1. Object Oriented Analysis and Design Class and Object Diagrams

2. Contents 2  Class Diagrams  Object diagrams

3. Class Diagrams 3  Shows the relationships between classes  We have several types of relationships  A class implements an interface  A class inherits from another class  A class is composed of another class  A class is an aggregate of another class

4. Sample Class Diagram 4 implements composition inheritance aggregation

5. Why Use Class Diagrams? 5  When classes are defined as text  You must read the text to find the associations  You must remember all the associations  You can then assimilate all the class relationships  With Diagrams  The associations are shown as lines  They are obvious and understood more quickly  As a result, you understand the program more quickly

6. Uses of Class Diagrams 6  As a notation for program design alternatives  As a way to capture the design of a program  As a way of explaining how a program works  As a way for programmers to discuss a design  As part of the documentation for a program

7. Modeling Classes 7  Select a class name  Define the attributes  Define the methods

8. Method Categories 8  Constructors – create objects  Destructors – destroy objects  Accessors – return all or part of object state  Modifiers – change all of part of object state  Iterators – visit objects in a collection of objects

9. Defining Methods 9  Define  Accessors to get and/or set private variables  Methods necessary to manipulate the class  Methods which are convenient to manipulate the class  Keep the number of methods to a minimum  offer convenience methods only when warrented

10. Associations 10  An association indicates a structural relationship between two classes  Normally, this means that one class has-a instance of the other class  You can specify various amounts of information about an association  The most general association is represented as a line between two classes

11. Associations 11  This is a bidirectional association that indicates  A car has an engine,  And an engine has a car

12. Directed Associations 12  A directed association indicates that the association is only in one direction  We can use a directed association to indicate that a car has an engine, but an engine does not have a car  This is indicated by adding an arrow head

13. Roles 13  Each entity in an association can play a role  You can label none, either, or both roles  For example, an engine acts in the role of the power source for a car

14. Multiplicity 14  Sometimes the relationship between two entities is not 1 to 1  For example, a car has 4 wheels  This is indicated by placing numbers at the ends of the associations

15. Multiplicity 15  In addition to a fixed number you can have  1- single instance  1.. *- one or more instances  0.. 1- zero or one instance  *- zero or more instances  You can label both ends of the association with their multiplicity if desired

16. Multiplicity 16  When the multiplicity is greater than one, it indicates the use of a collection to hold the members  A collection of members can be ordered or unordered  If ordered, this annotation is added

17. Labeling Associations 17  You can label an association to  Provide additional information about the association  Give it a name for convenience

18. Labeling Associations 18  An association name can have an optional arrow indicating the direction of the association Person Car owns

19. Association Roles 19  The ends of an association can also be labeled with the role one or both participants plays in the association Employee Project Participates in programmer Role played by employee

20. Qualified Associations 20  An association can be qualified to show how one object can access another  This usually involves specifying how one object can index or locate another  We use a rectangle on the association to indicate this Customer Order -orderNum: integer -quantity:integer = 0 orderNum:integer places

21. Composition 21  Composition is a special type of association that indicates a part-of or has-a relationship  By using composition, we are specifying that one object is physically contained in the other  For example, the bank account and the customer are related by composition since the account has-a owner  The symbol for composition is an solid diamond at the composite end of the association line

22. Composition 22

23. Composition 23 Account Customer Branch Composition indicates physical containment

24. Aggregation 24  Aggregation is like composition, but one object is not physically contained in the other  It is represented as an empty diamond

25. Aggregation 25 Account Customer Branch In aggregation, the objects in the aggregate are not physically contained, but are external to the aggregate and referenced by the aggregate

26. Inheritance 26  Inheritance is one of the key features of the object- oriented approach  This allows us to take a class and create a specialized subtype of the class  Specialization is usually done by adding attributes to the parent class  A specialized class is often called a derived class  The class from which it is derived is the parent class or base class

27. Inheritance 27  A derived class inherits  The attributes of the base class  The methods of the base class  A derived class adds  Attributes to represent its special information  Methods to handle the new attributes and capabilities

28. Inheritance 28  We can create a special type of account – the savings account  This has everything an account has, plus more  An empty arrow head is used to indicate inheritance

29. Inheritance 29  Several classes can be derived from the same base class  They each inherit the same attributes and methods of the base class

30. Generalization 30  Generalization is the opposite of specialization  It allows you to  take several classes,  abstract what they have in common,  Move this to a superclass  This has the advantage that the declarations for the common attributes and methods only need to be done once  This is one example of code reuse, although a minor one

31. Abstract Classes 31  During the process of class design we sometimes create a superclass that can never be instantiated  Such classes are called abstract classes  Abstract classes exist purely to act as base classes  The names of abstract classes are italicized

32. Abstract Classes 32  Banks have chequing accounts and savings accounts  They do not just have plain accounts  Therefore, we can make the Account class abstract

33. Visibility 33  Visibility determines where the members of a class can be seen  Visibility is indicated by a symbol in front of the member name +PublicVisible to all classes #ProtectedVisible to derived classes ~PackageVisible only with the package -PrivateVisible only within the class

34. Attribute Type 34  Denotes the type of data that will be stored in this attribute  Analysis – How the user sees the data  Design – represents the data type used in the specific programming language  Example  String  Integer

35. Attribute Initial Value 35  An optional initial value can be assigned to an attribute  Balance:float = 0.0

36. Derived Attribute 36  A slash “/” preceding the name of the attribute indicates that it is a derived attribute that comes from the parent class

37. Class Variables 37  A class variable is a single variable shared among all instances of the class  All members of the class can access the variable  A class variable can have any degree of visibility  Class variables are like globals, but are not in the global name space  You can also have class level methods which can be invoked on the class, rather than an object  Class-level objects have their names underlined

38. Object Diagrams 38  An object diagram shows several objects and their associations  This shows a snapshot of how objects are connected in an actual system  This is a static snapshot at a specific point in time  Object diagrams are useful to explain how the classes work together by showing an example of their use  The object symbols are represented with a name and class name

39. Object Diagrams 39

40. Using Object Diagrams 40  Object diagrams can be used for  Sketching the relationship between objects to help discover classes  As examples of how classes can be combined  To show the relationship of objects before and after an operation

41. Object Notation 41  Object format is  Object Name : Class Name  : Class Name  Indicates an anonymous object  Attributes have actual values  Address = 123 Anywhere St.  Operations are omitted

42. Additional Object Notation 42  The name of an object can be omitted if it is not needed  You can add the values of key variables if this is important to the example

43. Class/Object Diagram Differences 43