1. Graphical User Interface (GUI) Programming II

2. Lecture Objectives
Understand the Event-Handling Processes in Java

3. Caution Note!!!
This lecture presentation contains a number of "hidden" slides. Feel free to read, study and learn from them on your own!!!

4. Outline for Today's Fun Events: Swing Components: What is an event?
Simple (?) Example
Swing Components:
JFrames
JComponents
An example
Swing Component Design (MVC/UI-Delegate??)

5. Events
Behind the scenes, the Java runtime environment is monitoring many things
When any of a number of things happen an event is said to occur. Sometimes the terminology is “an event gets fired“
Examples of the types of things that can "fire" events:
Pressing a key on the keyboard
Clicking on a component (like a button)
Entering a component with the mouse pointer
Have a timer "time-out"

6. Events (Cont’d)
Moving the mouse around any reasonably complicated GUI can literally cause hundreds if not thousands of events to occur.
Events will be ignored except for the ones that you tell Java that you are interested in doing something about.
Java maintains a data structure of all the events that you have decided to handle and looks up events and does what you tell it to do.

7. Code from the Past: Remember???
import java.awt.*; public class HelloGUI { public static void main (String[ ] args) { System.out.println("About to make GUI"); Frame f = new Frame ("Hello GUIs"); f.setSize( 200, 200 ); f.show(); System.out.println("Finished making GUI"); }// main }// class HelloGUI

8. Code from the Past: Remember???
What didn't work???

9. Making It Work (MIT Approach)!
Determine which event occurs when the "Close the Window" button is pressed:
The API is your friend
The lecture notes are your friend
Hint: In this case it's an event called "Window Closing"
You decide what class is going to handle this event
It might be the actual class which has the window.
It can be any other class.

10. Making It Work (MIT Approach)!
Write the method (and class?) that will handle the event: When this event occurs, Java is going to go to the class that you identify as the event handler or Listener. It will look for a method called:
public void windowClosing(WindowEvent e)
Java will report an error to you if this class doesn't have this method. How might the designers of Java be guaranteed that you will implement this method?

11. Interfaces: Back to School!
// Note: found in java.awt.event public interface WindowListener extends EventListener { void windowActivated(WindowEvent e); void windowClosed(WindowEvent e); void windowClosing(WindowEvent e); void windowDeactivated(WindowEvent e); void windowDeiconified(WindowEvent e); void windowIconified(WindowEvent e); void windowOpened(WindowEvent e); }

12. So we could write a class as shown below:
Using Event Interfaces
Outline:
Import
Class definition
Implementation
Dummy (lazy) implementation
So we could write a class as shown below:
import java.awt.*; import java.awt.event.*; public class Handler implements WindowListener { public void windowActivated(WindowEvent e) {} public void windowClosed(WindowEvent e) {} public void windowClosing(WindowEvent e) { Window w = e.getWindow(); w.setVisible(false); w.dispose(); System.exit(0); } public void windowDeactivated(WindowEvent e) {} public void windowDeiconified(WindowEvent e) {} public void windowIconified(WindowEvent e) {} public void windowOpened(WindowEvent e) {}

13. Using Event Interfaces (Cont’d)
Register the listener with Java. That is, tell Java in which class the method will be located to run when the Window Closing Event occurs.

14. Event Handler Registration
import java.awt.*;
public class HelloGUI {
public static void main (String[] args) {
Handler h = new Handler();
System.out.println ("About to make GUI");
Frame f = new Frame ("Hello GUIs");
f.setSize( 200, 200 );
f.addWindowListener(h);
f.show();
System.out.println("Finished making GUI");
} // main
} // class HelloGUI

15. Interface WindowListener
Demonstration
Class Pool
class HelloGUI
main {
Frame f
Handler h }
class Frame
class Handler
Interface WindowListener
Frame
Instance
Handler
Instance

16. Event Handling: Key Ideas
Determine which event occurs
Decide what class is going to handle this event
Write the method (and class?) that will handle the event.
Register the listener with Java.

17. Today’s First Wisdom
Very important that you understand this simple example to understand the concepts that follow.

18. Potential Points of Confusion
What exactly is the listener? Is it the component getting clicked on?
No, the listener is the object that contains the method that Java will call when the event happens.
You must tell the component getting clicked which object that is by registering: addWindowListener...
As we will see it could be the same!!!
What type of listener do I use?
There are only so many.
See the Java API.
Lecture/Instructor/Lab/WebCT/Classmates/Friends.
Try yourself and experience the excitement!

