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Www.ddss.arch.tue.nl 7M822 UML Class Diagrams 7 October 2010.

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1 7M822 UML Class Diagrams 7 October 2010

2 7M822 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.

3 7M822 Classes and Objects Objects looks like modules in some ways Object = Identity + State + Behaviour Objects provide encapsulation of data object is described by a classAn object is described by a class. A class may define a number of objects with identical properties

4 7M822 About an Object operationsAn object has a public interface defining the operations (methods) it will support attributesAn object has private data called attributes, which only its own operations can access An object can have private operations for its own use associationsAn object may know about other objects by means of associations

5 7M822 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 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

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7 7M822 Essentials of UML Class Diagrams The 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

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9 7M822 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

10 7M822 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.

11 7M822 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!

12 7M822 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. Nouns in the bank account example

13 7M822 Redundant Redundant - overdraft, account Vague Vague - amount, money An event or an operation An event or an operation - transaction, deposit Outside scope of system Outside scope of system - bank, days An attribute An attribute - interest, rate of interest, maximum amount, current balance, overdraft Meta-language Meta-language - transaction, correct times, record Nouns eliminated

14 7M822 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

15 7M822 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). PersonCompany employee employer works for PersonCompany employee employer  has employment for

16 7M822 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 CarPerson transports passenger CarPerson transports passenger 5 CarPerson transports passenger * CarPerson transports passenger 1..* CarPerson transports passenger 2..5

17 7M822 Association classes Sometimes an attribute that concerns two associated classes cannot be placed in either of the classes StudentCourseSection ** Registration grade StudentCourseSection Registration grade **

18 7M822 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.

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20 7M822 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

21 7M822 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.

22 7M822 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.

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24 7M822 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

25 7M822 Aggregation Aggregations are special associations that represent ‘part- whole’ relationships –the ‘whole’ side is often called the assembly or the aggregate VehicleVehiclePart * CountryRegion *

26 7M822 Composition A composition is a strong kind of aggregation –if the aggregate is destroyed, then the parts are destroyed as well BuildingRoom *

27 7M822 Aggregation and Composition: example AB Question: What is OK? 1.A 2.B 3.A & B 4.None

28 7M822 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.

29 7M822 Object diagram : example Computer name: String memory: String Author name: String age: Integer 0..*1..*

30 7M822 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”

31 7M822 References Sommerville, Ian (2001) Software Engineering, 6 th edition Timothy Lethbridge & Robert Laganière (2005) Object-Oriented Software Engineering, 2 nd edition Martin Fowler (2000, 2004 ) Object-Oriented Software Engineering, 2 nd edition; 3 rd edition


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