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


Most systems have large numbers of
objects that come and go throughout the
systems’ lifetimes
Humans naturally classify similar things by
their observable properties
– Watch how little children reference things

Classes are abstractions that allow us to
deal with whole collections of objects that
share some commonalties
Coming up: Introduction to Classes
Introduction to Classes

Examples of classes in a Student Records
Management System
Student
Class
Section
Course
Semester
College
Instructor
Department
Class
Grade
Coming up: Three Perspectives
Major
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: Class Name Syntax
Class Name Syntax



A class name may be text consisting of any
numbers, letters, and punctuation marks
(except “:”)
Capitalize each word that makes up the
name
A class name can include a path
specification, if the class is part of a
package.
– “Records Management Package::Student”
– More on packages later
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: Object Name Syntax
joe: Student
major: String = “CS”
gpa: Real = 4.0
standing: String = “”
add(Class Section)
drop(Class Section)
Object Name Syntax





The name includes an the instance name
and a class path, separated by a “:”
The instance name is typically lower case
The instance name be may blank,
meaning an arbitrary object from the
designated class
The class path name may include a path
The whole name is underlined
Coming up: Attributes
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
Attributes from an
Implementation Perspective




Attributes from an implementation
perspective are similar data members in
C++ or Java
They are place holders with value
semantics
Each object can have different values
Constraints can be placed on the attributes
to restrict how and when the values can be
changed
Coming up: Attribute Syntax
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)] [:return-type]
[{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 words that make up the name are
typically all lowercase
– The name should include an arrow
indicating the direction in which the name
should be read
Some kinds of associations have implied
names
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
N-ary Associations


Associations can connect more than one class
Notation:
Student
Advisor
Major

How should we go about naming an n-ary
association?
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
Person
Coming up: Generalization
Student
Graduate
Student
Generalization

From a specification perspective,
generalization can represent sub-typing,
interface inheritance, type capability, and
substitutability
Student
Person
name: String
address: String
changeAddress(new_address)
Coming up: Generalization
major: String
GPA: Real
standing: String
add(Class Section)
drop(Class Section)
Generalization

From an implementation perspective, a
generalization can represent
implementation inheritance (depending on
developing environment)
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

Class Diagrams are similar to entityrelationship (ER) data models, but show
classes instead of data entities
Coming up: Class Diagrams
Class Diagrams

Class diagrams are collections of
– Classes and individual objects
– Associations, generalizations, and other
kinds of relationships

They can also include
– Grouping concepts like packages
– Constraints
– Various kinds of annotations
Coming up: Class Diagrams
Class Diagrams

Class Diagrams are like the paragraphs of
a technical paper
– each diagram should focus on a specific
topic
– a diagram provides supporting detail for the
main concept(s) that it is trying to
communicate
– the level of the abstraction used in the
diagrams should be consistent

Together, all the diagrams for a system
comprise a “model” of that system
Coming up: Class Diagrams
Class Diagrams

Pitfalls of Class Diagrams:
– Using class diagrams alone can cause
developers to focus too much on structure
and ignore behavior
– Using the wrong (or a mixed) perspective
can lead to misunderstanding
– Using the wrong level of abstraction can be
confusing to the target audience
– Using mixed levels of abstraction can
reduce the usefulness of diagram
Coming up: Multiplicity Constraints
Multiplicity Constraints
is registered for>
Student
1..*
0..*
1
0..8
teaches>
<works for
Instructor
1..3
1..*
1
Department
Semester
1
Coming up: Multiplicity Constraints
0..6
sponsors>
1..*
Class
Section
1..*
Course
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
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.

Some kinds of dependencies can be
inferred if the component definitions are
complete
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: Using a class diagram
Building
Validating a class diagram

One of the most important, and often
overlooked issues is how to validate a class
diagram.

Given a specification or a use-case, can
you look at the class diagram and use
features of it to manually “execute” the use
case.

Lets try it for the WeGrow class diagram
Coming up: Questions
Questions




What do classes and associations in class
diagrams really mean?
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”?
How can we show relationships involving
more than two classes?
When during the software engineering life
cycle should you build classes diagrams?
Coming up: More Questions
More Questions



How does one decide how to break up the
responsibilities of a system into meaningful
classes?
How do you know if you have identified the
best classes for a system?
How do you know where in a class
hierarchy a particular attribute or operation
bests fit?
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: More Questions