Download Event-Driven Programming Event-Driven Programming

Event-Driven Programming
Reading: Chapter 16 (except 16.5)
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(c) 2011 Pearson Education, Inc. & Robert F. Kelly
Motivations
 Suppose you write a GUI program in which the user


enters the loan amount, annual interest rate, and number of years
click the Compute Loan button to obtain the monthly payment and
t t l paymentt
total
 How do you accomplish the task?
 You have to use event-driven programming to write the
code to respond to the button-clicking event.
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(c) 2011 Pearson Education, Inc. & Robert F. Kelly
Motivations
 Suppose you wish to write a program that animates a rising
flag
 How do yyou accomplish
p the task?
 One way is to use a timer in event-driven programming
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Objectives
 To describe events, event sources, and event classes (§16.2)
 To define listener classes, register listener objects with the source
object, and write the code to handle events (§16.3)
 To define listener classes using inner classes (§16.4)
 To get input from text field upon clicking a button (§16.7)
 To write programs to deal with

WindowEvent (§16.8)


MouseEvent (§16.10)
KeyEvent (§16.11)
 To simplify
p y codingg for listener classes usingg listener interface
adapters (§16.9)
 To use the Timer class to control animations (§16.12)
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Procedural vs. Event-Driven Programming
 Procedural programming is executed in procedural order.
 In event-driven programming, code is executed upon
activation of events
 Example – button click
Webb interfaces
W
i
f
are essentially
i ll
event-driven programming,
usually written in JavaScript
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Events
 An event can be defined as a type of signal to the program
that something has happened.
 The event is ggenerated byy
external user actions such as mouse movements, mouse clicks, and
keystrokes, or
 by the operating system, such as a timer

