1. Copyright © 2002, Systems and Computer Engineering, Carleton University. 94.204-17-EventModel.ppt 1 94.204* Object-Oriented Software Development Part 17 The Java Event Model revised March 2002

2. Copyright © 2002, Systems and Computer Engineering, Carleton University. 94.204-17-EventModel.ppt 2 The Java Event Model Java 1.1 has introduced a full-blown Java Event Model, which addresses the same type of needs as Observer/Observable while solving some of the latter’s limitations An event source can now encapsulate all relevant info in an event object that it sends to its event listeners. –all listeners implement a common interface defined for the particular event –the event source maintains a list of listeners –when a change occurs, the event source instantiates an event object and sends it by iterating over the list of listeners and invoking one of the methods defined in the listener interface The Java Event Model is heavily used in GUI design and also in the JavaBeans component model.

3. Copyright © 2002, Systems and Computer Engineering, Carleton University. 94.204-17-EventModel.ppt 3 The Java Event Model Let’s rearrange the Prof-Student example so that it implements the Event Model: –define a CourseEvent class that extends EventObject –define a CourseListener interface –have the Student class implement CourseListener –have the Prof class (the event source) manage a vector of courseListeners The naming conventions are important!

4. Copyright © 2002, Systems and Computer Engineering, Carleton University. 94.204-17-EventModel.ppt 4 The Java Event Model

5. Copyright © 2002, Systems and Computer Engineering, Carleton University. 94.204-17-EventModel.ppt 5 Java Event Model: sequence diagrams 1- Subscription

6. Copyright © 2002, Systems and Computer Engineering, Carleton University. 94.204-17-EventModel.ppt 6 Java Event Model: sequence diagrams 2- Notification

7. Copyright © 2002, Systems and Computer Engineering, Carleton University. 94.204-17-EventModel.ppt 7 The Java Event Model Implementation: take a look at the full signature of the CourseListener interface methods: void midtermAnnounced(CourseEvent e); void midtermPostponed(CourseEvent e); void assignmentPosted(CourseEvent e); –they all contain a CourseEvent, which can contain useful information –many objects can potentially send a CourseEvent. How can we determine the source of an event?

8. Copyright © 2002, Systems and Computer Engineering, Carleton University. 94.204-17-EventModel.ppt 8 The Java Event Model EventObject, the superclass of CourseEvent, provides a getSource() method. The source can be set when instantiating the event, by passing the event source as an argument: public void setMidterm(Date date) { midterm = date; //the prof creates the event CourseEvent e = new CourseEvent(this); //now send it… … }

9. Copyright © 2002, Systems and Computer Engineering, Carleton University. 94.204-17-EventModel.ppt 9 The Java Event Model The event source (the prof) has to manage its list of listeners (usually a Vector ) by providing a addCourseListener and a removeCourseListener method: public synchronized void addCourseListener (CourseListener cl){ courseListeners.addElement(cl); } public synchronized void removeCourseListener (CourseListener cl) { courseListeners.removeElement(cl); }

10. Copyright © 2002, Systems and Computer Engineering, Carleton University. 94.204-17-EventModel.ppt 10 The Java Event Model To send an event, the event source has to iterate through its list of listeners, and call the appropriate method: public void setMidterm(Date date) { midterm = date; //the prof creates the event and sends it CourseEvent e = new CourseEvent(this); Iterator iter=courseListeners.iterator(); while (iter.hasNext()) { ((CourseListener)iter.next()). midtermAnnounced(e); }

11. Copyright © 2002, Systems and Computer Engineering, Carleton University. 94.204-17-EventModel.ppt 11 The Java Event Model handling the event: public void assignmentPosted(CourseEvent e) { //examine the event source String name = ((Prof)e.getSource()).getName()); … // ooh it’s Prof E. !!! System.out.println("please push it back!"); } CourseEvent could also potentially have other interesting instance variables (the assignment content, deadline, …) that could be examined here.

12. Copyright © 2002, Systems and Computer Engineering, Carleton University. 94.204-17-EventModel.ppt 12 The Java Event Model Forces / Consequences: all the relevant info is gathered into an event object. the listener interface allows us to be more specific about the particular nature of the event, by providing different methods for different cases. adding a new method to handle a new case means that all the concrete listeners have to implement one new method. the Event source has to manage its list of listeners, which is a hassle.