graphical user interface (gui) programming ii. lecture objectives understand the event-handling...

Graphical User Interface (GUI) Programming II

Lecture Objectives

• Understand the Event-Handling Processes in Java

Caution Note!!!

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

Outline for Today's Fun

• Events: What is an event? Simple (?) Example

• Swing Components: JFrames JComponents An example Swing Component Design (MVC/UI-Delegate??)

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"

• 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.

Events (Cont’d)

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

What didn't work???

Code from the Past: Remember???

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.

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?

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);

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) {}

Using Event InterfacesOutline:

Import

Class definition

Implementation

Dummy (lazy) implementation

• 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.

Using Event Interfaces (Cont’d)

Event Handler Registrationimport 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

Demonstration

Class Pool

class HelloGUI main { Frame f Handler h }

class Frame

class Handler

FrameInstance

HandlerInstance

Interface WindowListener

Event Handling: Key Ideas

1. Determine which event occurs

2. Decide what class is going to handle this event

3. Write the method (and class?) that will handle the event.

4. Register the listener with Java.

Today’s First Wisdom

Very important that you understand this simple example to

understand the concepts that follow.

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!

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!

• Events: What is an event? Simple (?) Example

• Swing Components: JFrames JComponents An example Swing Component Design (MVC/UI-Delegate)

Outline for Today's Fun: Reminder

A Call from the Past!

Earlier, we cautioned aboutthe existence of two toolkitsin Java for creating GUIs:

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.

Java’s GUI Capabilities

Java provides essentially two related toolkits for making GUIs:

The Abstract Windowing Toolkit (“AWT”), and

The Java Foundation Classes (“Swing”)

Swing is merely a new, improved version of the AWT, and still uses some of AWT’s features.

Java’s GUI Capabilities

Java provides essentially two related toolkits for making GUIs:

The Abstract Windowing Toolkit (“AWT”), and

The Java Foundation Classes (“Swing”)

Swing is merely a new, improved version of the AWT, and still uses some of AWT’s features.

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 . . .

Welcome to JFC/Swing Planet

“Now it gets interesting . . . “

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

WinNTButtonPeer

MacOSButtonPeer

MotifButtonPeer

ButtonPeer

AWT components requirednative “peer” methods torender and operate--many steps!

CLICK ME

File Edit Help

Slow & Inflexible!

• 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

CLICK ME

File Edit Help

Tooltip

(Tooltips requiredthreads,window subclasses &extensive event handling)

(Image buttons requiredcomponent subclassing,methods to handle ‘click’look, as well as eventhandlers)

(Menu bars limited;no images possiblewithout *extensive*simulation throughcomponents)

(Layering of componentsrequires layoutsubclasses)Bottom line: Making stuff look cool

or satisfying a client’s requestcould be a nightmare!

Bottom line: Making stuff look coolor satisfying a client’s requestcould be a nightmare!

Fixing AWT Problems

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)

javax.swing.JButton

Fast, flexible, extensible!

CLICK ME

File Edit Help

javax.swing.*javax.swing.*

Swing Packages• 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:

Denotes ‘extension’ package thathas migrated to core status

Why “javax”?

* logical grouping

* minimizes transitioncosts

* most developershappy with it

* helps maintainexisting JDK 1.1 code (cf. MFC lib breaks)

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

Overview of the Overview

Overview of J FC/S w ing P ac kagesOverview of J FC/S w ing P ac kages

• 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

Text-based widgets(including html/rtf display) New event packages

Components, including“aggregate” or complexcomponents

Packages tocontrol the“look and feel”of Swing

Short Examples

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 APIfor useful behaviors.

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

Cool Buttons

Sets icon androllover Icon.

Note: Icon constructortook String argument,and automaticallyloaded image

Images from disk:

Why getContentPane() ?

• The HelloWorld example required us to call getContentPane() before “add()”ing an object to the JFrame:

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 . . .

Required ofJFrame, JDialogand JInternalFrameinstances

myFrameInstance.getContentPane().add(myComponent);

A JFrame Autopsy

A java.awt.Frame is composed of a single container--the Frame object itself. It is flat as a pop tart.

JLayeredPane

JPanelContentPane

A javax.swing.JFrame is composed of a transparent “glassPane” surface, and an inner JPanel with Contents and Menu

“The Pop Tart / Sandwich Duality”

JFrame Class View

JRootPane JMenuBar

JLayeredPane

JFrame

Container

Component

ContentPane

GlassPane

manages

contains

JComponent

The JRootPane is a container with a JLayeredPane (holding the Menu and ContentPane) and a Component GlassPane. It serves as base for numerous classes.

AWTAWT

JFCJFC

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.

JFrame Blue PrintWe 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 Component getGlassPane();public void setGlassPane(Component);

public JPanel getContentPane();public JPanel getContentPane();

JRootPane: The Glass Pane

JFrame Disposal• 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.

