1 Introduction to Classes

2 Terms and Concepts A class is... –The most important building block of any object- oriented system. –A description of a set of objects that share the same attributes and behaviors. –A blueprint for creating an object. –An abstraction (simplification) of reality. –A representation of a software thing, a hardware thing, or even a conceptual thing. –Graphically represented as a rectangle.

3 Terms and Concepts All classes have … –Names - Used to distinguish one class from another. A class must have a name. –Attributes - Member data. –Operations (behaviors) - Member functions. –Stereotypes - Documentation used to categorized class members. –Responsibilities - Documents the purpose of the class. Attributes and operations are the most important features of a class definition. A class usually collaborate with other classes.

4 Terms and Concepts Class names are … –Extracted from the problem domain (statement). –Nouns or noun phrases. –Concise and descriptive. Class attributes are … –Usually nouns or noun phrases. –Extracted or inferred from the problem statement. –Used to represent properties of the enclosing class. –Determine the state of an object.

5 Terms and Concepts Class operations (behaviors) are … –Verbs or verb phrases. –Inferred from the problem statement. –Provide a requested service. Class responsibility… –Is a contract or obligation of a class to the users of the class. –Maps directly to the Responsibility field of a CRC card. CRC - Class Responsibility Collaboration

6 Class Artifact

7 Common Modeling Techniques Identify the classes that are to be included in the design. –Formulate a problem statement. –Focus on the nouns in the problem statement. –Use CRC (Class, Responsibility, Collaboration) cards or use-cases to isolate each class.

8 Common Modeling Techniques Determine the responsibilities of each class. –Even out the work load between classes. A class with too much responsibility should be broken up into multiple classes. A class with too little responsibility should be absorbed into another class. –Group together classes that work toward a common goal. Packages (directories) may be used to group related classes into a higher level of abstraction. Packages may contain other packages.

9 Common Modeling Techniques Classes should exhibit high cohesion and low coupling. –A class should have a single well-defined purpose. This promotes software reusability –A class should interact with a limited number of other classes. This simplifies modifications to the program.

10 Common Modeling Techniques Feel free to model abstract (non-software) things (such as people) that are outside of the system boundaries. Occasionally built-in data types (int, float, char, etc.) are modeled as classes, although this is rare.

11 Class Relationships

12 Terms and Concepts

13 Terms and Concepts A dependency is... –A type of relationship that can exist between two classes. –An indication that one class ‘uses’ another class. –If the ‘used’ class changes it can have an impact on the ‘using’ class. –Graphically represented as a dashed line with an open arrowhead on one end. –Rarely labeled. The ‘used’ class is frequently an argument to one of the member functions of the ‘using’ class. The “used” class has no knowledge of the “using” class.

14 Terms and Concepts

15 Terms and Concepts A generalization is … –A type of relationship that can exist between two classes. One of the classes is the base (or parent) class; the other class is the derived (or child) class. –Used to show a ‘kind-of’ relationship. –Another name for inheritance. –All OO programming languages support single inheritance; some (C++) also support multiple inheritance. –Graphically represented by a solid line with an open triangular arrowhead on the base class end. The parent class has no knowledge of the child class.

16 Terms and Concepts

17 Terms and Concepts An association is … –A type of relationship that can exist between one or more classes. –Used to show a ‘knows-a’ relationship. –Either unidirectional or bi-directional. –Graphically represented by a solid line which may optionally be labeled and have a name direction indicator. –A peer relationship. Association names are verbs or verb phrases. The same class can be on both ends of a binary association.

18 Terms and Concepts Associations may optionally have role names and multiplicity symbols on either end of the association line (next to the class icon). Role –The face that a class on one end of an association presents to the class on the other end of the association. –A class can participate in many associations and thus have multiple (different) roles. –Role names are nouns. –Role names are usually used in place of association names.

19 Terms and Concepts Multiplicity –Indicates how many object may be connected across an instance of an association. Aggregation is … –A relationship between two classes. –A form of association. –Used to show a ‘has-a’ or a ‘whole-part’ relationship. –Graphically represented as a solid line with an open diamond on the ‘whole’ end. Aggregations and associations are implemented in exactly the same way. The difference is entirely conceptual.

20 Terms and Concepts

21 Terms and Concepts Composition is … –A relationship between two classes. –A strong form of association. –Used to show a ‘contains-a’ or a ‘whole-part’ relationship. –Graphically represented as a solid line with a solid diamond on the ‘whole’ end. Composition relationships and associations are not implemented in exactly the same way. If the ‘whole’ object is destroyed, the part object will also be destroyed. –The whole and part have concurrent lifetimes.

22 Terms and Concepts

23 Common Modeling Techniques Modeling inheritance –Make it a priority to look for possible inheritance relationships. This will reduce code redundancy. Use the “ClassA ‘is-a-kind-of’ ClassB” test. –Do not model relationships using multiple inheritance unless the implementation language supports multiple inheritance. This will save programmers having to develop messy work-arounds. –Look for classes that have similar attributes and behaviors and then factor those attributes and behaviors into a common base class.

24 Common Modeling Techniques Modeling dependency, association, aggregation and composition relationships. –It is not always easy to determine which is the best relationship to use. It all has to do with your perspective of the problem domain. –There is not always one best solution. –Personal experience is your most valuable modeling tool. –CRC cards are helpful.