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UML Class Diagrams

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  1. UML Class Diagrams 7 September 2009

  2. UML Class Diagrams 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. 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

  3. 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

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

  5. Rectangle Rectangle Rectangle height width height width height: int width: int getArea resize getArea(): int resize(int,int) 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 The complete signature of an operation is: operationName(parameterName: parameterType …): returnType

  6. 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

  7. 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

  8. 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.

  9. 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!

  10. Nouns in 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 intereston 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 onetransaction. 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. Nouns eliminated 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

  12. 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

  13. employee employer Person Company works for employee employer Person Company  has employment for 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).

  14. transports Car Person passenger transports 5 Car Person passenger transports * Car Person passenger transports 1..* Car Person passenger transports 2..5 Car Person passenger 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

  15. * * Student CourseSection Registration grade Registration * * Student CourseSection grade Association classes Sometimes an attribute that concerns two associated classes cannot be placed in either of the classes

  16. 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.

  17. Generalization Customer 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. Corporate Customer Personal Customer

  18. 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.

  19. 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.

  20. 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

  21. Vehicle Country * * VehiclePart Region Aggregation Aggregations are special associations that represent ‘part-whole’ relationships • the ‘whole’ side is often called the assembly or the aggregate

  22. Building * Room Composition A composition is a strong kind of aggregation • if the aggregate is destroyed, then the parts are destroyed as well

  23. Aggregation and Composition: example A B • Question: What is OK? • A • B • A & B • None

  24. 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.

  25. Computer Author 0..* 1..* name: String memory: String name: String age: Integer Object diagram : example

  26. Computer Author 0..* 1..* name: String memory: String name: String age: Integer Brian: Author Brian’s PC: Computer Brian’sLaptop: Computer name = “Brain Jones” age = 35r name = “Dell 486” memory = “256MB” name = “Toshiba CT” memory = “512MB” Object diagram : example

  27. References • Sommerville, Ian (2001) Software Engineering, 6th edition http://www.software-engin.com • Timothy Lethbridge & Robert Laganière (2005) Object-Oriented Software Engineering, 2nd edition http://www.lloseng.com • Martin Fowler (2000, 2004) Object-Oriented Software Engineering, 2nd edition; 3rd edition