19. Potential Points of Confusion (Cont’d)
What about all those other window things (e.g. windowActivated)
We actually did implement them (with empty bodies) – Lazy or dummy implementation!
We said: Don't do anything!

20. Outline for Today's Fun: Reminder
Events:
What is an event?
Simple (?) Example
Swing Components:
JFrames
JComponents
An example
Swing Component Design (MVC/UI-Delegate)

21. A Call from the Past! Earlier, we cautioned about
the existence of two toolkits
in Java for creating GUIs:
AWT
Swing
Today, we examine a few Swing components.
The goal is to learn how Swing components in general are designed, so that you can make better use of the API.

22. Swing: Sun’s Response to Microsoft?
In 1997, Sun announced a new graphical toolkit for Java called the “Java Foundation Classes”, or “JFC”.
This is usually called “Swing”.
The JFC/Swing classes provide well designed, powerful widgets for GUI developers.
Let’s take a look . . .

23. Welcome to JFC/Swing Planet
“Now it gets interesting “

24. Historical Problems with AWT
All AWT components required runtime peer resources:
Slow on some platforms (notably Windows)
Portability problems (slightly different look, some behaviors different)
Least common denominator phenomenon: If one OS (e.g., Windows) did not support a widget, the entire AWT had to suffer without it.
Limited AWT widget library
Addressed somewhat by JDK 1.1b3+, which allowed subclassing of components, or “lightweights” (not covered in this course)
java.awt.Button
WinNT
ButtonPeer
MacOS
Motif
AWT components required
native “peer” methods to
render and operate--many steps!
CLICK ME
File Edit Help
Slow & Inflexible!

25. Fixing AWT Problems (Layering of components requires layout
Developers avoided a few AWT limitations through:
Pervasive use of lightweights (again, not covered in cs1312).
e.g., tooltip simulation through threads/windows/components
extensive coding around rigid look
use of layered gifs
(Layering of
components
requires layout
subclasses)
Bottom line: Making stuff look cool
or satisfying a client’s request
could be a nightmare!
(Menu bars limited;
no images possible
without *extensive*
simulation through
components)
CLICK ME
File Edit Help
Tooltip
(Tooltips required
threads,
window
subclasses &
extensive
event handling)
(Image buttons required
component subclassing,
methods to handle ‘click’
look, as well as event
handlers)

26. Introducing Swing/JFC
Sun addressed these problems by producing Java Foundation Classes (JFC) i.e., Swing
Key elements:
No reliance on native peers; the JVM does the work, and is faster
Swing completely controls look and feel of all components:
PLAF, or “pluggable look and feel”
Superior design: MVC-esque (Model View Control)
CLICK ME
File Edit Help
javax.swing.JButton
Fast, flexible, extensible!

27. Swing Packages javax.swing.*
All the new Swing components and classes need a home. Where? A subject of great debate!
For JDK 1.1, Sun used “com.sun.java.swing”; developers revolted.
Problem: developers complained this “com” designation was not appropriate for “core” class--something part of language.
Solution:
Why “javax”?
* logical grouping
* minimizes transition
costs
* most developers
happy with it
* helps maintain
existing JDK 1.1 code
(cf. MFC lib breaks)
javax.swing.*
Denotes ‘extension’ package that
has migrated to core status

28. Overview of JFC/Swing Packages
javax.swing.plaf
javax.swing.plaf.basic
javax.swing.plaf.metal
javax.swing.plaf.multi
javax.swing.text
javax.swing.text.html
javax.swing.text.html.parser
javax.swing.text.rtf
javax.swing
javax.swing.table
javax.swing.tree
javax.swing.border
javax.swing.colorchooser
javax.swing.filechooser
javax.swing.event
javax.swing.undo

29. Overview of the Overview
Components, including
“aggregate”
or complex
components
Packages to
control the
“look and feel”
of Swing
Text-based widgets
(including html/rtf display)
New event packages

30. Short Examples 1.

31. Widget Example: JButtons
The java.swing.JButton class implements a “state version” of a java.swing.AbstractButton. Many methods come from the abstract class:
A variety of constructors (Strings, Icons, etc.);
setRolloverEnabled(boolean);
setIcon(Icon);
setRolloverIcon(Icon);
setActionCommand(String); -- an extra String tacked onto the event that gets fired!
setContentAreaFilled(boolean) -- transparency for icons!
setModel(ButtonModel); -- sets the ‘type’ of Button (you can define your own Button behaviors!)
setMnemonic(char/int); -- set mnemonics for button
Lesson: Check the API
for useful behaviors.

