3. UML Object Models

Transcription

1 3. UML Object Models 3.1. Introduction UML Object models capture the static structure of a system, as a: class architecture, or static object structure at some time instant. Both are represented graphically.

2 3.2. UML Class diagrams Class architectures lead to UML class diagrams which show: the template structure of a class, inheritance relations between classes, other relations between classes.

3 A UML class diagram contains 3 kinds of information: (1) The attributes and methods of a class, perhaps with visibility constraints, typing information and initialisation values. (2) UML defined relations between classes, mainly: (a) is_a and, (b) has_a (shared or unshared) with multiplicity (1-to-1, many-to-1, etc) (3) User defined relations between classes, e.g. x father_of y, with multiplicity

4 Class Attributes A class definition provides a template for creating objects. This information includes: a class name, names and types of attributes, names and types of methods, initialization values at object creation time, visibility attributes of members.

5 UML class name Employee <<abstract>> - identity : String # status : int - pay : float + fire() : void + promote( increment : int ) : void UML attributes UML methods UML visibility Optionally can mention code inside a comment UML Stereotypes include: <<abstract>>, <<singleton>> and <<interface>>

6 Visibility - : private +: public #: protected ~: package A class attribute has the general format <name> [ : <type> ] [ = <initialisation value> ] where the information in brackets [ ] is optional.

7 UML 2.0 allows C++ style grouping public a(): int b(): void private c():void UML 2.0 allows properties {readonly}, {ordered},{bag}, {sequence}, {composite}, {union} e.g. area : int {readonly}

8 Inheritance Information After individual classes have been identified by analysis as Nouns we try to identify inheritance relationships, e.g. subtypes employee office manager employee office worker

9 This will help us to: re-use code refactor designs and code clarify definitions is every x really a y? what s the difference between x and y? understand the structure of the domain spot incomplete models ( x??? z )

10 A UML inheritance hierarchy Class_A Class_B multiple inheritance Class_C Class_D Class_E Class_F single inheritance Important Note: no multiplicity!!!

11 class_c and class_d inherit from class_a. class_c also inherits from class_b (multiple inheritance, in C++ but not Java!) Also class_e and class_f inherit from Class_C. Tip don t overextend class hierarchies, Max Depth <= 7?

12 Aggregation Aggregation is an anti-symmetric and transitive relation between two classes: a container class a component class. possibly with a multiplicity. The simplest example is class composition, where one class is contained in another, e.g...

13 Book * 1 Chapter Multiplicity 1 = one * = many n = exactly n 2 Section Important Note: multiplicity must respect inheritance!

14 This relation is often called the has_a relation e.g. each Book object has_a chapter object, each Chapter object has_a Section object. A chapter is not a special kind of book, and so this relation differs from inheritance.

15 Semantically, composition is very similar to extending a class by an attribute, e.g. Book Chapter_1 : Chapter The differences are rather subtle, e.g. does every book have exactly one chapter?no? How many? Generally has_a is more expressive than class extension by attributes, but try to minimize diagram complexity?

16 UML distinguishes shared objects with another type of aggregation arrow, e.g. Polygon Line Point Name x, y : int text : String

17 Notice that unshared has_a is a special case of shared has_a ( the number of objects we share with just happens to be one ) Thus it is often said that composition is a special kind of aggregation. When in doubt about sharing we can use aggregation, can fill in diamond later!

18 UML also allows user-defined associations between classes. Think of these as relations between sets. Student * teaches * Teacher studies * result * * Course Exam_Result 1 assigns