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.

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.

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.

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.

Events (Cont’d)

Events: General Overview

ExplanationExplanationWhen the Java VM createdour Frame, it entered into akind of ‘infinite loop’,waiting for input andevents. (This is commonof graphical toolkits.)

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(); System.out.println (“Finished making GUI”); }// main}// class HelloGUI

while(true){ //get user input // handle event}

Since we didn’twrite any eventhandlers, not eventhe “windowdisposal” x works.

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.

The Real Story

We usually think of our program as a single, linear set of steps being executed. But something special happens when we create graphical objects.

When Java sees that you’ve created a GUI, your program gets a second “set” of linear instructions.

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.)

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 Clickoccurs

The code trappingthis 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.

The Real Story (Cont’d)

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”.

callbackcallback

Our eventhandling

The code trappingthis event appears

in the graphics thread

The Real Story: “Call backs”

callbackcallback

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.

But Wait!But Wait!

We can useWe can useinterfaces, right?interfaces, right?

The Real Story: How?

Event InterfacesJava 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.

callbackcallbackpublic void actionPerformed (ActionEvent e) { // code doing something }

This method This method MUSTMUSTbe there, so Java knowsbe there, so Java knowsit can “callback” to itit can “callback” to it

ActionListener

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.

Where toWhere tocallback?callback?

An Examplepublic 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

Another Examplepublic class Handler2 implements ActionListener {

public void actionPerformed(ActionEvent e) {

System.out.println (“Button was clicked”);

} // Handler2

Why doesn’t this work?

public class DemoFrame extends JFrame {

JButton b = new JButton (“Click me”);

b.addActionListener(h);

add(b);

show();

} // Constructor

DemoFrame df;

} // main

} // DemoFrame

Another Example (Cont’d)

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?

Another Examplepublic class DemoFrame extends Frame implements ActionListener {

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”);

DemoFrame df;

} // main

} // DemoFrame

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:

Keyboard events

Timing events

Mouse events

Other user action inputs

Java Event Handling Strategies

With this basic understanding, we can investigate theFOUR primary means of event handling in Java

Event Adapters

Semantic Events

Event Listeners

Inheritance-based event handling

Very similar

Verygeneral

Very old

We’ll not talk about this one…

Listeners Strategy

StrategyNo. 1No. 1

From the discussion about callbacks, we notedthat 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. . .

Most commonlyused in this course

Listeners: So Many Choices

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...

MouseListenerThe 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

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 whenwe run this theconstructor willrun and terminate

MouseListener: An Example

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”

MouseListener: An Example (Cont’d)

Event Listener Summary

We need a class that implements the appropriate listener type.

We need to “register” a component as interested in receiving events:

addXYZXYZListener ( <listener instance> );

Whatever listener we’re working with. E.g.:

addMouseListener(this);addMouseMotionListener(myEventHandler);

There’s another strategy using “adapters”, using inheritance that could have saved us some trouble...

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.

Event Listener: Observations

Adapters

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) {}}

Java has built-in classes called “event adapters” that implement each of the various event listeners.

But all of these methods are “no-ops”.

WHY???

Strategy

StrategyNo. 2No. 2

Key to Adapters: Inheritance

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”

MouseFrame

MouseAdapter

Parentclass takes

care ofthese

import java.awt.*;import java.awt.event.*;public class MouseFrame extends MouseAdapterextends 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) {}

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 overridethe one or

two methodswe care about

Same behavior; less code;but we use up our single inheritance

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 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

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!

Big Picture Time

So far, we’ve tinkered with different ways of codingvery low-level event handling.

But what if our event handling needs are very general.

Consider this simple dialog box:

cancel

Are you sure you wishto proceed ?

There’s not much interactionthat needs to be supported.

Mouse entry/exit might notbe needed at all.

Event Listeners(interfaces)

Event Adapters(inheritance)

Costs Benefits

Must code allmethods; wastefulno-ops result

Uses up singleinheritanceopportunity

Keep allevents insingle class

Good abstraction;override those methodsyou need

Event Handling Options: How to Decide?

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!

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.

Debugging Event Handlers (Cont’d)

To compare the three event handling techniques, let’s see a *brief* example how all three might work on a common problem.

Events: A Short Example

My Program

BUTTON

TEXT AREA

Panel subclass

Goal: Create asimple Frame thatholds a TextAreaand Button.

The Button togglesthe ability to editthe TextArea

The Panel holding the Button and TextArea is placed in a Frame subclass, which handles its own disposal

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 firsteliminate flicker.)

Events: A Short Example (Cont’d)

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

Advanced feature:“Anonymous Inner Class”:

used as a shortcut. For your

code, use listeners

Frames are not self-disposing!(Setting Frame

invisible firsteliminate flicker.)

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

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