32. Cool Buttons Images from disk: Sets icon and rollover Icon.
import javax.swing.*; public class HelloWorld2 extends JFrame { public JButton bNew; public JPanel p; public HelloWorld2() { bNew = new JButton("New Document", new ImageIcon("Document.gif")); bNew.setRolloverEnabled(true); bNew.setRolloverIcon(new ImageIcon("New.gif")); p = new JPanel(); p.add(bNew); getContentPane().add(p); this.pack(); } public static void main(String arg[ ]) { new HelloWorld2().show(); } }// class HelloWorld2
Images from disk:
Sets icon and
rollover Icon.
Note: Icon constructor
took String argument,
and automatically
loaded image

33. Why getContentPane() ?
The HelloWorld example required us to call getContentPane() before “add()”ing an object to the JFrame:
myFrameInstance.getContentPane().add(myComponent);
Required of
JFrame, JDialog
and JInternalFrame
instances
Usually
“this”
E.g.,
“myJButton”
This differs from traditional AWT container additions, where we simply call “add”, passing in the component.
Let’s cut a JFrame open to find out why . . .

34. A JFrame Autopsy “The Pop Tart / Sandwich Duality” JLayeredPane Menu
click
JLayeredPane
JPanel
ContentPane
Menu
A javax.swing.JFrame is composed of a transparent “glassPane” surface, and an inner JPanel with Contents and Menu
click
A java.awt.Frame is composed of a single container--the Frame object itself. It is flat as a pop tart.

35. JFrame Class View AWT JFC Component GlassPane Frame Container
contains
manages
ContentPane
JComponent
JFC
contains
manages
JMenuBar
JRootPane
JFrame
contains
contains
The JRootPane is a container with a JLayeredPane (holding the Menu and ContentPane) and a Component GlassPane. It serves as base for numerous classes.
contains
JLayeredPane

36. JRootPane: The Content Pane
The JRootPane contains only two components:
the JLayeredPane and
the Component GlassPane
Its layout manager ignores all attempts to add new components.
Instead, one must add to the JRootPane’s ContentPane, found inside the JLayeredPane.
A call to getContentPane() returns an instance of the ContentPane.

37. JRootPane: The Glass Pane
public Component getGlassPane();
public void setGlassPane(Component);
JFrame Blue Print
We can use the top “glassPane”
as a drawing area. Since it spans the entire JFrame, we can draw on top of menu bars, and every component.
The JPanel has a remarkable layering feature, allowing us to stack and shuffle components.
public JPanel getContentPane();

38. JFrame Disposal E.g.,: /* behaves just like java.awt.Frame */
JFrame allows you to configure how it responds to closure.
Default: hides JFrame on closure attempt.
To modify: invoke setDefaultCloseOperation().
E.g.,:
MyJFrameInstance.setDefaultCloseOperation(WindowConstants.DO_NOTHING_ON_CLOSE);
/* behaves just like java.awt.Frame */
other constants in javax.swing.WindowConstants:
HIDE_ON_CLOSE - invokes any registered WindowListener object, then hides. This is default behavior.
DISPOSE_ON_CLOSE - invokes any registered WindowListener object, and then disposes.

39. JComponent: The Generic Widget
The JComponent provides the basis for all Swing components.
JComponent extends java.awt.Container, making all Swing components large, powerful widgets. (Also, all Swing components are also containers--even if you wouldn’t normally place things in them. E.g., JButton)
In turn, JComponent is subclassed by numerous widgets. Thus, composition is favored over inheritance for widget manipulation.

