Class Diagrams 1 Overview Class diagrams are the most commonly used diagrams in UML Class diagrams are for visualizing specifying and documenting the system from a static perspective Class diagrams indicate which classes know about other classes and if they do what type of relationship exists Class diagrams will have different levels of detail abstraction depending on where we are in the software development process 2 Overview Class diagrams commonly contain classes interfaces collaborations and associations Class diagrams help in showing the functional requirements of the system the services the system should provide to its end users 3 A Class Diagram 4 Common Modeling Techniques To model a collaboration a group of classes working toward a common purpose Use scenarios to see which classes are actually involved in carrying out a particular operation Scenarios will also aide in establishing relationships between classes Fill in the responsibilities section of each class icon The responsibilities of each class will eventually evolve into actual attributes and behaviors A complex system typically requires multiple class diagrams Each diagram is devoted to a particular functionality of the system 5 Modeling Simple Collaborations 6 Common Modeling Techniques To model a logical database schema Identify the persistent objects objects that are stored on disk between runs of the program Label persistent objects as such in the class diagrams using tags or stereotypes 7 Modeling a Database Schema 8 Common Modeling Techniques Multiple class diagrams are required to model large systems Each individual class diagram Shows a single aspect of the system Contains only elements that are essential to understanding that aspect Provide details consistent with its level of abstraction Uses meaningful class and member names Pointers to other classes are modeled as associations 9 Advanced Class Diagram Features 10 Overview A classifier is a mechanism that describes structural and behavioral features Types of classifiers are classes interfaces datatypes signals components nodes use cases and subsystems Classes are the most important kind of classifier Classes have a number of features beyond attributes and behaviors that allow you to model some of the more subtle advanced features of a system 11 Advanced Class Features 12 Terms and Concepts Other classifier definitions Interface A collection of operations that are used to specify a service of a class or component Datatype Modeled as a class with the strerotype type May be primitive or user defined Signal A class used for communicating information The class in its entirety is a kind of message Component A physical and replaceable part of a system that conforms to and provides the realization of a set of interfaces Node A physical element that exists at run time and that represents a computational resource 13 Terms and Concepts Other classifier definitions Use case A description of a set of a sequence of actions that yields an observable result of value to a particular actor Actor An external user of the system Subsystem A grouping of element that carry out a subset of the entire systems functionality Modeled as a package with the stereotype subsystem 14 Classifier Icons 15 Terms and Concepts Visibility Class members attributes and behaviors may be specified as public private or protected In UML the single character visibility indicator is placed to the left of the member myProtectedFunction myPrivateFunction myPublicFunction Restricting visibility is the same as restricting accessibility By restring accessibility you are limiting the number of entry points into an object By restricting the number of entry points you are facilitating debugging and code re use 16 Terms and Concepts Scope Individual member data attributes may have either class scope or instance scope Class scope A single copy of an attribute is shared by all instances of a class In UML you underline the attribute to indicate class scope productCount int In C members with class scope would be declared as static static int productCount Instance scope Each instance of a class would have its own copy of the attribute All attributes have instance scope by default 17 Terms and Concepts Abstract A abstract class cannot have any direct instances Not all OO programming languages directly support abstract classes In C abstract classes contain one or more pure virtual functions int myFunction int x 0 Pure virtual functions have no function body An abstract class is thought to be so general as to be useless by itself Abstract classes only occur in the context of an inheritance hierarchy In UML you specify that a class is abstract by writing its name in italics 18 Terms and Concepts Root and Leaf Tags may be used to indicate that a class can have no parents root or children leaf There is rarely an occasion to use this feature 19 Terms and Concepts Polymorphism Polymorphic behavior exists in the context of inheritance Polymorphism applies to behavior member functions only Polymorphism is synonymous with dynamic binding In UML a behavior name in italics is used to indicate polymorphism In C a polymorphic call would most likely appear as baseClassPointer poylmorphicFunction baseClassPointer poylmorphicFunction 20 Abstract and Concrete Classes and Operations 21 Terms and Concepts Multiplicity Class multiplicity In ULM it can be indicated by placing a number in the upper right corner of the class icon Most commonly expressed in the context of associations between classes Attribute multiplicity In UML it is indicated as an expression appearing in square brackets just after the attribute name Looks like an array declaration in C attributeName 4 attributeType 22 Terms and Concepts Attributes May be expressed using various levels of detail The syntax for an attribute is visibility name multiplicity type initialValue propertyTag There are three predefined property tags changeable the attribute may be read and modified default addOnly when multiplicity 1 additional objects may be added but not removed frozen read only constant value The only feature of an attribute that is required in a class icon is its name 23 Terms and Concepts Operations behaviors May be expressed using various levels of detail The syntax for an operation is visibility name parameterList returnType propertyTag Predefined propertyTags are isQuery cannot change the state of the object Sequential only one thread of control in the object at a