7M900
UML Class Diagrams UML Class Diagrams
11 November 2015
www.isbe.tue.nl
UML Class Diagrams UML Class Diagrams
Class diagrams are used in:
• Analysis To build a conceptual domain model with semantic associations between concepts
• Design Structural model of a design in terms of class interfaces
• Implementation Source code documentation, exposing the implementation
The class diagram provides a static structure of all the classes
that exist within the system. Classes are arranged in hierarchies
sharing common structure and behaviour and are associated
with other classes.
7M900
Classes and Objects Classes and Objects
• Objects looks like modules in some ways Object = Identity + State + Behaviour
• Objects provide encapsulation of data.
• An object is described by a class. A class may define a number of objects with identical properties.
3
www.isbe.tue.nl
About a Class About a Class
• An class has a public interface defining the operations(methods) it will support
• A class has private data called attributes, which only its own operations can access
• A class can have private operations for its own use
• A class may know about other classes by means of associations
4
7M900
Classes Classes
A class is simply represented as a box with the name of the class inside
– The diagram may also show the attributes and operations Rectangle Rectangle
height width
Rectangle height width getArea resize
Rectangle height: int width: int getArea(): int resize(int,int)
The complete signature of an operation is:
operationName(parameterName: parameterType …): returnType
5
www.isbe.tue.nl
From UML Distilled 2ndEd.
– Martin Fowler (Covers UML 1.4)
7M900 7 From UML Distilled 3rdEd.
– Martin Fowler (Covers UML2.0)
www.isbe.tue.nl
Essentials of UML Class Diagrams
1of2Essentials of UML Class Diagrams
1of2The main symbols shown on class diagrams are:
• Classes
– Represent the types of data themselves
• Associations
– Represent linkages between instances of classes
• Attributes
– Are simple data found in classes and their instances
• Operations
– Represent the functions performed by the classes and their instances
• Generalizations
– Group classes into inheritance hierarchies
8
7M900 9
www.isbe.tue.nl
Essentials of UML Class Diagrams
2of2Essentials of UML Class Diagrams
2of2• Constraints
– Extensions of the semantics of a UML element, allowing you to add new rules or modify existing ones.
– A constraint specifies conditions that a run-time configuration must satisfy to conform to the model.
– A constraint is rendered as a string enclosed by brackets and placed near the associated element.
• Navigation
– Given a plain, unadorned association between two classes, it is possible to navigate from objects of one kind to objects of the other kind. Unless otherwise specified, navigation across an association is bidirectional.
However there are circumstances in which you will want to be able to limit navigation to just one direction.
– Specifying that an association is navigable is a statement that, given an object at one end, you can easily and directly get to objects at the other end, usually because the source object store some references to objects of the target.
7M900
Navigation: Example Navigation: Example
11 Bidirectional
Unidirectional
association
association navigation
www.isbe.tue.nl
12
7M900
Identifying objects ? Identifying objects ?
A possible recipe
– We can identify objects in our problem context by looking for nouns and noun phrases
– Each of these can be underlined and becomes a candidate for an object in our solution
– Eliminate irrelevant objects
• Redundant, vague, event, outside scope, attribute, meta-language
13
www.isbe.tue.nl
The Bank Account Example The Bank Account Example
You are asked to design a system to handle current and savings accounts for a bank.
Accounts are assigned to one or more customers, who may make deposits or withdraw money. Each type of account earns interest on the current balance held in it. Current accounts may have negative balances (overdrafts) and then interest is deducted. Rates of interest are different for each type of account. On a savings account, there is a maximum amount that can be withdrawn in one transaction.
Bank employees may check any account that is held at their branch. They are
responsible for invoking the addition of interest and for issuing statements at the correct times.
A money transfer is a short lived record of an amount which has been debited from one account and has to be credited to another. A customer may create such a transfer from their account to any other. Transfers within a branch happen immediately, while those between branches take three days.
