Applets and Event Handling in Java

5/24/2018 Applets and Event Handling in Java

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Session 3 - Exception-Handling

Java Programming

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Applets and EventHandling

Java Programming


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Java Programming

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AppletsApplet Basics

The Applet Architecture

Applet Initialization & Termination

Applet Restrictions

Writing a Simple AppletVisualizing How an Applet Works


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Applets Simple Applet Display Methods

Overriding update()

Requesting Repainting - Threadsin Applets

Using the Status WindowThe HTML APPLET Tag

Passing Parameters to Applets


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Applet Basics

Applet

Applets are small application thatare accessed on an internetserver, Transported over the

Internet, Automatically installedand run as part of an webdocument.


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Applet Basics

Component

Container

Panel

Applet

All applets


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Applet Basics

Run Examples

From appletviewerFrom Browser


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Applet Basics

Provide support for

Applet execution (start and stop)

Load and display images

load and play audio clips.

Applets must import java.applet.*and java.awt.*

Execution by Web browser or anapplet viewer provided by JDK


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The Applet Architecture

AWTMethods

SOPO/P

Only FewAllmain()

init()main()Begin with

AppletNormal

Class

Execution of


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The Applet Architecture ...

This is a crucial point.

An applet should not maintaincontrol for an extended period.

An Additional thread must be started

for repetitive task.


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Second

The user initiates interaction with

an applet not the other wayround.

These interactions are sent to theapplet as events to which theapplet must respond.


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Control applets execution by

overriding set of methods.

init

start

stop

destroy


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Applet Initialization and

TerminationBegins order

init()

start()

paint()

Termination order

stop()

destroy()


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Termination init():

First method

Variables initialization

Called only once


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Termination start():

It is called after init();

Called to restart an applet

User comes back to a web page


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paint():

It is called each time

Restoring the applet window

Window overwritten

When applet begins execution;

paint() has one parameter of type

Graphics.


Termination


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Termination stop():

When a web browser leaves the

HTML document containing theapplet and goes to another page

destroy():When the environment

determines that your applet needsto be removed completely frommemory.


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Applet Restrictions

Which are not enforced at compile

time.Result in run-time exception.


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Applet Restrictions

Applets may not

Read, write or delete filesCreate or list directories

Check for the existence of a

file/properties


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Applet Restrictions ...

Applets may not invoke a local

program

Create a network connection

Applets can read the file systemon the server it has been

downloaded from.


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Applet Restrictions ...

These restrictions can be

overwhelming.

Be lifted for applets from a trusted

source.

Signing of an applet

Information is encoded that proves

that the applet can be trusted.


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Java Programming


Demo of SampleApplet.java// Writing a Simple Appletpublic class SampleApplet extends Applet {

String msg;

// Initial settings.public void init() {msg = "In init() -- ";

setBackground(Color.cyan);setForeground(Color.red);

}public void start(){msg += "In start() -- "; }public void stop() {msg = ""; }

// Display msgpublic void paint(Graphics g) {msg += "In paint().";g.drawString(msg, 50, 150); }

}


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Compiling/Running an Applet

The applet subclass must bedeclared as publ ic, because it will

be accessed by code that is outsidethe program.

Compile the program in the sameway used for other Java programs:

javac SampleApplet.java

There are three ways to run theapplet.


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Compiling/Running an Applet

1.Executing within a Java-compatible Web browser:

Create file apv.html


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Compiling/Running an Applet ...

2.Above html file can also beexecuted from command line as

follows at the DOS prompt:D:\suryoday> appletviewer

apv.html


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Compiling/Running an Applet ...

3. Include the contents of the html fileat the head (after includes) of your

Java source code file.D:\suryoday> appletviewer

MovingBanner.javaApplet Viewer: SampleApplet.class

In init() -- In start() -- In paint()

Applet

Applet started.


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Visualizing How an Applet Works

Applet CodeApplet Code

Javac compilerJavac compiler bytecode

.class file

byte code

.class fileHTML

envelope

Hello.java

Hello.class

Hello.html

Java enabled

Browser

OUTPUT

Java enabled

BrowserOUTPUT:

Hello,World!

Java Run-time

Environment

Classes.zip

Java Run-time

Environment

Classes.zip


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Applet Display Methods

void drawString(String msg, int

x, int y)

Member of Graphics class.

update()or paint().


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void setBackground(Color

newColor)

void setForeground(Color newColor)

The colors can be changed during

execution.


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Current setting of

background/foreground color can

be obtained by

getBackground()/getForeground().


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Overriding update()

Change default color in the Update

methods instead of Paint method.

public void update(Graphics g) {

// Redisplay the window, here. }

public void paint(Graphics g) {

update(g); }


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Requesting Repainting

One of architectural constraintsimposed on applet is that

it must quickly return control to theAWT run-time system.