40. Jcomponent (Cont’d)
Since JComponent is the basis for most Swing components, all Swing widgets have the following general features:
Borders
-- JComponent derived classes can have borders
Accessibility
-- JComponents use Swing’s accessibility features to provide additional information about the widget.
Tooltips
-- JComponents can have time sensitive tooltips.
Double Buffering
-- By default, Swing components have double buffering built in
Serialization
-- Ability to save state to a file.

41. Events Here, we review event handling one more time.
To understand how events work in Java, we have to look closely at how we use GUIs.
When you interact with a GUI, there are many events taking place each second. Only a few of these, however, may actually be ‘delivered’ to the application.

42. Events (Cont’d)
Java uses a “delegation” event model found in many other toolkits.
Under the delegation model, components fire events, which can be caught and acted on by listeners. A listener is linked to a component through a registration process.
The delegation event model is contrasted to an event filtration model where all events are delivered to target components regardless of whether they asked for them.

43. Events: General Overview
Recall our first consideration of events, where our first frame would not close, even when the end of main() was reached.
We explained this behavior by thinking of our program as entering an “infinite loop” when the graphics are shown. This ‘infinite loop’ is actually an event-driven cycle, but we can think of it as a “while (true)” structure that periodically polls for user input.

44. The Real Story
import java.awt.*;
public class HelloGUI {
public static void main (String[ ] arg) {
System.out.println
(“About to make GUI”);
Frame f = new Frame (“Hello GUIs”);
f.setSize( 200, 200 );
f.show();
(“Finished making GUI”);
}// main
}// class HelloGUI
We usually think of our program as a single, linear set of steps being executed. But something special happens when we create graphical objects.

45. The Real Story (Cont’d)
When Java sees that you’ve created a GUI, your program gets a second “set” of linear instructions.
import java.awt.*;
public class HelloGUI {
public static void main (String[ ] arg) {
System.out.println
(“About to make GUI”);
Frame f = new Frame (“Hello GUIs”);
f.setSize( 200, 200 );
f.show();
(“Finished making GUI”);
}// main
}// class HelloGUI
This is actually a separate “thread”, but don’t worry if that’s unclear for now. We can think of this as a second part of our program than handles special graphics-related tasks (such as drawing the window, etc.)

46. The Real Story (Cont’d)
This model is very important to understand because as it turns out, when an event occurs--such as mouse click, it happens in the “graphics side” of the model.
Mouse Click
occurs
import java.awt.*;
public class HelloGUI {
public static void main (String[ ] arg) {
System.out.println
(“About to make GUI”);
Frame f = new Frame (“Hello GUIs”);
f.setSize( 200, 200 );
f.show();
(“Finished making GUI”);
}// main
}// class HelloGUI
The code trapping
this event appears
in the graphics thread
Actually, there’s a separate “event queue” that handles incoming events. But this is already complicated enough. Let’s just generalize and imagine that all events arrive in the ‘graphics side’ of things.

47. The Real Story: “Call backs”
Since the event arrived in the ‘graphics half’ of our program, we need a way to have it call a method in our program. This is known as a “call back”.
import java.awt.*;
public class HelloGUI {
public static void main (String[ ] arg) {
System.out.println
(“About to make GUI”);
Frame f = new Frame (“Hello GUIs”);
f.setSize( 200, 200 );
f.show();
(“Finished making GUI”);
}// main
}// class HelloGUI
The code trapping
this event appears
in the graphics thread
Our event
handling
code
callback

48. The Real Story: How?
So Java needs to call some event handling code that we write. The trouble is, how will Java know what we called out method? We can name them anything we want, and Java won’t necessarily know what methods handle events.
import java.awt.*;
public class HelloGUI {
public static void main (String[ ] arg) {
System.out.println
(“About to make GUI”);
Frame f = new Frame (“Hello GUIs”);
f.setSize( 200, 200 );
f.show();
(“Finished making GUI”);
}// main
}// class HelloGUI
callback
But Wait!
We can use
interfaces, right?