7M900
The Bank Account Example The Bank Account Example
You are asked to design a system to handle current and savings accounts for a bank.
Accounts are assigned to one or more customers, who may make deposits or withdraw money. Each type of account earns interest on the current balance held in it. Current accounts may have negative balances (overdrafts) and then interest is deducted. Rates of interest are different for each type of account. On a savings account, there is a maximum amount that can be withdrawn in one transaction.
Bank employees may check any account that is held at their branch. They are
responsible for invoking the addition of interest and for issuing statements at the correct times.
A money transfer is a short lived record of an amount which has been debited from one account and has to be credited to another. A customer may create such a transfer from their account to any other. Transfers within a branch happen immediately, while those between branches take three days.
Question ? Recognize the nouns!
15
www.isbe.tue.nl
You are asked to design a system to handle current and savings accountsfor a bank.
Accountsare assigned to one or more customers, who may make depositsor withdraw money. Each type of account earns intereston the current balanceheld in it. Current accounts may have negative balances (overdrafts) and then interest is deducted. Rates of interestare different for each type of account. On a savings account, there is a maximum amountthat can be withdrawn in onetransaction.
Bank employeesmay check any account that is held at their branch. They are
responsible for invoking the addition of interest and for issuing statementsat the correct times.
A money transferis a short lived recordof an amountwhich has been debited from one account and has to be credited to another. A customer may create such a transfer from their account to any other. Transfers within a branch happen immediately, while those between branches take three days.
Nouns in the bank account example
16
7M900
Redundant - overdraft, account Vague - amount, money
An event or an operation - transaction, deposit Outside scope of system - bank, days
An attribute - interest, rate of interest, maximum amount, current balance, overdraft
Meta-language - transaction, correct times, record
Nouns eliminated Nouns eliminated
17
www.isbe.tue.nl
Nouns left Nouns left
• current account, savings account, customer, branch, statement, transfer
Remark:
The Bank Account is derived from
Pauline Wilcox – ‘The Unified Modelling Language’ in
Msc in Systems Level Integration – Systems Partitioning Module
7M900
Association Association
Association is a relationship between two classes (objects).
Whenever a class use another class, there exists a relationship between the two classes, this relationship is known
as association.
Association talks about relationship between objects in general.
Example
:
www.isbe.tue.nl
Association Adornments: Name, Role Association Adornments: Name, Role
• The association has a name - the descriptive term, often a verb, for the association.
• Each association has two association ends; each end is attached to one of the classes in the
association. An end can be explicitly named with a label. This label is called a role name (association ends are often called roles).
Person Company
employee employer
works for
Person Company
employee employer
has employment for
20
7M900
Association: Multiplicity Association: Multiplicity
Multiplicity defines the number of objects associated with an instance of the association.
– Default of 1 (1: 1) – 0 or 1: 0..1
– Zero or more (0..infinite): * – 1 or more (1..infinite): 1..*
– n..m; range from n to m inclusive
Car Person
transports
passenger
Car Person
transports
passenger 5
Car Person
transports
passenger
*
Car Person
transports
passenger 1..*
Car Person
transports
passenger 2..5
21
www.isbe.tue.nl
Association classes Association classes
Sometimes an attribute that concerns two associated classes cannot be placed in either of the classes
Student
* *
CourseSectionRegistration grade
Student CourseSection
Registration grade
* *
7M900
Attribute Analysis Attribute Analysis
It is not always clear which attributes belongs to which classes by finding out the class attributes. An attribute is assigned to that class where it is certainly a feature.
For example:
a project leader has the attributes name, department and age.
But what to do with the attributes project number, project duration, starting time, and budget? These attributes clarifies something about the relation between project leader and type of project.
This often happens if a n:m relation refers to an association between two classes. In that case, we can define a new class, for instance project management.
23
www.isbe.tue.nl
24
7M900
Generalization Generalization
A specialization / generalization relationship, in which objects of the specialized element (child) are substitutable for objects of the generalized element (parent).
