Chapter 12 Event-Driven Programming

Chapter 8 Inheri tance and Polymorphism Chapter 9 Abstract Classes and Interfaces Chapter 11 Getting Started with GUI Programming Chapter 12 Event - Driven Programming Chapter 13 Creating User Interfaces Chapter 14 Applets, Images, Audio Chapter 12 Event-Driven Programming

Objectives • To explain the concept of event-driven programming (§12.2). • To understand event, event source, and event classes (§12.2). • To declare listener classes and write the code to handle events (§12.3). • To register listener objects in the source object (§12.3). • To understand how an event is handled (§12.3). • To write programs to deal with ActionEvent (§12.3). • To write programs to deal with MouseEvent (§12.4). • To use the Timer class to control animations (§12.6).

Procedural vs. Event-Driven Programming • Procedural programming is executed in procedural order. • In event-driven programming, code is executed upon activation of events.

Events • An event can be defined as a type of signal to the program that something has happened. • The event is generated by external user actions such as mouse movements, mouse clicks, and keystrokes, or by the operating system, such as a timer.

Event Classes

Event Information An event object contains whatever properties are pertinent to the event. The subclasses of EventObject deal with special types of events, such as button actions, window events, component events, mouse movements, and keystrokes. Table 12.1 lists external user actions, source objects, and event types generated.

Selected User Actions Source Event TypeUser Action Object Generated Click a button JButtonActionEvent Click a check box JCheckBoxItemEvent, ActionEvent Click a radio button JRadioButtonItemEvent, ActionEvent Press return on a text field JTextFieldActionEvent Select a new item JComboBoxItemEvent, ActionEvent Window opened, closed, etc. WindowWindowEvent Mouse pressed, released, etc. ComponentMouseEvent Key released, pressed, etc. ComponentKeyEvent

The Delegation Model

The Delegation Model: Example ListenerClass listener = new ListenerClass(); JButton jbt = new JButton("OK"); jbt.addActionListener(listener);

Selected Event Handlers Event Class Listener Interface Listener Methods (Handlers)ActionEvent ActionListener actionPerformed(ActionEvent) ItemEvent ItemListener itemStateChanged(ItemEvent) WindowEvent WindowListener windowClosing(WindowEvent) windowOpened(WindowEvent) windowIconified(WindowEvent) windowDeiconified(WindowEvent) windowClosed(WindowEvent) windowActivated(WindowEvent) windowDeactivated(WindowEvent) ContainerEvent ContainerListener componentAdded(ContainerEvent) componentRemoved(ContainerEvent) MouseEvent MouseListener mousePressed(MouseEvent) mouseReleased(MouseEvent) mouseClicked(MouseEvent) mouseExited(MouseEvent) mouseEntered(MouseEvent) KeyEvent KeyListener keyPressed(KeyEvent) keyReleased(KeyEvent) keyTypeed(KeyEvent)

java.awt.event.ActionEvent

Example 12.1Handling Simple Action Events • Objective: Display two buttons OK and Cancel in the window. A message is displayed on the console to indicate which button is clicked, when a button is clicked. TestActionEvent

Example 12.3 Multiple Listeners for a Single Source • Objective: This example modifies Example 12.1 to add a new listener for each button. The two buttons OK and Cancel use the frame class as the listener. This example creates a new listener class as an additional listener for the action events on the buttons. When a button is clicked, both listeners respond to the action event. TestMultipleListener

MouseEvent

Handling Mouse Events • Java provides two listener interfaces, MouseListener andMouseMotionListener, to handle mouse events. • The MouseListener listens for actions such as when the mouse is pressed, released, entered, exited, or clicked. • The MouseMotionListener listens foractions such as dragging or moving themouse.

Handling Mouse Events

Example 12.4 Moving Message Using Mouse Objective: Create a program to display a message in a panel. You can use the mouse to move the message. The message moves as the mouse drags and is always displayed at the mouse point. MoveMessageDemo

Example 12.5 Handling Complex Mouse Events Objective: Create a program for drawing using a mouse. Draw by dragging with the left mouse button pressed; erase by dragging with the right button pressed. ScribbleDemo

The Timer Class Not all source objects are GUI components. The javax.swing.Timer class is a source component that fires an ActionEvent at a predefined rate. The Timer class can be used to control animations. For example, you can use it to display a moving message. AnimationDemo

Clock Animation In Section 11.12, you drew a StillClock to show the current time. The clock does not tick after it is displayed. What can you do to make the clock display a new current time every second? The key to making the clock tick is to repaint it every second with a new current time. You can use a timer to control how to repaint the clock. ClockAnimation

Chapter 12 Event-Driven Programming