49. Event Interfaces
Java uses interfaces as its primary event handling scheme. If you implement an event-related interface, Java will know which methods to call. This is because the contract nature of interfaces requires all methods to appear in the implementing class.
ActionListener
import java.awt.*;
public class HelloGUI {
public static void main (String[ ] arg) {
System.out.println
(“About to make GUI”);
Frame f = new Frame (“Hello GUIs”);
f.setSize( 200, 200 );
f.show();
(“Finished making GUI”);
}// main
}// class HelloGUI
callback
public void actionPerformed
(ActionEvent e) {
// code doing something }
This method MUST
be there, so Java knows
it can “callback” to it

50. Why “Registration”?
We are told that “event registration” must occur before event handling will occur. What does this mean?
Well, since we can have any class handle events, we need to tell Java which object implements the proper event handling interface.
This “registers” the component as being interested in receiving callbacks.
import java.awt.*;
public class HelloGUI {
public static void main (String[ ] arg) {
System.out.println
(“About to make GUI”);
Frame f = new Frame (“Hello GUIs”);
f.setSize( 200, 200 );
f.show();
(“Finished making GUI”);
}// main
}// class HelloGUI
Where to
callback?

51. An Example
public class DemoFrame extends Frame { public DemoFrame( ) { super (“A poor use of inheritance, but simple”); Handler2 h = new Handler2(); this.setSize(400,400); this.setLayout(new FlowLayout()); Button b = new Button (“Click me”); this.add(b); this.show(); } // Constructor public static void main(String[] args) { DemoFrame df; df = new DemoFrame(); } // main } // DemoFrame

52. Another Example Why doesn’t this work?
public class Handler2 implements ActionListener { public void actionPerformed(ActionEvent e) { System.out.println (“Button was clicked”); } } // Handler2
Why doesn’t this work?

53. Another Example (Cont’d)
public class DemoFrame extends JFrame { public DemoFrame( ) { super (“A poor use of inheritance, but simple”); Handler2 h = new Handler2(); this.setSize(400,400); this.setLayout(new FlowLayout()); JButton b = new JButton (“Click me”); b.addActionListener(h); add(b); show(); } // Constructor public static void main(String[] args) { DemoFrame df; df = new DemoFrame(); } // main } // DemoFrame

54. Question???
We said we had to have a Listener to handle the event and it had to be an object. Does it have to be a separate object?

55. Another Example
public class DemoFrame extends Frame implements ActionListener { public DemoFrame( ) { super (“A poor use of inheritance, but simple”); Handler2 h = new Handler2(); this.setSize(400,400); this.setLayout(new FlowLayout()); Button b = new Button (“Click me”); b.addActionListener(this); this.add(b); this.show(); } // Constructor public void actionPerformed(ActionEvent e) { System.out.println (“Button was clicked”); } public static void main(String[] args) { DemoFrame df; df = new DemoFrame(); } // main } // DemoFrame

56. Is There Any Other Event There?
Anything can be an event. Including general protection faults.
But for the most part, good programming dictates that handled events should come from the following area of input: a
Mouse events b
Keyboard events c
Timing events d
Other user action inputs

57. Java Event Handling Strategies
With this basic understanding, we can investigate the
FOUR primary means of event handling in Java
Very
similar
Event Adapters
Semantic Events
Event Listeners
Inheritance-based event handling 1 2 3 4
Very
general
Very
old
We’ll not talk about this one…

58. Listeners Strategy No. 1 From the discussion about callbacks, we noted
that interfaces were the primary mechanism for structuring our event handling.
There are numerous event interfaces we can implement, roughly divided around categories of events.
The next slide lists many of them. Don’t freak out because there are so many.
We’ll highlight the most commonly used ones. . .

59. Listeners: So Many Choices
Most commonly
used in this course
Package java.awt.event features:
ActionListener
MouseListener
MouseMotionListener
AdjustmentListener
ComponentListener
FocusListener
ContainerListener
ItemListener
KeyListener
WindowListener
TextListener
As it turns out, the ActionListener is part of the “semantic” event group, even though it’s an interface. So let’s focus on simple events like MouseListener...

