JavaEvent Driven Programming
Writing GUI applications requires that program control be driven by the user’s interaction with the GUI. When the user moves the mouse, clicks on a button, or selects an item from a menu an “event” occurs.
JavaFX EventsJavaFX Events
The javafx.event
package provides the basic framework for FX events. The
Event
class serves as the base class for JavaFX events. Associated with each
event is an event source, an event target, and an event type.
- Source — GUI objects (e.g., a
Buttonobject) which generate an ActionEvent (e.g., when the button is clicked). The source implements theEventTargetinterface. - Listener — An object that subscribes (listens) for a particular type of event to occur. The listener must implement an interface like
EventHandlerorInvalidationListener. - Type — An additional classification that are used to specify the specific type of event. For example, a
MouseEventhas the following types:ANY,MOUSE_CLICKED,MOUSE_PRESSED,MOUSE_RELEASED,MOUSE_MOVED,MOUSE_DRAGGED,MOUSE_ENTERED,MOUSE_EXITED,MOUSE_ENTERED_TARGET,MOUSE_EXITED_TARGET, andDRAG_DETECTED.
The operating system or windowing system captures generated events and notifies the listener objects when a type of event they are listening for has occurred. The operating system notifies the listener object by sending a message to (calling) the event handling method. If no listener object is subscribed to listen for the event that occurred, the event is ignored.
Types of EventsTypes of Events
There are many subclasses of the Event class:
ActionEvent— widely used to indicate things like when a button is pressed.MouseEvent— indicates mouse activity.DragEvent— indicates a drag-and-drop gesture.KeyEvent— indicates that a keystroke has occurred.ScrollEvent— indicates scrolling by mouse wheel, track pad, touch screen, etc…TouchEvent— indicates a touch screen action
Event Handling in JavaFXEvent Handling in JavaFX
Here we will just consider ActionEvents. Other types of events are handled
in a similar way. When a Button is pressed, it generates an ActionEvent.
If I want my program to respond to a button click, I need to tell the button
what code should run.
We need to pass an object to setOnAction() that implements the EventHandler
interface. The EventHandler interface is a functional interface with one method:
void handle(T event).
Implementation as an Inner ClassImplementation as an Inner Class
It seems silly to need a separate class for ever event we want to handle. Also, if we want to access private attributes of the GUI, we can’t do that if we are in a different class. Solution: implement the interface as an inner class.
Solve: JavaFX Inner Class (Practice)
Implement a class, Question that contains a start(Stage stage) method.
In the start() method, you must create a Button object and set the
onAction event to an object from an inner class of the Question class.
The inner class must implement the EventHandler<ActionEvent> interface
causing "clicked" to be printed to the console whenever the event is triggered.
Implementation as an Anonymous Inner ClassImplementation as an Anonymous Inner Class
Since we will only create one instance of this handler class, we could declare it as an anonymous inner class.
Solve: JavaFX Anonymous Inner Class (Practice)
Create an EventHandler<ActionEvent> reference called handler and
assign it an object from an anonymous inner class that implements the
EventHandler<ActionEvent> interface. The handler must display
"clicked" to the console whenever triggered.
Lambda ExpressionLambda Expression
Functional interfaces can be implemented with a lambda expression which have a more concise syntax.
Solve: JavaFX Lambda (Practice)
Create an EventHandler<ActionEvent> reference called handler and
assign it to a lambda expression that causes the handler to
display "clicked" to the console whenever triggered.
Method ReferenceMethod Reference
Method references provide even more concise syntax. Here we pass a
method reference to setOnAction().
More Practice
Need more practice? Head over to the practice page.