CLASS DIAGRAMS A classy approach to objects

The Basic Class Diagram  Class Name  Attributes (- indicates private each would have properties or accessor/mutator methods)  Class Methods Customer ClassElements

Class Relationships  Classes have relationships with other classes  These fall into 4 different kinds  Associations  Inheritance (generalization/specialization)  Aggregation (parts to whole, where parts have a separate existence)  Composition (parts to a whole where the parts do not have a separate existence

Associations  Associations means that a class interacts with another class  A customer for instance interacts with a purchase  These associations can also contain a property called “multiplicity”  Multiplicity specifies how many instances of each class an association a class can have  For instance one customer can have many (1..* ) purchases, or one class can have 25 students (1..25)

Association Example

Inheritance  In Inheritance child classes inherit from a parent class  This means they get all the public properties and methods of the parent without having to rewrite them  Usually the parent is a generalized class that contains all the elements the child classes have in common, and the child classes represent more specialized instances of that general class

Inheritance Example

Aggregation  Aggregation exists when classes have a whole, part relationship but the parts have an independent existence of their own  For instance a company exists of many parts, but each of those parts can be treated as an object in themselves

Aggregation Example

Composition  Composition describes a relationship where the parts cannot have a separate existence from the whole.  An example is the.Net DataTable class. It contains the classes DataRow and DataColumn, but they can only exist in the DataTable class

Composition Example

Interfaces  Interfaces contain only method definitions.  They are often referred to as “contracts”  When a class implements an interface it “contracts” to provide an implementation for all the methods in the interface  Interfaces are useful for ensuring the consistency of some kinds of common functionality. A developer knows that if a class implements a particular interface then it will contain definitions for the interfaces methods

Interface Example Both classes must implement the three methods, as well as their own methods