60. MouseListener
The MouseListener interface has several methods we have to code:
public void mouseClicked(MouseEvent e) { }
-- a timing-based determination; else
the events are processed as
pressed/releases
public void mouseEntered(MouseEvent e) { }
-- entry into component
public void mouseExited(MouseEvent e) { }
-- exit from component
public void mousePressed(MouseEvent e) { }
-- simply a press . . .
public void mouseReleased(MouseEvent e){ }
the corresponding release

61. MouseListener: An Example
import java.awt.*;
import java.awt.event.*;
public class MouseFrame implements MouseListener{
Color highlight, normal;
boolean bHighlight = true;
Frame fr;
public MouseFrame () {
fr = new Frame(“For demonstration only”);
highlight = Color.red;
normal = Color.gray;
frame.setSize(400,400);
Button b = new Button("Click");
b.addMouseListener(this);
fr.setBackground(normal);
fr.setLayout(new FlowLayout());
fr.add(b);
fr.show(); }
public static void main(String[] args) {
new MouseFrame();
To keep it simple,
we ignore
WindowEvents
Note that when
we run this the
constructor will
run and terminate

62. MouseListener: An Example (Cont’d)
public void mouseReleased(MouseEvent e){
System.out.println ("Changing color");
if (bHighlight)
frame.setBackground(highlight);
else
frame.setBackground(normal);
bHighlight = !bHighlight; }
public void mouseClicked(MouseEvent e) {}
public void mouseEntered(MouseEvent e) {}
public void mouseExited(MouseEvent e) {}
public void mousePressed(MouseEvent e) {}
} // MouseFrame
“click”
“click”

63. Event Listener Summary
We need a class that implements the appropriate listener type.
We need to “register” a component as interested in receiving events:
addXYZListener ( <listener instance> );
Whatever listener we’re working with. E.g.:
addMouseListener(this);
addMouseMotionListener(myEventHandler);

64. Event Listener: Observations1
The “WindowListener” interface required numerous methods.
But only one was important to us.
All the rest were coded as “no-op” or no operation methods. 2 3
There’s another strategy using “adapters”, using inheritance that could have saved us some trouble...

65. WHY??? Strategy No. 2 Adapters Java has built-in classes called “event
adapters” that implement each of the
various event listeners.
But all of these methods are “no-ops”.
public class MouseAdapter implements MouseListener {
public void mouseClicked(MouseEvent e) {}
public void mouseEntered(MouseEvent e) {}
public void mouseExited(MouseEvent e) {}
public void mousePressed(MouseEvent e) {}
public void mouseReleased(MouseEvent e) {} }
WHY???

66. Key to Adapters: Inheritance
MouseAdapter
MouseFrame
Why a bunch of no-op methods?
Well, if you subclass the adapter, your class IS-A type of event listener.
And you then only have to override the one or two methods you care about. The rest can be inherited as “no-ops”

67. Event Adapters (Cont’d)
import java.awt.*;
import java.awt.event.*;
public class MouseFrame extends MouseAdapter
implements MouseListener{
Color highlight, normal;
boolean bHighlight = true;
Frame frame;
public MouseFrame () {
frame = new Frame(“For demonstration only”);
highlight = Color.red;
normal = Color.gray;
frame.setSize(400,400);
Button b = new Button("Click");
b.addMouseListener(this);
frame.setBackground(normal);
frame.setLayout(new FlowLayout());
frame.add(b);
frame.show(); }
public void mouseClicked(MouseEvent e) {}
public void mouseEntered(MouseEvent e) {}
public void mouseExited(MouseEvent e) {}
public void mousePressed(MouseEvent e) {}
Parent
class
takes
care of
these

68. Event Adapters (Cont’d)
public void mouseReleased(MouseEvent e){
System.out.println ("Changing color");
if (bHighlight)
frame.setBackground(highlight);
else
frame.setBackground(normal);
bHighlight = !bHighlight; }
public static void main(String[] args) {
new MouseFrame();
} // MouseFrame
We override
the one or
two methods
we care about
Same behavior; less code;
but we use up our
single inheritance

69. public class MouseAdapter implements MouseListener {
public void mouseClicked(MouseEvent e) {}
public void mouseEntered(MouseEvent e) {}
public void mouseExited(MouseEvent e) {}
public void mousePressed(MouseEvent e) {}
public void mouseReleased(MouseEvent e) {} }
This comes with Java!
import java.awt.*;
import java.awt.event.*;
public class MouseFrame extends MouseAdapter implements MouseListener{
Color highlight, normal;
boolean bHighlight = true;
Frame frame;
public MouseFrame () {
frame = new Frame(“For demonstration only”);
highlight = Color.red;
normal = Color.gray;
frame.setSize(400,400);
Button b = new Button("Click");
b.addMouseListener(this);
frame.setBackground(normal);
frame.setLayout(new FlowLayout());
frame.add(b);
frame.show(); }
public void mouseReleased(MouseEvent e) {
System.out.println ("Changing color");
if (bHighlight)
frame.setBackground(highlight);
else
bHighlight = !bHighlight;
public static void main(String[] args) {
new MouseFrame();
} // MouseFrame

