11 November

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1 UML Class Diagrams 11 November 2015 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 2 1

2 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 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 2

3 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 Rectangle Rectangle height width height width getarea resize The complete signature of an operation is: height: int width: int getarea(): int resize(int,int) operationname(parametername: parametertype ): returntype 5 From UML Distilled 2 nd Ed. Martin Fowler (Covers UML 1.4) 6 3

4 From UML Distilled 3 rd Ed. Martin Fowler (Covers UML2.0) 7 Essentials of UML Class Diagrams 1of2 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 8 4

5 9 Essentials 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. 10 5

6 Navigation: Example Bidirectional association navigation Unidirectional association

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 13 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. 14 7

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. Question? Recognize the nouns! 15 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 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. 16 8

9 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 17 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 18 9

10 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: 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 employee works for employer Company Person employee has employment for employer Company 20 10

11 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 Car Car Car Car transports transports transports transports transports passenger 5 passenger * passenger 1..* passenger 2..5 passenger Person Person Person Person Person 21 Association classes Sometimes an attribute that concerns two associated classes cannot be placed in either of the classes Student * * CourseSection Registration grade Student Registration * * grade CourseSection 22 11

12 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

13 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 Personal Customer Customer 25 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

14 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

15 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 Aggregation Aggregations are special associations that represent partwhole relationships the whole side is often called the assembly or the aggregate ( has-a relationship) Vehicle * VehiclePart Country * Region 30 15

16 Composition A composition is a strong kind of aggregation if the aggregate is destroyed, then the parts are destroyed as well Building * Room 31 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. 16

17 Association, Aggregation, Composition? What is OK? A or B or None A B 33 Association, Aggregation, Composition? What is OK? A or B or None A B 34 17

18 Association, Aggregation, Composition? What is OK? A or B or None A B 35 Association, Aggregation, Composition? What is OK? A or B or None A B 36 18

19 Aggregation and Composition: example A B Question: What is OK? 1. A 2. B 3. A & B 4. None 37 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

20 Class diagram Object diagram : example Author name: String age: Integer 0..* 1..* Computer name: String memory: String Derive from this class diagram an object diagram consisting of 1 author and 2 computers. 39 Class diagram Object diagram : example Author name: String age: Integer 0..* 1..* Computer name: String memory: String Brian: Author name = Brain Jones age = 35r Brian s PC: Computer name = Dell 486 memory = 256MB Brian slaptop: Computer name = Toshiba CT memory = 512MB 40 20

21 References 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 41 21