If the applet is displaying a movingbanner, how can it itself update the

window for its changing contents?


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Requesting Repainting

It cannot create a loop inside paint()

Well, it can simply call repaint()

whenever it needs to update theinformation to be displayed.

In turn, update() is called which then

calls paint() by default.Since the scrolling of message is a

repetitive task, it should be performed by

a separate thread created by applet whenit is initialized.


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Demo of MovingBanner.javaThreads in applets: This applet scrolls a

message, from right to left, across the appletswindow with the help of a thread.

import java.awt.*;import java.applet.*;

public class MovingBanner extends Applet implementsRunnable {

String msg = HELLO WORLD;Thread t = null; boolean stopFlag;

public void init() {// set colors

setBackground(Color.cyan);setForeground(Color.red);

}// end of init


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MovingBanner.java ...public void start( ) { // start thread

t = new Thread(this);stopFlag = false; t.start();

} // end of startpublic void run() { char ch;

for ( ; ; ) {try {repaint();

Thread.sleep(250);

ch = msg.charAt(0);msg = msg.substring(1, msg.length());msg += ch;if (stopFlag) break;

} catch(InterruptedException e) { }}

}


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MovingBanner.java ...public void stop() {

stopFlag = true; t = null;

} // end of stop

public void paint(Graphics g) {g.drawString(msg,100,100);

} // end of paint

} // end of class MovingBanner

Applet Viewer: MovingBanner.class

WORLD HELLO

Applet

Applet started.

SampleOutput:


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Using the Status Window

Also output a message to status window of thebrowser or applet viewer.

By calling showStatus().

Lets write paint() again:


g.drawString(msg,10,10);

showStatus(This is shown in statuswindow.);

}Applet Viewer: MovingBanner.class

HELLO WORLD

Applet

This is shown in status window.


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The HTML APPLET Tag

The APPLET tag is used to start

an applet.

An applet viewer will execute

each APPLET tag in a separate

window

Browsers allow many applets on

a single page.

Syntax of Applet tag


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The HTML APPLET Tag

[]

[< PARAM NAME = pname VALUE = pvalue >

[HTML displayed in the absence of Java]


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Demo of ParamDemo.java// Passing Parameters to Appletsimport java.awt.*;

import java.applet.*;

/*

*/

public class ParamDemo extends Applet {

String fontName;

int fontSize;float leading;


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ParamDemo.java ...

public void start( ) { String param;

fontName = getParameter(font);

if (fontName.equals("")) fontName=Not Found;

param = getParameter(size);try { if (param.equals("")) fontSize = 0;

else fontSize = Integer.parseInt(param);

} catch(NumberFormatException e) {fontSize=-1;}param = getParameter(leading);

try { if (param.equals("")) leading = 0;

else leading=Float.parseFloat();

} catch(NumberFormatException e) {leading=-1;}

}


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ParamDemo.java ...


g.drawString(Font name: + fontName, 40, 50);

g.drawString(Font size: + fontSize, 40, 150);

g.drawString(Leading: + leading, 40, 200);}

}Applet Viewer: ParamDemo

Font name: TimesNewRoman

Font size: 24

Leading: 25.68

Applet

Applet started.


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Java Programming


Event Handling

Event Classes

Sources of Events

Event Listeners and Interfaces

Handling Mouse Events

Handling Keyboard events


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Session 3 - Exception-HandlingJava Programming


Event Handling

Applets are event-driven programs.The most-commonly handled events

are clicking of a mouse, pressingkeyboard keys and controls likepush button. Events are supported

by java.awt.event package. In oldJava 1.0 approach, an event waspropagated up the containment

hierarchy until it was handled by acomponent, wasting valuable time.


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Event Handling

The modern approach to handlingevents is based on the delegat ion

event modelin which listeners mustregister with a source in order toreceive an event notification. A

Source generates an event andsends it to one or more listeners Thelistener processes the event and

then returns.

E t H dli


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Event Handling ...

An event is an object that describes astate change in a source. It can begenerated due to interaction with the

elements of a graphical user interface -pressing a button, keying a character,selecting an item in a list, and clicking

the mouse. Other events may also occurlike a timer expires, counter exceeds avalue, soft/hardware failure occurs, etc.

sourcelistener

listener

E t H dli


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Event Handling ...

Source is an object that

generates an event. Sourcesmay generate more than onetype of events.

A listener is an object that isnotified when an event occurs.

E t Cl


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Event Classes

The classes that represent eventsare the core of Javas event

handling mechanism. Thesuperclass of all events isEventObjectwhich is in java.util.

AWTEvent, defined within java.awt,is a subclass of EventObject. It is asuperclass of all AWT events that

are handled by the delegation eventmodel.