70. Big Picture Time So far, we’ve tinkered with different ways of coding
very low-level event handling.
But what if our event handling needs are very general.
Consider this simple dialog box:
There’s not much interaction
that needs to be supported.
Mouse entry/exit might not
be needed at all.
Are you sure you wish
to proceed ?
cancel ok

71. Event Handling Options: How to Decide?
Costs Benefits
Event Listeners
(interfaces)
Event Adapters
(inheritance)
Must code all
methods; wasteful
no-ops result
Keep all
events in
single class
Uses up single
inheritance
opportunity
Good abstraction;
override those
methods
you need 33

72. Debugging Event Handlers
Debugging an event-driven program (whether applet or graphical application) is more tricky than debugging a non-event-driven program.
With an event-driven Java program, you don't explicitly code any kind of event-handling loop that "polls" for occurring events, then calls the appropriate handler(s) for those events.
Instead, the Java internals handle this polling action for you. Debugging becomes trickier because now you have to make sure that your event handling code works correctly.
You also have to make sure you're handling the correct events in the first place! For example, your code for mouseEntered( ) may work perfectly, but if you're expecting it to get called when the user clicks a mouse button, it won't be! 34

73. System.out.println is still your friend...
Debugging Event Handlers (Cont’d)
So, in debugging event-driven programs written
with Java, the steps are:
Be sure you're handling the appropriate events:
Map out on paper what events get thrown from what components, and what class(es) handle them.
Handle the events appropriately: This is the kind of debugging you're already familiar with: Once you're sure the appropriate events are getting handled, the rest is being sure the event-handling code (and the code that the event handlers call) work.
System.out.println is still your friend... 35

74. Events: A Short Example
To compare the three event handling techniques, let’s see a *brief* example how all three might work on a common problem.
My Program
Goal: Create a
simple Frame that
holds a TextArea
and Button.
The Button toggles
the ability to edit
the TextArea
The Panel holding the Button and TextArea is placed in a Frame subclass, which handles its own disposal
TEXT AREA
BUTTON
Panel subclass 24

75. Events: A Short Example (Cont’d)
import java.awt.*;
import java.awt.event.*;
public class MyFrame extends Frame implements WindowListener{
public static final int iWidth = 300, iHeight = 500;
public MyFrame() {
this.setSize(iWidth, iHeight);
this.addWindowListener(this);
BorderLayout border = new BorderLayout();
this.setLayout(border); }
public void windowClosing (WindowEvent e) {
e.getWindow().setVisible(false);
e.getWindow().dispose();
System.exit(0);
public void windowActivated(WindowEvent e) {}
public void windowClosed(WindowEvent e) {}
public void windowDeactivated(WindowEvent e) {}
public void windowDeiconified(WindowEvent e) {}
public void windowIconified(WindowEvent e) {}
public void windowOpened(WindowEvent e) {}
}// class MyFrame
Constructor
WindowListener
Frames are not
self-disposing!
(Setting Frame
invisible first
eliminate flicker.) 25

76. Events: A Short Example (Cont’d)
Advanced feature: “Anonymous
Inner Class”:
used as a short
cut. For your
code, use listeners
import java.awt.*;
import java.awt.event.*;
public class MyFrame extends Frame {
public static final int
iWidth = 300,
iHeight = 500;
public MyFrame() {
this.setSize(iWidth, iHeight);
this.addWindowListener
(new WindowAdapter()
public void windowClosing (WindowEvent e)
e.getWindow().setVisible(false);
e.getWindow().dispose();
System.exit(0); }
});
BorderLayout border = new BorderLayout();
this.setLayout(border);
}// class MyFrame
Frames are not
self-disposing!
(Setting Frame
invisible first
eliminate flicker.) 25

77. Events: A Short Example (Cont’d)
import java.awt.*;
public class Driver {
public static void main (String arg[]){
Notepad note = new Notepad();
MyFrame f = new MyFrame();
f.add(note, BorderLayout.CENTER);
f.show();
}//main
}//class Driver 26