Factory Methods

Factory Methods You can modify CourseSession so that it supplies a static-side factory method to create CourseSession objects. By doing so, you will have control over what happens when new instances of CourseSession are created. Modify the CourseSessionTest method createCourseSession to demonstrate how you will use this new factory method. private CourseSession createCourseSession() { return CourseSession.create("ENGL", "101", startDate); } In CourseSession, add a static factory method that creates and returns a new CourseSession object: public static CourseSession create( String department, String number, Date startDate) { return new CourseSession(department, number, startDate); } Find all other code that creates a CourseSession via new CourseSession(). Use the compiler to your advantage by first making the CourseSession constructor private: private CourseSession( String department, String number, Date startDate) { // ... Replace the CourseSession constructions you find (there should be one in RosterReporterTest and one in CourseSessionTest) with message sends to the static factory method. With the CourseSession constructor declared as private, no client code (which includes test code) will be able to create an instance of CourseSession directly using a constructor. Clients must instead use the static factory method. Joshua Kerievsky names the above refactoring "Replace [Multiple[4]] Constructors with Creation Methods."[5] The class methods are the creation methods. The significant benefit is that you can provide descriptive names for creation methods. Since you must name a constructor the same as the class, it cannot always impart enough information for a developer to know its use. [4] Java allows you to code multiple constructors in a class. The descriptive names of creation methods are far more valuable in this circumstance, since they help a client developer determine which to choose. [5] [Kerievsky2004]. Now that you have a factory method to create CourseSession objects, tracking the total count can be done on the static side, where it belongs. Much of good object-oriented design is about putting code where it belongs. This doesn't mean that you must always start with code in the "right place," but you should move it there as soon as you recognize that there is a better place for it to go. It makes more sense for incrementCount message sends to be made from the static side: private CourseSession( String department, String number, Date startDate) { this.department = department; this.number = number; this.startDate = startDate; } public static CourseSession create( String department, String number, Date startDate) { incrementCount(); return new CourseSession(department, number, startDate); }