• Superclass – the generalization of another class, the child.
• Subclass – the specialization of another class, the parent.
Customer
Corporate Customer
Personal Customer
25
www.isbe.tue.nl
Generalization - characteristics Generalization - characteristics
• Identify common features concerning behaviour and knowledge. Define these common features on a higher level in the inheritance hierarchy.
• The aim is at behaviour more than knowledge when combining classes.
• Generalization is a bottum-up process.
• A superclass includes all common properties of
its subclasses.
7M900
Specialization - characteristics Specialization - characteristics
• Define a new class which is a special appearance of an existing class.
• Specialization is a top-down process.
• A subclass can have attributes and operations that are specific for that sub-class.
• A subclass may redefine operations of its super-class.
27
www.isbe.tue.nl
28
7M900
Associations vs. Generalizations Associations vs. Generalizations
Associations describe the relationships that will exist between instances at run time
– when you show an instance diagram generated from a class diagram, there will be an instance of both classes joined by an association
Generalizations describe relationships between classes in class diagrams
– they do not appear in instance diagrams at all
– an instance of any class should also be considered to be an instance of that class’s super classes
29
www.isbe.tue.nl
Aggregation Aggregation
Aggregations are special associations that represent ‘part- whole’ relationships
– the ‘whole’ side is often called the assembly or the aggregate (“has-a” relationship)
Vehicle
*
VehiclePartCountry
*
Region7M900
Composition Composition
A composition is a strong kind of aggregation
– if the aggregate is destroyed, then the parts are destroyed as well
Building
*
Room31
www.isbe.tue.nl
Difference between Aggregation and Composition Difference between Aggregation and Composition
Association talks about relationship between two classes (objects) in general. Any type of relationship between two objects is called association.
Aggregation is more restricted form of association. When a object has a ownership of another object, there exists aggregation between objects. However, this ownership does not mean that: without Parent object child object would not exist.
Composition is again the more restricted form of aggregation. When a object has a ownership of another object and without Parent object, child object can not exist, there exists composition.
7M900
Association, Aggregation, Composition ? Association, Aggregation, Composition ?
33
What is OK?
A or B or None
A B
www.isbe.tue.nl
Association, Aggregation, Composition ? Association, Aggregation, Composition ?
What is OK?
A or B or None
A B
7M900
Association, Aggregation, Composition ? Association, Aggregation, Composition ?
35
What is OK?
A or B or None
A B
www.isbe.tue.nl
Association, Aggregation, Composition ? Association, Aggregation, Composition ?
36
What is OK?
A or B or None
A B
7M900
Aggregation and Composition: example Aggregation and Composition: example
A B
Question: What is OK?
1. A 2. B 3. A & B 4. None
37
www.isbe.tue.nl
Object diagram Object diagram
An object diagram is shown as a class, and the name is underscored, although an object’s name can be shown optionally preceding the class name as:
objectname: classname.
The object does not have to be named, in which case only
the classname is shown underscored.
7M900
Class diagram Object diagram : example Class diagram Object diagram : example
Computer name: String memory: String Author
name: String age: Integer
0..* 1..*
39
Derive from this class diagram an object diagram consisting of 1 author and 2 computers.
www.isbe.tue.nl
Class diagram Object diagram : example Class diagram Object diagram : example
Computer name: String memory: String Author
name: String age: Integer
0..* 1..*
Brian’s PC: Computer name = “Dell 486”
memory = “256MB”
Brian: Author name = “Brain Jones”
age = 35r
Brian’sLaptop: Computer name = “Toshiba CT”
memory = “512MB” 40
7M900
References References
• Timothy Lethbridge & Robert Laganière
(2005) Object-Oriented Software Engineering, 2ndedition http://www.lloseng.com• Martin Fowler (2000, 2004
)Object-Oriented Software Engineering, 2ndedition; 3rdedition
41
