Slide 5 Class Diagram

What is a Class Diagram?
• A Class Diagram is a diagram describing
the structure of a system
• shows the system's
• classes
• Attributes
• Operations (or methods),
• Relationships among the classes.

What is a Class Diagram?
• A class diagram depicts classes and their
interrelationships
• Used for describing structure and behavior in the
use cases
• Provide a conceptual model of the system in
terms of entities and their relationships
• Used for requirement capture, end-user
interaction
• Detailed class diagrams are used for developers

Essential Elements of a UML Class Diagram
• Class
• Attributes
• Operations
• Relationships
– Associations
– Generalization
– Realization
– Dependency
• Constraint Rules and Notes

Class
• Describes a set of objects having
similar:
– Attributes (status)
– Operations (behavior)
– Relationships with other classes
• Graphically, a class is rendered as
a rectangle, usually including its
name, attributes, and operations
in separate, designated
compartments.

Class Names
The name of the class is the only
required tag in the graphical
representation of a class. It always
appears in the top-most
compartment.

Class Attributes and Operations
An attribute is a named property of a class that describes the object
being modeled.
In the class diagram, attributes appear in the second compartment
just below the name-compartment.
Attributes can be:
+ public
# protected
- private
Attributes are usually listed in the form:
attributeName : Type
Operations describe the class behavior and appear in the third
compartment.

Visibility and Access for attributes
and operations of a class

Class
Account_Name
- Customer_Name
- Balance
+addFunds( )
+withDraw( )
+transfer( )
Name
Attributes
Operations

Class Attributes and Operations
Class Attributes:
 The attribute type is shown after
the colon.
 Attributes map onto member
variables (data members) in code.
Class Operations (Methods):
 The return type of a method is
shown after the colon at the end of
the method signature.
 The type of method parameters
are shown after the colon following
the parameter name.
 Operations map onto class
methods in code

Operation (Method) Parameter Directionality
Parameter
direction
Description
in states that p1 and p2 are passed to
op1 by the caller.
They are both in parameters.
inout states that p3 is passed to op2 by
the caller and is then possibly
modified by op2 and is passed back
out.
p3 is an inout parameter.
out states that p6 is not set by the caller
but is modified by op3 and is passed
back out.
p6 is an out parameter.

Association
• In UML, object interconnections (logical or
physical), are modeled as relationships.
• An association between two classes indicates
that objects at one end of an association
“recognize” objects at the other end and may
send messages to them.
• Example: “An Employee works for a
Company”
Employee Company

Association Relationships
If two classes in a model need to communicate with
each other, there must be link between them.
An association denotes that link.
InstructorStudent

We can indicate the multiplicity of an association by
adding multiplicity adornments to the line denoting
the association.
The example indicates that a Student has one or more
Instructors:
InstructorStudent
1..*

The example indicates that every Instructor has one or
more Students:
InstructorStudent
1..*

We can also indicate the behavior of an object in an
association (i.e., the role of an object) using
rolenames.
InstructorStudent
1..*1..*
learns fromteaches

We can also name the association.
TeamStudent
membership
1..* 1..*

We can specify dual associations.
TeamStudent
member of
1..*
president of1 1..*
1..*

Associations
• Multiplicity
– the number of objects that participate in the association.
– Indicates whether or not an association is mandatory.
Exactly one 1
Zero or more (unlimited) * (0..*)
One or more 1..*
Zero or one (optional association) 0..1
Specified range 2..4
Multiple, disjoint ranges 2, 4..6, 8
Multiplicity Indicators

A class can have a self association.
LinkedListNode
next
previous

Aggregation
• A special form of association that models a
whole-part relationship between an aggregate
(the whole) and its parts.
• A directional association between objects.
• When an object ‘has-a’ another object, then
you have got an aggregation between them.
• Direction between them specified which
object contains the other object.
• It is also called ‘Has-a’ relationship.

Aggregation
 Class2 is part of Class1.
 Many instances (denoted by the *) of Class2
can be associated with Class1.
 Objects of Class1 and Class2 have separate
lifetimes.

Aggregation
• Note: If you delete the parent object, even
then the child object may exist. One object
can contain the other, but there is no
restriction that the composed object has to
exist in order to have existence of child object.

Aggregation
Whole Part
Car Door
2..*

Composition
• A strong form of aggregation
• In a more specific manner, a restricted
aggregation is called composition.
– The whole is the sole owner of its part.
• The part object may belong to only one whole
– The life time of the part is dependent upon the
whole.
• The composite must manage the creation and
destruction of its parts.

Composition
 Objects of Class2 live and die with
Class1.
 Class2 cannot stand by itself.

Composition
• A class contains students. A student cannot
exist without a class. There exists composition
between class and students.

Difference between aggregation
and composition
• Composition is more restrictive.
• When there is a composition between two objects,
the composed object cannot exist without the other
object.
• This restriction is not there in aggregation. Though
one object can contain the other object, there is no
condition that the composed object must exist.
• The existence of the composed object in aggregation
is entirely optional.

Difference between aggregation
and composition
• Example: A Library contains students and books.
Relationship between library and student is
aggregation.
• Relationship between library and book is
composition.
– A student can exist without a library and therefore it is
aggregation.
– A book cannot exist without a library and therefore its a
composition.

Generalization Relationship
o A generalization connects a
subclass to its superclass. It denotes
an inheritance of attributes and
behavior from the superclass to the
subclass and indicates a
specialization in the subclass of the
more general superclass.
o “is kind of” relationship.
Subtype2Subtype1
Supertype

Generalization
• A sub-class inherits from its super-class
– Attributes
– Operations
– Relationships
• A sub-class may
– Add attributes and operations
– Add relationships
– Refine (override) inherited operations
Person
Student

Generalization Relationships
UML permits a class to inherit from multiple superclasses,
although some programming languages (e.g., Java) do not
permit multiple inheritance.
Student
TeachingAssistant
Employee

Inheritance is indicated by a solid line with a closed,
unfilled arrowhead pointing at the super class

An example of inheritance using tree notation

Packages
 UML provides an organizing element called
a package.
 Packages enable modelers to organize the
model's classifiers into namespaces, which is sort
of like folders in a filing system.
 Dividing a system into multiple packages
makes the system easier to understand,
especially if each package represents a specific
part of the system.

Drawing Packages
• There are two ways of drawing packages on
diagrams.
1. If the modeler decides to show the package's
members within the large rectangle, then all
those members need to be placed within the
rectangle.

Drawing Packages
2. If the modeler decides to show the package's
members outside the large rectangle then all
the members that will be shown on the
diagram need to be placed outside the
rectangle. To show what classifiers belong to
the package, a line is drawn from each
classifier to a circle that has a plus sign inside
the circle attached to the package

Realization
• A realization relationship indicates that one
class implements a behavior specified by
another class (an interface or protocol).
• An interface can be realized by many
classes.
• A class may realize many interfaces.
LinkedList
<<interface>>
List ArrayList

Constraint Rules and Notes
• Constraints and notes annotate among other things
associations, attributes, operations and classes.
• Constraints are semantic restrictions noted as
Boolean expressions.
– UML offers many pre-defined constraints.
id: long { value > 0 }
Customer
Order
*1
{ total < $50 }
may be
canceled
Constraint Note

What is a Object Diagram?
• Object diagrams represent an instance of a
class diagram.
• The purposes of object diagrams are similar to
class diagrams.
• The difference is that a class diagram
represents an abstract model consisting of
classes and their relationships. But an object
diagram represents an instance at a particular
moment which is concrete in nature.

Slide 5 Class Diagram

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