Objects, Classes, and
Basic Class Diagrams
Classifier
Generalizable
Element
isRoot
Model
Element
Namespace
name
visibility
isSpecification
CS/SWE 421
Introduction to Software Engineering
Dan Fleck
(Slides adapted from Dr. Stephen Clyde with permission)
Coming up: Introduction to
Objects
Constraint
Body
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: Introduction to Objects
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: Exercise - Object Hunt
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: Introduction to Classes
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
Course
Class
Section
Instructor
Semester
Class
Grade
Coming up: Three Perspectives
College
Student
Major
Department
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: Classes from different Perspectives
Classes from different Perspectives

Meaning from three perspectives
– Analysis: sets of objects
– Specifications: interfaces to encapsulated
software representations of objects
– Implementations: abstract data types
Analysis
Specification
Implementation
Student
Student
Student
Interface Student
{…}
class Student
{…}
{Joe, Sue, Mary,
Frank, Tim, …}
Coming up: Class Syntax
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 Names
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: Exercise – Class Identification
Exercise – Class Identification

Identify meaningful classes in the Elevator
System
Coming up: Return to Objects – Object 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
Coming up: Attributes
joe: Student
major: String = “CS”
gpa: Real = 4.0
standing: String = “”
add(Class Section)
drop(Class Section)
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 an Analysis 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
Coming up: Attributes from a Specification Perspective
name
1
Is basically the
same as ...
Joe Jones : String
Attributes from a Specification
Perspective

An attribute represents an obligation to
provide or manage a certain (single) piece
of information

For example, each
Student object must
be able to
encapsulate a
major, a GPA, and a
standing
Coming up: Attributes from an Implementation Perspective
Student
major: String
gpa: Real
standing: String
Attribute Syntax
[visibility] name [multiplicity] [:type] [=initial-value]
[{property-string}]
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
Coming up: Exercise – Attributes
Operations

Meaning from three perspectives
– Analysis: Ways in which objects interaction
– Specification: An obligation to provide a
service
– Implementation: A function member, a
method
Coming up: Operations
Operations
Student
major: String
GPA: Real
standing: String
add(Class Section)
drop(Class Section)
Course
Coming up: Operation Syntax
Class Section
takes>
name: String
capacity: Integer
add(Student)
drop(Student)
checkPrerequisites(Students)
Prerequisite
Operation Syntax
[visibility] name [(parameter-list)] [:returntype] [{property-strong}]
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
Coming up: Type of Relationships in Class Diagrams
Type of Relationships in Class
Diagrams
A consolidated snippet of the UML Meta-model
Relation
Generalization
Association
Binary Association
Coming up: Associations
Aggregation
Dependency
N-ary Association
Associations


An association is a structural relationship
that specifies that objects of class may be
connected to objects of another class
Meaning from three perspectives
– Analysis: Links between objects
– Specification: Obligation to provide a
connection between objects
– Implementation: Object pointers,
references, linking structures, etc.
Coming up: Associations
Associations
is registered for>
Student
<works for
Instructor
Department
Coming up: Association Names
Semester
teaches>
Class
Section
sponsors>
Course
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
<works for
Instructor
Department
Coming up: Navigation
teaches>
Class
Section
sponsors>
Course
Navigation


The navigation of association without an
arrowhead is assumed to be undefined
Navigation has little value when modeling
from a conceptual perspective
– Why?

Navigation is more important from
specification and implementation
perspectives
– Why?
Coming up: N-ary Associations
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
Person
Coming up: Generalization
Student
Graduate
Student
Generalization

From a specification/implementation
perspective, generalization can represent subtyping, inheritance, type capability, and
substitutability (depends on the language)
Student
Person
name: String
address: String
changeAddress(new_address)
Coming up: Exercise – Simple Associations
major: String
GPA: Real
standing: String
add(Class Section)
drop(Class Section)
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: Class Diagrams
Multiplicity Constraints
is registered for>
Student
1..*
0..*
1
0..8
teaches>
<works for
Instructor
1..*
1
Department
Coming up: Questions
Semester
1..3
1
0..6
sponsors>
1..*
Class
Section
1..*
Course
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: Multiplicity Constraints
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
This is also called the
cardinality constraint
Coming up: Dependencies
Dependencies
A consolidated snippet of the UML Meta-model
Relation
Generalization
Association
Binary Association
Coming up: Dependencies
Aggregation
Dependency
N-ary Association
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)
Coming up: Dependencies
Prerequisite
Dependencies

Meaning from three perspectives
– 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: Dependencies
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: Aggregations (is part of)
Aggregations (is part of)
A consolidated snippet of the UML Meta-model
Relation
Generalization
Association
Binary Association
Coming up: Aggregation
Aggregation
Dependency
N-ary Association
Aggregation


Aggregation: is a special kind of association
that means “part of”
Aggregations should focus on single type of
composition (physical, organization, etc.)
Crust
1
1
Sauce Serving
1
1
Cheese Serving
Topping Serving
*
Pizza
1..3 1
1
0..9 1
4..*
Coming up: Composition (very similar to aggregation)
Slice
1
Order
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.
Room
Coming up: Lets look at BookstoreExample4.jpg
Building
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?
Coming up: Class Exercise
Class Exercise

Lets create the WeGrow class diagram
Coming up: Validating a class diagram
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: Some User Stories for WeGrow
Some User Stories for WeGrow

As an analyst I need to get a report on
sales statistics on a per store basis.

As a store manager I want to change prices
for an item in my store only.

As a sales clerk I want to determine the
closest store that has the item in stock and
request a transfer of the merchandise
Coming up: 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?
Coming up: Bonus Slide!
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
End of presentation