Synapseindia dotnet development chapter 14 event-driven programming

1
Chapter 14 Event-Driven
Programming
Chapter 12 GUI Basics
Chapter 14 Event-Driven Programming
Chapter 15 Creating User Interfaces
§10.2, “Abstract Classes,” in Chapter 10
Chapter 13 Graphics
Chapter 16 Applets and Multimedia
§10.4, “Interfaces,” in Chapter 10

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Objectives
To start with event-driven programming with a simple example
(§14.1).
To explain the concept of event-driven programming (§14.2).
To understand events, event sources, and event classes (§14.2).
To declare listener classes and write the code to handle events
(§14.3).
To register listener objects in the source object (§11.3).
To understand how an event is handled (§14.3).
To write programs to deal with ActionEvent (§14.3).
To write programs to deal with MouseEvent (§14.4).
To write programs to deal with KeyEvent (§14.5).
To use the Timer class to control animations (§14.6 Optional).

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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.

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Taste of Event-Driven Programming
 The example displays a button in the frame. A
message is displayed on the console when a button
is clicked.
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RRuunn

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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 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 Information
An 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 14.1 lists external user actions,
source objects, and event types generated.

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Selected User Actions
Source Event Type
User Action Object 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

+actionPerformed(event: ActionEvent)
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The Delegation Model
source: SourceClass
+addXListener(listener: XListener)
listener: ListenerClass
User
Action
Trigger an event
XListener
+handler(event: XEvent)
Register by invoking
source.addXListener(listener);
(a) A generic source component
with a generic listener
source: JButton
+addActionListener(listener: ActionListener)
ActionListener
listener: CustomListenerClass
Register by invoking
source.addActionListener((b) A JButton source component listener);
with an ActionListener

Keep it a list
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Internal Function of a Source
Component
source: SourceClass
+addXListener(XListener listener)
Keep it a list
event: XEvent listener1
listener2
…
listenern
(a) Internal function of a generic source object
+handler(
Invoke
listener1.handler(event)
listener2.handler(event)
…
listenern.handler(event)
An event is
triggered
source: JButton
+addActionListener(ActionListener listener)
event:
ActionEvent
(b) Internal function of a JButton object
listener1
listener2
…
listenern
+handler(
Invoke
listener1.actionPerformed(event)
listener2.actionPerformed(event)
…
listenern.actionPerformed(event)
An event is
triggered

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The Delegation Model: Example
JButton jbt = new JButton(OK);
ActionListener listener = new OKListener();
jbt.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
+getModifiers(): 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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Inner Class Listeners
A listener class is designed specifically to
create a listener object for a GUI
component (e.g., a button). It will not be
shared by other applications. So, it is
appropriate to define the listener class
inside the frame class as an inner class.

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Inner Classes
Inner class: A class is a member of another class.
Advantages: In some applications, you 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.
SShhoowwIInnnneerrCCllaassss

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Inner Classes, cont.
public class Test {
...
}
public class A {
...
}
public class Test {
...
// Inner class
public class A {
...
}
}
(a)
(b)
// OuterClass.java: inner class demo
public class OuterClass {
private int data;
/** A method in the outer class */
public void m() {
// Do something
}
// 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();
}
}
}
(c)

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Inner Classes (cont.)
 Inner classes can make programs simple and
concise.
 An inner class supports the work of its
containing outer class and is compiled into a
class named
OuterClassName$InnerClassName.class. For
example, the inner class InnerClass in
OuterClass is compiled into
OuterClass$InnerClass.class.

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Inner Classes (cont.)
 An inner class can be declared public,
protected, or private subject to the same
visibility rules applied to a member of the
class.
 An inner class can be declared static. A static
inner class can be accessed using the outer
class name. A static inner class cannot access
nonstatic members of the outer class

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Revising SimpleEventDemo Using
Inner Classes
SSiimmpplleeEEvveennttDDeemmooIInnnneerrCCllaassss
RRuunn

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Anonymous Inner Classes
 An anonymous inner class must always extend a superclass or
implement an interface, but it cannot have an explicit extends
or implements clause.
 An anonymous inner class must implement all the abstract
methods in the superclass or in the interface.
 An anonymous inner class always uses the no-arg constructor
from its superclass to create an instance. If an anonymous inner
class implements an interface, the constructor is Object().
 An anonymous inner class is compiled into a class named
OuterClassName$n.class. For example, if the outer class Test
has two anonymous inner classes, these two classes are
compiled into Test$1.class and Test$2.class.

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Anonymous Inner Classes (cont.)
Inner class listeners can be shortened using anonymous
inner classes. An anonymous inner class is an inner
class without a name. It combines declaring an inner
class and creating an instance of the class in one step.
An anonymous inner class is declared as follows:
new SuperClassName/InterfaceName() {
// Implement or override methods in superclass or interface
// Other methods if necessary
}

22
Revising SimpleEventDemo Using
Anonymous Inner Classes
SSiimmpplleeEEvveennttDDeemmooAAnnoonnyymmoouussIInnnneerrCCllaassss
RRuunn

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Example: Handling 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.
TTeessttAAccttiioonnEEvveenntt
RRuunn

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Interaction Between Source and
Listener
: TestActionEvent jbtCancel: JButton
jbtOK: JButton btListener: ButtonListener
1. addActionListener
2. addActionListener
3. actionPerformed
4. actionPerformed
1. jbtOK registers btListener by invoking
addActionListener(btListner).
2. jbtCancel registers btListener by invoking
addActionListener(btListner).
3. jbtOK invokes btListener’s actionPerformed method to process
an ActionEvnet.
4. jbtCancel invokes btListener’s actionPerformed method to
process an ActionEvent.

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: Handling Window Events
TTeessttWWiinnddoowwEEvveenntt RRuunn

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Example: Multiple Listeners for a Single
Source
Objective: This example modifies Listing 14.1 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.
TTeessttMMuullttiipplleeLLiisstteenneerr RRuunn

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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 for
actions such as dragging or moving the
mouse.

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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: 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.
MMoovveeMMeessssaaggeeDDeemmoo RRuunn

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Example: (Omitted)
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.
SSccrriibbbblleeDDeemmoo RRuunn

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Handling Keyboard Events
To process a keyboard event, use the following
handlers in the KeyListener interface:
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 then
released.

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The KeyEvent Class
Methods:
getKeyChar() method
getKeyCode() method
Keys:
Home VK_HOME
End VK_END
Page Up VK_PGUP
Page Down VK_PGDN
etc...

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

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The Timer Class
Optional
Some non-GUI components can fire events. The javax.swing.Timer
class is a source component that fires an ActionEvent at a predefined
rate.
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.
AAnniimmaattiioonnDDeemmoo RRuunn

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Clock Animation
In Chapter 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.
CClloocckkAAnniimmaattiioonn RRuunn

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