EVENT-DRIVEN PROGRAMMING AND ANIMATIONS - Introduction to Java Programming: Comprehensive Version - page 588

OKHandlerClass is invoked to process the event. The object handler2 is an instance
of CancelHandlerClass (line 20), which is registered with the button btCancel in line
21. When the Cancel button is clicked, the handle(ActionEvent) method (line 41) in
CancelHandlerClass is invoked to process the event.
You now have seen a glimpse of event-driven programming in JavaFX. You prob-
ably have many questions, such as why a handler class is defined to implement the
EventHandler<ActionEvent> . The following sections will give you all the answers.

15.2 Events and Event Sources
An event is an object created from an event source. Firing an event means to create an
event and delegate the handler to handle the event.

Key
Point

When you run a Java GUI program, the program interacts with the user, and the events drive
its execution. This is called event-driven programming. An event can be defined as a signal
to the program that something has happened. Events are triggered by external user actions,
such as mouse movements, mouse clicks, and keystrokes. The program can choose to respond
to or ignore an event. The example in the preceding section gave you a taste of event-driven
programming.
The component that creates an event and fires it is called the event source object , or simply
source object or source component . For example, a button is the source object for a button-
clicking action event. An event is an instance of an event class. The root class of the Java
event classes is java.util.EventObject . The root class of the JavaFX event classes is
javafx.event.Event . The hierarchical relationships of some event classes are shown in
Figure 15.4.

event-driven programming
event

fire event
event source object
source object

ActionEvent

MouseEvent

EventObject Event InputEvent

KeyEvent

JavaFX event classes are in
the javafx.event package

WindowEvent

F IGURE 15.4

An event in JavaFX is an object of the javafx.event.Event class.

An event object contains whatever properties are pertinent to the event. You can identify
the source object of an event using the getSource() instance method in the EventObject
class. The subclasses of EventObject deal with specific types of events, such as action
events, window events, mouse events, and key events. The first three columns in Table 15.1
list some external user actions, source objects, and event types fired. For example, when click-
ing a button, the button creates and fires an ActionEvent , as indicated in the first line of this
table. Here, the button is an event source object, and an ActionEvent is the event object fired
by the source object, as shown in Figure 15.3.

event object
getSource()

Note
If a component can fire an event, any subclass of the component can fire the same type of
event. For example, every JavaFX shape, layout pane, and control can fire MouseEvent
and KeyEvent since Node is the superclass for shapes, layout panes, and controls.