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Chapter 3 Event-Driven Programming

Prerequisites for Part III

Chapter 11 Getting Started with GUI Programming

Chapter 8 Inheritance and Polymorphism

Chapter 13 Creating User Interfaces

Chapter 14 Applets, Images, Audio

Chapter 9 Abstract Classes and Interfaces

Chapter 12 Event-Driven Programming

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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

(§11.3). To register listener objects in the source object (§11.3). To understand how an event is handled (§11.3). To write programs to deal with ActionEvent (§11.3). To write programs to deal with MouseEvent (§11.4). To write programs to deal with KeyEvent (§11.5). To use the Timer class to control animations (§11.6 Optional).

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Procedural vs. Event-Driven Procedural vs. Event-Driven ProgrammingProgramming

Procedural programming is executed in procedural order.

In event-driven programming, code is executed upon activation of events.

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EventsEvents 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.

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Event ClassesEvent Classes

The root class of the event classes is java.util.EventObject. The hierarchical relationships of some event classes are shown in Figure 3.1.

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Event InformationEvent InformationAn event object contains whatever properties are pertinent to the event. You can identify the source object of the event using the getSource() instance method in the EventObject class. The subclasses of EventObject deal with special types of events, such as button actions, window events, component events, mouse movements, and keystrokes. Table 3.1 lists external user actions, source objects, and event types generated.

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Selected User ActionsSource Event Type

User Action Object GeneratedClicked on a button JButton ActionEvent

Double-clicked on a list item JList ActionEvent

Selected or deselected an item JList ItemEvent with a single clickSelected or deselected an item JComboBox ItemEvent

Press the mouse on a component Component MouseEvent

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The Delegation Model

source: SourceClass

+addXListener(XListener listener)

listener: ListenerClass User Action

Trigger an event

XListener +handler(XEvent event)

Internal function of the source object

event: XEvent listener1 listener2 … listenern

+handler(XEvent

Register by invoking source.addXListener(listener);

Keep it a list

Invoke listener1.handler(event) listener2.handler(event) … listenern.handler(event)

FIGURE 3.2 An event is triggered by user actions on the source object; the source object generates the event object and invokes the handler of the listener object to process the event.

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The Delegation Model: Example

source: JButton

+addActionListener(ActionListener listener)

listener: ListenerClass

ActionListener +actionPerformed(ActionEvent event)

Register by invoking source.addActionListener(listener);

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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)

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java.awt.event.ActionEvent

java.awt.event.ActionEvent +getActionCommand(): String

+getModifier(): int +getWhen(): long

Returns the command string associated with this action. For a button, its text is the command string.

Returns the modifier keys held down during this action event. Returns the timestamp when this event occurred. The time is

the number of milliseconds since January 1, 1970, 00:00:00 GMT.

java.util.EventObject +getSource(): Object

Returns the object on which the event initially occurred.

java.awt.event.AWTEvent

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Example 3.1Handling 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

Run

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Example 3.2Handling Window Events

TestWindowEvent Run

Objective: Demonstrate handling the window events. Any subclass of the Window class can generate the following window events: window opened, closing, closed, activated, deactivated, iconified, and deiconified. This program creates a frame, listens to the window events, and displays a message to indicate the occurring event.

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Example 3.3 Multiple Listeners for a Single Source

TestMultipleListener Run

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.

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MouseEvent

java.awt.event.MouseEvent +getButton(): int +getClickCount(): int +getPoint(): java.awt.Point +getX(): int +getY(): int

Indicates which mouse button has been clicked. Returns the number of mouse clicks associated with this event. Returns a Point object containing the x and y coordinates. Returns the x-coordinate of the mouse point. Returns the y-coordinate of the mouse point.

java.awt.event.InputEvent +getWhen(): long +isAltDown(): boolean +isControlDown(): boolean +isMetaDown(): boolean +isShiftDown(): boolean

Returns the timestamp when this event occurred. Returns whether or not the Alt modifier is down on this event. Returns whether or not the Control modifier is down on this event. Returns whether or not the Meta modifier is down on this event Returns whether or not the Shift modifier is down on this event.

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Handling Mouse Events Java provides two listener interfaces, MouseListener and MouseMotionListener, 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.

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Handling Mouse Events java.awt.event.MouseListener

+mousePressed(e: MouseEvent): void

+mouseReleased(e: MouseEvent): void

+mouseClicked(e: MouseEvent): void

+mouseEntered(e: MouseEvent): void +mouseExited(e: MouseEvent): void

Invoked when the mouse button has been pressed on the source component.

Invoked when the mouse button has been released on the source component.

Invoked when the mouse button has been clicked (pressed and released) on the source component.

Invoked when the mouse enters the source component. Invoked when the mouse exits the source component.

java.awt.event.MouseMotionListener

+mouseDragged(e: MouseEvent): void +mouseMoved(e: MouseEvent): void

Invoked when a mouse button is moved with a button pressed. Invoked when a mouse button is moved without a button

pressed.

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Example 3.4 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 Run

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Example 3.5 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 Run

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Handling Keyboard Events

keyPressed(KeyEvent e)Called when a key is pressed.

keyReleased(KeyEvent e) Called when a key is released.

keyTyped(KeyEvent e) Called when a key is pressed and thenreleased.

To process a keyboard event, use the following handlers in the KeyListener interface:

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The KeyEvent Class Methods:getKeyChar() method

getKeyCode() method

Keys:Home VK_HOMEEnd VK_EndPage Up VK_PGUPPage Down VK_PGDNetc...

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The KeyEvent Class, cont.

java.awt.event.KeyEvent +getKeyChar(): char +getKeyCode(): int

Returns the character associated with the key in this event. Returns the integer keyCode associated with the key in this event.

java.awt.event.InputEvent

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Example 4.6Keyboard Events Demo

Objective: Display a user-input character. The user can also move the character up, down, left, and right using the arrow keys.

KeyboardEventDemo Run

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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.

Optional

javax.swing.Timer

+Timer(delay: int, listener: ActionListener)

+addActionListener(listener: ActionListener): void

+start(): void +stop(): void +setDelay(delay: int): void

Creates a Timer with a specified delay in milliseconds and an ActionListener.

Adds an ActionListener to the timer. Starts this timer. Stops this timer. Sets a new delay value for this timer.

The Timer class can be used to control animations. For example, you can use it to display a moving message.

AnimationDemo Run

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Clock Animation

In Section 3.12, 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 Run