Event-Driven Programming Event:  A signal to the program that something has happened.  It can be triggered either by external user actions, such as mouse movements, button clicks, and keystrokes, or by the operating system, such as a timer.  The program can choose to respond to or ignore an event. Source Object:  The component on which an event is generated is called the source object. button, button click

User Action, Source Object and Event type User ActionSource ObjectEvent Click a buttonJButtonActionEvent Press returnJTextFieldActionEvent Select itemsJListListSelectionEvent Click check boxJCheckBoxItemEvent Window openedWindowWindowEvent

A simple example Write a program that display two buttons. OK and Cancel, in a window. A message is displayed on the console to indicate which button and when it is clicked

Listeners, Registrations, and Handling Events An external user action on a source object triggers an event, and an listener receives the event. The listener must implement the corresponding event-listener interface The listener must be registered by the source object

Listener interface Java provides a listener interface for every type of GUI event. The listener interface is usually names XListener for XEvent, with excpetion of MouseMotionListener For example, listener interface for ActionEvent is ActionListener Each listener for ActionEvent should implement the ActionListener interface The listener interface contains the methods, know as the handlers, which process the events

Events, Event Listeners, and Listener Methods Event ClassListener interfaceMethods(handlers) ActionEventActionListenerActionPerformed ItemEventItemListeneritemStateChanged WindowEventWindowListenerwindowClosing KeyEventKeyListenerkyPressed MouseEventMouseListener MouseMotionListe ner mousePressed mouseDragged

Registration The listener object must be registered by the source object. Registration methods are dependent on the event type. For the ActionEvent, the method is addActionListener In general, the method is named addXListener for the XEvent ListenerClass lis = new ListenerClass(); JButton jbt – new JButton(“OK”); Jbt.addActionListener(lis);

Handling the Event When you click the button, the JButton object generates an ActionEvent and passes it to invoke the actionPerformed method to handle the event The event object contains information pertinent to the event type