Chapter 9: Relationships Chapter 10: Inheritance and Polymorphism [Arlow and Neustadt, 2005] CS 426 Senior Projects in Computer Science University of Nevada, Reno Department of Computer Science & Engineering

2  Relationships Links Associations Dependencies  Inheritance and Polymorphism Generalization Class inheritance Polymorphism Advanced generalization

3 Relationships = semantic connections between modeling elements Links = connections between objects Associations = connections between classes Links are instances of associations

4  A link is a connection between objects that communicate via messages  Links can be: Unidirectional, if only one object (the source) contains a reference to the other (the target). Bidirectional, if both objects have a reference to the other  Object diagrams show objects and their relationships at some point in time. Thus, links are dynamic connections between objects

5  Example of object diagram, that shows objects, links, and roles. It also illustrates the oblique style of drawing paths in UML diagrams.

6 Example of unidirectional link The orthogonal style of drawing paths in UML diagrams

7  Associations are relationships between classes  Links depend on associations, and instantiate them

8  The syntax for an association includes:  Association name  Role names  Multiplicity  Navigability

9  Multiplicity constrains the number of objects of a class that can be involved in a given relationship at any point in time

10  There is no default value for multiplicity in UML. If not specified, multiplicity is undecided

11  Multiplicity constraints encode business rules of the model  It is important to “read the model as written”

12 Example of association hierarchy Example of association network

13  Navigability can be read “messages can only be sent in the direction of the arrow”  Relationships without arrows are bidirectional  Unidirectional associations reduce coupling between classes

14  One-to-one and one-to-many associations can be modeled using attributes

15 Many-to-many associations can lead to association classes Corresponding association class

16  However, many-to-many associations can also be modeled using regular classes

17  “A dependency is a relationship between two elements where a change to one element (the supplier) may affect or supply information needed by the other element (the client)” [Rumbaugh et al., The UML Reference Manual]  Three types of dependency

18  Examples of different types of dependencies

19  The > dependencies are the most common type of dependency. They state that the client makes use of the supplier in some way. Java code for the above example

20  Abstraction dependencies are dependencies between model elements that are at different level of abstraction: >, >, > and >

21  Permission dependencies indicate the capability of one model element to access another model element.  Examples of permission dependencies: > > (formerly >)

22  Generalization applies to all classifiers and to some other elements such as associations, states, and events

23  Example of class hierarchy

24  Example of overriding

25  Example of abstract classes and operations

26  In generalization hierarchies it is important to maintain a uniform level of abstraction at each level  Incorrect example

27  Polymorphism means “many forms”. A polymorphic operation has several different implementations

28  Example of polymorphism “in action”

29  Object diagram with polymorphism

30  Another example of polymorphism

31  An example of generalization set

32  Constraints on generalization sets