 The program can respond to or ignore an event
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Event Classes
 An event is an instance of an event class
Sub-classes of EventObject deal
with
ith special
i l ttypes off events,
t suchh as
button actions
EventObject
AWTEvent
ActionEvent
ContainerEvent
AdjustmentEvent
FocusEvent
ComponentEvent
InputEvent
ItemEvent
PaintEvent
TextEvent
WindowEvent
MouseEvent
KeyEvent
ListSelectionEvent
ChangeEvent
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Selected User Actions
 You can identify the source object of the event using the
getSource() instance method in the EventObject class
Selected User Actions
Source
Object
Event Type
Generated
Click a button
JButton
ActionEvent
Click a check box
JCheckBox
ItemEvent, ActionEvent
Click a radio button
JRadioButton
ItemEvent, ActionEvent
Press return on a text field
JTextField
ActionEvent
Select a new item
JComboBox
ItemEvent, ActionEvent
Window opened, closed, etc.
Window
WindowEvent
Mouse pressed, released, etc.
Component
MouseEvent
Key released, pressed, etc.
Component
KeyEvent
Subclasses inherit the ability to fire events – e.g., all subclasses
of Component can fire MouseEvent, KeyEvent, etc.
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The Delegation Model
 A source object fires an event
and a listener handles it
Text UML notation is nonstandard (and possibly
incomplete). This is likely a
non-exclusive association
Trigger an event
User
Action
source: SourceClass
XListener
+addXListener(listener: XListener)
+handler(event: XEvent)
Register by invoking
source.addXListener(listener);
(a) A generic source component
with a generic listener
listener: ListenerClass
sou rce: J Button
B tt
Acti on List ener
+addActionListener(l istener: Act ionListener)
(b) A JButton source component
with an Act ionListener
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+acti onPerfor med(event: ActionEvent)
Register by invoking
source.addActi onListen er(li stener);
lis tener: CustomLi stenerClass
ActionListener interface defines an actionPerformed method
(c) 2011 Pearson Education, Inc. & Robert F. Kelly
Listener Interfaces
 Java provides a listener interface for every type of event
Notice the naming: e.g., ActionListener is
associated with ActionEvent
Listener interface methods are
referred to as handlers
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Source Component Internals
 Diagram on left shows general case
 JButton example in diagram on right
source: SourceClass
source: JButton
+addXListener(XListener listener)
An event is
triggered
event: XEvent
Invoke
listener1.handler(event)
listener2 handler(event)
listener2.handler(event)
…
listenern.handler(event)
+addActionListener(ActionListener listener)
Keep it a list
An event is
triggered
listener1
listener2
…
li t
listenern
event:
ActionEvent
(a) Internal function of a generic source object
Keep it a list
Invoke
listener1.actionPerformed(event)
listener2 actionPerformed(event)
listener2.actionPerformed(event)
…
listenern.actionPerformed(event)
listener1
listener2
…
li t
listenern
(b) Internal function of a JButton object
An event causes the actionPerformed method
of all registered listeners to be invoked
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The Delegation Model: Example
 Textbook illustrates listeners with the code below
JButton jbt = new JButton(
JButton("OK");
OK );
ActionListener listener = new OKListener();
jbt.addActionListener(listener);
 More often, you just have some class you are using implement the
listener interface and then
JButton jbt = new JButton("OK");
jb
jbt.addActionListener(this);
dd
i
i
( hi )
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(c) 2011 Pearson Education, Inc. & Robert F. Kelly
Selected Event Handlers
Event Class
Listener Interface
Listener Methods (Handlers)
ActionEvent
ItemEvent
WindowEvent
ActionListener
ItemListener
WindowListener
ContainerEvent
ContainerListener
MouseEvent
MouseListener
KeyEvent
KeyListener
actionPerformed(ActionEvent)
itemStateChanged(ItemEvent)
windowClosing(WindowEvent)
windowOpened(WindowEvent)
windowIconified(WindowEvent)
windowDeiconified(WindowEvent)
windowClosed(WindowEvent)
windowActivated(WindowEvent)
windowDeactivated(WindowEvent)
componentAdded(ContainerEvent)
componentRemoved(ContainerEvent)
mousePressed(MouseEvent)
mouseReleased(MouseEvent)
mouseClicked(MouseEvent)
mouseExited(MouseEvent)
mouseEntered(MouseEvent)
keyPressed(KeyEvent)
keyReleased(KeyEvent)
keyTypeed(KeyEvent)
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java.awt.event.ActionEvent
 A high-level event generated by a component (such as a Button) when
the component-specific action occurs (such as being pressed)
java.util.EventObject
java.ut
. ve tObject
+getSource(): Object
Returns the object on which the event initially occurred.
java.awt.event.AWTEvent
java.awt.event.ActionEvent
+getActionCommand():
getActionCommand(): String
Returns the command string associated with this action. For a
button, its text is the command string.
+getModifiers(): int
Returns the modifier keys held down during this action event.
+getWhen(): long
Returns the timestamp when this event occurred. The time is
the number of milliseconds since January 1, 1970, 00:00:00
GMT.
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Example: ControlCircle2
 Program uses two buttons to control the size of a circle
 Pressing the “Enlarge” button causes events that increase the size of
the circle
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ControlCircle2.java …
public class ControlCircle2 extends JFrame {
private JButton jbtEnlarge = new JButton("Enlarge");
private JButton jbtShrink = new JButton("Shrink");
private CirclePanel canvas = new CirclePanel();
public ControlCircle2() {
JPanel panel = new JPanel(); // panel groups buttons
panel.add(jbtEnlarge);
panel.add(jbtShrink);
this.add(canvas, BorderLayout.CENTER);
thi
this.add(panel,
dd(
l BorderLayout.SOUTH);
B d L
t SOUTH)
jbtEnlarge.addActionListener(new Listener());
jbtShrink.addActionListener(new Listener());
}
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(c) 2011 Pearson Education, Inc. & Robert F. Kelly
… ControlCircle2.java …
class CirclePanel extends JPanel {
private int radius = 5; // Default circle radius
public void enlarge() {
Calls
C
ll tto enlarge
l
andd shrink
hi k
change the radius of the circle
radius++;
repaint();
}
public void shrink() {
radius--;
repaint();
}
We have not covered this in
class, but it is not essential to
understand this program
protected
t t d void
id paintComponent(Graphics
i tC
t(G
hi
g)
) {
super.paintComponent(g);
g.drawOval(getWidth() / 2 - radius, getHeight() / 2 radius,
2 * radius, 2 * radius);
}
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}
(c) 2011 Pearson Education, Inc. & Robert F. Kelly
… ControlCircle2.java …
public static void main(String[] args) {
JFrame frame = new ControlCircle2();
frame.setTitle("ControlCircle2");
frame.setLocationRelativeTo(null); // Center the frame
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(200, 200);
frame.setVisible(true);
}
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(c) 2011 Pearson Education, Inc. & Robert F. Kelly
… ControlCircle2.java
class Listener implements ActionListener {
public void actionPerformed(ActionEvent e) {
System.out.println(new java.util.Date(e.getWhen()));
if (e.getSource() == jbtEnlarge)
canvas.enlarge();
else if (e.getSource() == jbtShrink)
canvas.shrink();
}
}
Note that it is not good code to instantiate
two anonymous Listener objects
Alternate approach (not recommended by Liang) is to have ControlCircle2
implement ActionListener and then register the listener with:
jbtEnlarge.addActionListener(this);
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(c) 2011 Pearson Education, Inc. & Robert F. Kelly
Multiple Classes per File
 Notice in the ControlCircle2 example,
 the ControlCirle2.java file contained ControlCircle2, Listener,
and CirclePanel classes
 Listener and CirclePanel classes are contained within the
ControlCircle2 class
 Initially, Java allowed multiple classes in a file (if at most one
was public) because it was convenient to include some helper
classes in the same file with a larger class
 Later versions of Java allowed classes to be contained within
other classes (inner classes)
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Inner Classes
 Inner class: A class is a member of another class
 Advantages:
g In some applications,
pp
, yyou can use an inner
class to make programs simple
 An inner class can reference the data and methods
defined in the outer class in which it nests, so you do not
need to pass the reference of the outer class to the
constructor of the inner class
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Inner Class Features
 Inner class can reference the data and methods defined in the





