defined object
property-string: e.g, changeable, addOnly, frozen
[visibility] name [multiplicity] [:type] [=initial-value]
[{property-string}]

Coming up: Operations
Operations
– Analysis: Ways in which objects interaction
– Specification: An obligation to provide a
service
– Implementation: A function member, a
method

Coming up: Operation Syntax
Operations
Student
major: String
GPA: Real
standing: String
add(Class Section)
drop(Class Section)
Class Section
name: String
capacity: Integer
add(Student)
drop(Student)
checkPrerequisites(Students)
Prerequisite
<has
takes>
Course

Coming up: Type of Relationships in Class Diagrams
Operation Syntax
visibility: “+”, “#”, “-”
name: verb or verb phase, capitalize first letter
of every word, except first
parameter-list: coma separated list of
parameters
return-type: primitive type or user-defined type
property-string: isQuery, sequential, guarded,
concurrent
[visibility] name [(parameter-list)] [:return-
type] [{property-strong}]

Coming up: Associations
Type of Relationships in Class
Diagrams
Relation
A consolidated snippet of the UML Meta-model
Association
Generalization Dependency
Aggregation
Binary Association N-ary Association

Coming up: Associations
Associations
 An association is a structural relationship
that specifies that objects of class may be
connected to objects of another class
– Analysis: Links between objects
– Specification: Obligation to provide a
connection between objects
– Implementation: Object pointers,
references, linking structures, etc.

Coming up: Association Names
Associations
Student
Class
Section
Course
Semester
Instructor
Department
takes>
is registered for>
teaches>
sponsors>
<works
for
is
instance
of>
i
s
h
e
l
d
d
u
r
i
n
g
>

Coming up: Navigation
Association Names
 Associations may be named
– The names should communicate the
meaning of the links
– The names are typically verb phases
– The name should include an arrow
indicating the direction in which the name
should be read

Coming up: Navigation
Navigation
 The navigation of associations can be
– uni-directional
– bi-directional
– unspecified
Class
Section
Course
Instructor
Department
teaches>
sponsors>
<works
for
is
instance
of>

Coming up: Generalization
Generalization
 Generalization is another kind of
relationship in UML – see Meta Model
 From an analysis perspective, it is a pure
generalization/specialization concept, i.e.,
the specialization is a subset of the
generalization
Student
Person
Graduate
Student

Coming up: Generalization
Generalization
 From a specification/implementation
perspective, generalization can represent sub-
typing, inheritance, type capability, and
substitutability (depends on the language)
Student
major: String
GPA: Real
standing: String
add(Class Section)
drop(Class Section)
Person
name: String
address: String
changeAddress(new_address)

Coming up: Class Diagrams
Exercise – Simple Associations
 From an analysis perspective:
– Identify meaningful associations and
generalization/specializations among
classes in the Elevator System

Coming up: Class Diagrams
Class Diagrams
 Class Diagrams describe
– the types of objects in a system
– their properties (attributes and operations)
– relationships between objects
 They can also include
– Grouping concepts like packages
– Constraints
– Various kinds of annotations

Coming up: Multiplicity Constraints
Multiplicity Constraints
Student
Class
Section
Course
Semester
Instructor
Department
takes>
is registered for>
teaches>
sponsors>
<works
for
is
instance
of>
i
s
h
e
l
d
d
u
r
i
n
g
>
1..*
1
1..*
1..*
1
1
1..*
0..8
0..*
0..6
1..3

Questions
 From the previous diagram
– How many classes can a student take?
– Do you have to be registered in any classes
to be a student?
– Do I need to teach this class to be an
Instructor? Do I need to teach ANY
classes?

Coming up: Dependencies
Multiplicity Constraints
 A multiplicity constraint can be
– a single number
– a “*”, meaning an arbitrarily large number or
simply “many”
– a range, denoted by “min..max”
– a sequence of single numbers and ranges

Dependencies
Relation
Association
Aggregation

Dependencies
 A dependency is a type of relationship
 It says that one modeling component “uses”
another.
 If the later changes then, the former may
have to change as well
Student
add(Course)
drop(Course)
Prerequisite

Dependencies
– Analysis: physical or logical dependency
between the connected classes
– Specification: a “uses” relationship at an
interface level
– Implementation: a “uses” relationship at an
implementation level.
Just think: uses!

Coming up: Aggregations (is part of)
Dependencies
 Syntax:
– a dashed link with an straight-line
arrowhead point to a component on which
there is a dependency
 Dependencies can be defined among:
classes, notes, packages, and other types
of components
 Can dependencies go both ways?
 Any problems with having lots of
dependencies?

Coming up: Aggregation
Aggregations (is part of)
Relation
Association
Aggregation

Coming up: Composition (very similar to aggregation)
Aggregation
 Aggregation: is a special kind of association
that means “part of”
 Aggregations should focus on single type of
composition (physical, organization, etc.)
1 1
*
4..*
1
1
1 1
1..3 1
0..9 1
Pizza Order
Slice
Crust
Sauce Serving
Cheese Serving
Topping Serving

Coming up: Using a class diagram
Composition (very similar to aggregation)
 Think of composition as a stronger form of
aggregation. Composition means
something is a part of the whole, but cannot
survive on it’s own.
Building
Room

Lets look at
BookstoreExample4.jpg
 Does John McCain (who has 7 houses) have
a problem using this system?
 If Barack Obama decides to create a Federal
sales tax, how would we change the system?
 Why is there a display method in Item, Book,
MusicCD and Software?
 An ItemOrder is part of how many Orders?
 Can you explain how a search works using
this diagram?

Class Exercise
 Lets create the WeGrow class diagram

Coming up: Questions
Validating a class diagram
 One of the most important, and often
overlooked issues is how to validate a class
diagram. (Usually best for an diagram at the
implementation perspective)
 Given a specification or a use-case, can you
look at the class diagram and use attributes
and methods to “execute” a use case?
 Lets try it for the WeGrow class diagram

Coming up: More Questions
Questions
 What’s the difference between an attribute and an
association with another class? For example, should
“grade” be an attribute or a link to another class called
“Grade”?
 When during the software engineering life cycle should
you build classes diagrams?

Coming up: More Questions
More Questions
 How do you know when a class diagram is
complete?
 How can you manage change control on all
the class diagrams for project?
 What do you do with class diagrams after a
system has been built and delivered?

Bonus Slide!
 If you’re interested in Auto-generating UML,
Netbeans has an option to do it.
– Install the UML plugin
– Right-click on a project
– Choose “Reverse Engineer”
– Go to the new UML project
– Select a package and choose to generate a
new UML diagram

Basic Class Diagrams in software engineering

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