Slide 5 Class Diagram

OOADClass

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

Class Attributes and OperationsAn 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 OperationsClass 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:

InstructorStudent1..*


The example indicates that every Instructor has one or more Students:

InstructorStudent1..*


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

InstructorStudent1..*1..*

learns fromteaches


We can also name the association.

TeamStudentmembership

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 Door2..*

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.

Packages

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

Packages

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 }

CustomerOrder*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.

Class and Objects

Education

Slide 5 Class Diagram