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outer class
I
Inner
class
l can bbe defined
d fi d static
t ti
Objects in the inner class are often created in the outer class
Reduces the number of source files
Compiled into a class named OuterClassName$InnerClassName
ControlCircle2 as it appears in the classes directory
(c) 2011 Pearson Education, Inc. & Robert F. Kelly
Inner Class Structure
// OuterClass.java: inner class demo
public class OuterClass {
private int data;
public class Test {
...
}
/** A method in the outer class */
/
/
public void m() {
// Do something
}
public class A {
...
}
(a) 2 classes
in a file
// An inner class
class InnerClass {
/** A method in the inner class */
public void mi() {
// Directly reference data and method
// defined in its outer class
data++;
m();
}
}
public class Test {
...
// Inner class
public class A {
...
}
}
}
(b) Class with
inner class
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(c)
(c) 2011 Pearson Education, Inc. & Robert F. Kelly
Window Events
 Previously, we examined action events
 Now we look at window events
 Any subclass of Window can fire:
 Window opened
 Window closing
 Window closed
 Window activated
 Window deactivated
 Window iconified
 Window deiconified
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Example
 TestWindowEvent.java
 When a window event occurs, a message is written to the console
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(c) 2011 Pearson Education, Inc. & Robert F. Kelly
TestWindowEvent …
public class TestWindowEvent extends JFrame {
public static void main(String[] args) {
TestWindowEvent frame = new TestWindowEvent();
frame.setLocationRelativeTo(null); // Center the frame
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setTitle("TestWindowEvent");
frame.setSize(220, 80);
frame.setVisible(true);
}
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(c) 2011 Pearson Education, Inc. & Robert F. Kelly
… TestWindowEvent
Anonymous class listener
public TestWindowEvent() {
addWindowListener(new WindowListener() {
public void windowActivated(WindowEvent event) {
System.out.println("Window activated"); }
public void windowDeactivated(WindowEvent event) {
System.out.println("Window deactivated"); }
public void windowOpened(WindowEvent event) {
System.out.println("Window opened"); }
...
});
}
}
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(c) 2011 Pearson Education, Inc. & Robert F. Kelly
Listener Interface Adapters
 Methods in WindowsListener are abstract
 You must implement all the methods to satisfy the interface
((even th
thoughh you might
i ht nott needd some))
 Convenience adapters provide default implementations of all
the methods in the interface
 Example –WindowAdapter class implements all the methods
for WindowListener
 Instantiate the adapter and override the methods you need
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(c) 2011 Pearson Education, Inc. & Robert F. Kelly
Mouse Events
 Mouse event is fired when a mouse is
 Pressed
 Released
R l d
 Clicked
 Moved
 Dragged on a component
 Mouse event object captures the event (e.g., location)
 MouseListener handles mouse events
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(c) 2011 Pearson Education, Inc. & Robert F. Kelly
MouseEvent
java.awt.event.InputEvent
+getWhen(): long
Returns the timestamp when this event occurred.
+isAltDo n(): boolean
+isAltDown():
R t
Returns
whether
h th or nott the
th Alt modifier
difi is
i down
d
on this
thi event.
t
+isControlDown(): boolean
Returns whether or not the Control modifier is down on this event.
+isMetaDown(): boolean
Returns whether or not the Meta modifier is down on this event
+isShiftDown(): boolean
Returns whether or not the Shift modifier is down on this event.
java.awt.event.MouseEvent
+getButton(): int
Indicates which mouse button has been clicked.
+ Cli kC
+getClickCount():
() int
i
R
Returns
the
h number
b off mouse clicks
li k associated
i d with
i h this
hi event.
+getPoint(): java.awt.Point
Returns a Point object containing the x and y coordinates.
+getX(): int
Returns the x-coordinate of the mouse point.
+getY(): int
Returns the y-coordinate of the mouse point.
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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 for actions such as
dragging or moving the mouse
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(c) 2011 Pearson Education, Inc. & Robert F. Kelly
Handling Mouse Events
java.awt.event.MouseListener
+mousePressed(e: MouseEvent): void
Invoked when the mouse button has been pressed on the
source component.
+mouseReleased(e: MouseEvent): void
Invoked when the mouse button has been released on the
source component.
+mouseClicked(e: MouseEvent): void
Invoked when the mouse button has been clicked (pressed and
released) on the source component.
+mouseEntered(e: MouseEvent): void
Invoked when the mouse enters the source component.
+mouseExited(e: MouseEvent): void
Invoked when the mouse exits the source component.
java.awt.event.MouseMotionListener
+mouseDragged(e: MouseEvent): void
Invoked when a mouse button is moved with a button pressed.
+mouseMoved(e: MouseEvent): void
Invoked when a mouse button is moved without a button
pressed.
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Example: Moving Text Using Mouse
 Objective: Create a program to display a message in a panel.
And use the mouse to move the message
 The message moves as the mouse drags and is always
displayed at the mouse point.
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(c) 2011 Pearson Education, Inc. & Robert F. Kelly
MoveMessageDemo …
public class MoveMessageDemo extends JFrame {
public MoveMessageDemo() {
MovableMessagePanel p =
new MovableMessagePanel(
MovableMessagePanel("Welcome
Welcome to Java");
Java );
setLayout(new BorderLayout());
add(p);
}
public static void main(String[] args) {
MoveMessageDemo frame = new MoveMessageDemo();
frame.setTitle("MoveMessageDemo");
frame.setTitle(
MoveMessageDemo );
frame.setLocationRelativeTo(null); // Center the frame
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(200, 100);
frame.setVisible(true);
}
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… MoveMessageDemo
public MovableMessagePanel(String s) {
message = s;
addMouseMotionListener(new MouseMotionAdapter() {
/
/**
Handle mouse dragged event */
/
public void mouseDragged(MouseEvent e) {
// Get the new location and repaint the screen
x = e.getX();
y = e.getY();
repaint();
}});}
protected void paintComponent(Graphics g) {
super.paintComponent(g);
g.drawString(message, x, y);
}}}
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(c) 2011 Pearson Education, Inc. & Robert F. Kelly
Handling Keyboard Events

keyPressed(KeyEvent e) -

keyReleased(KeyEvent
y
( y
e)
) -

keyTyped(KeyEvent e) -
Called when a key is pressed
Called when a keyy is released
Called when a key is pressed and
then released.
Similar to other event
handling approaches
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(c) 2011 Pearson Education, Inc. & Robert F. Kelly
The Timer Class
 Some non-GUI components can fire events
 The javax.swing.Timer class is a source component that fires
an ActionEvent at a predefined
d f d rate.
javax.swing.Timer
+Timer(delay: int, listener:
ActionListener)
Creates a Timer with a specified delay in milliseconds and an
ActionListener.
+addActionListener(listener:
ActionListener): void
+start(): void
Adds an ActionListener to the timer.
+stop(): void
Stops this timer.
+setDelay(delay: int): void
Sets a new delay
d l value
l for
f this
hi timer.
i
Starts this timer.
 The Timer class can be used to control animations. For
example, you can use it to display a moving message.
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(c) 2011 Pearson Education, Inc. & Robert F. Kelly
Timer Example: Clock Animation
 Chapter 15 includes code for a StillClock to show the
current time, but the clock does not tick after it is displayed
 This
h example
l uses a Timer to animate the
h clock
l k
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(c) 2011 Pearson Education, Inc. & Robert F. Kelly
ClockAnimation
StillClock draws a clock
with the current time
public class ClockAnimation extends JFrame {
private StillClock clock = new StillClock();
public ClockAnimation() {
add(clock);
Timer timer = new Timer(1000, new TimerListener());
timer.start(); }
private class TimerListener implements ActionListener {
public void actionPerformed(ActionEvent e) {
clock.setCurrentTime();
clock.repaint(); } }
public static void main(String[] args) {
JFrame frame = new ClockAnimation();
frame.setTitle("ClockAnimation");
frame.setSize(200, 200);
frame.setLocationRelativeTo(null);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true); }
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}
(c) 2011 Pearson Education, Inc. & Robert F. Kelly
Have You Satisfied the Objectives
 To describe events, event sources, and event classes (§16.2)
 To define listener classes, register listener objects with the source
object, and write the code to handle events (§16.3)
 To define listener classes using inner classes (§16.4)
 To get input from text field upon clicking a button (§16.7)
 To write programs to deal with

WindowEvent (§16.8)


MouseEvent (§16.10)
KeyEvent (§16.11)
 To simplify
p y codingg for listener classes usingg listener interface
adapters (§16.9)
 To use the Timer class to control animations (§16.12)
40
(c) 2011 Pearson Education, Inc. & Robert F. Kelly