E t Cl


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Event Classes

Package java.awt.event definesseveral types of events. Below

are enumerated the mostimportant of the event classesof this package:ActionEvent, AdjustmentEvent,ComponentEvent,

ContainerEvent, FocusEvent, InputEvent,

ItemEvent, KeyEvent, MouseEvent,TextEvent,

WindowEvent.

S f E t


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Sources of Events

Button

CheckBox

Option1 Option2

Text Components

S f E t


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Sources of EventsHere are some of the user interface

components that can generate events:

Event Source & Description

Button: causes action events when button ispressed.

CheckBox: causes item events when

checkbox is (de)selected.Choice : causes item events when choice is

changed.

List: causes action events when item isdouble-clicked; causes item events when itemis (de)selected.

S f E t


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Sources of Events Menu Item: causes action events when

menu item is selected; causes itemevents when a checkable one is(de)selected.

Scrollbar: causes adjustment eventswhen it is manipulated.

Text Components: causes text events ona character entry.

Window: causes window events when awindow is activated, closed, deactivated,opened, quit, (de)iconified.



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Event listeners are objects thatare notified when an event

occurs. The methods thatreceive and process events aredefined in a set of interfacesfound in java.awt.event package:



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ActionListener

actionPerformed(ActionEvent)

AdjustmentListeneradjustmentValueChanged(AdjustmentEvent)

ComponentListenercomponentResized(ComponentEvent e)componentMoved(ComponentEvent e)componentShown(ComponentEvent e)componentHidden(ComponentEvent e)



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MouseListenermouseClicked(MouseEvent e)mousePressed(MouseEvent e)mouseReleased(MouseEvent e)mouseEntered(MouseEvent e)

mouseExited(MouseEvent e)MouseMotionListenermouseDragged(MouseEvent e)

mouseMoved(MouseEvent e)



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TextListenertextValueChanged(TextEvent e)

WindowListenerwindowOpened(WindowEvent e)windowClosing(WindowEvent e)

windowClosed(WindowEvent e)windowIconified(WindowEvent e)windowDeiconified(WindowEvent e)

windowActivated(WindowEvent e)windowDeactivated(WindowEvent e)



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FocusListenerfocusGained(FocusEvent e)focusLost(FocusEvent e)

ItemListeneritemStateChanged(ItemEvent e)

KeyListenerkeyTyped(KeyEvent e)keyPressed(KeyEvent e)

keyReleased(KeyEvent e)

Demo of MouseEvents java


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Demo of MouseEvents.java

Handling Mouse Events: The MouseListenerand the MouseMotionListener interfaces mustbe implemented to handle the mouse events.

import java.awt.*;import java.awt.event.*;


public class MouseEvents extends Appletimplements

MouseListener, MouseMotionListener {

String msg = " ";int mouseX = 0, mouseY = 0; // coordinates ofmouse

public void init() {addMouseListener(this);

MouseEvents java


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MouseEvents.java ...public void mouseClicked(MouseEvent me) {

msg = Mouse clicked.;

mouseX=0; mouseY=10; repaint();

} // Handling mouse clicked.

public void mouseEntered(MouseEvent me) {

msg = Mouse entered.;


} // Handling mouse entered.

public boolean mouseExited(MouseEvent me){

msg = Mouse exited.;


MouseEvents java


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MouseEvents.java ...public void mousePressed(MouseEvent me) {

mouseX = me.getX(); mouseY = me.getY();msg = Down"; repaint( );

} // Handling mouse pressed.

public void mouseReleased(MouseEvent me) {

mouseX = me.getX(); mouseY = me.getY();

msg = Up"; repaint( );} // Handling mouse released

public void mouseMoved(MouseEvent me) {

showStatus(Moving mouse at +me.getX() + , + me.getY()); // Handlingmouse moved.}

MouseEvents java


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MouseEvents.java ...

public void mouseDragged(MouseEvent me) {

mouseX = me.getX(); mouseY = me.getY();

msg = *";

showStatus(Dragging mouse at +mouseX + , + mouseY);

repaint( );

} // Handling Mose dragged


g.drawString(msg, mouseX, mouseY);

} // Display msg in applet window at current

} // X,Y location.



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On pressing a key, aKEY_PRESSED event is generated;this results in a call to keyPressed()event handler. When the key isreleased, a KEY_RELEASED event

is generated and the keyReleased()handler is executed. If a characteris generated by the keystroke, then

KEY_TYPED event is sent andkeyTyped() handler is invoked.



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Thus, on each key-press at least 2and often 3 events are generated. Ifwe care about only the actualcharacters, the key press andrelease events can be ignored, but

to handle special keys (arrow /function keys), the key press eventsmust watch for them through

keyPressed() handler. Next programdemonstrates keyboard input.

Demo of SimpleKey java


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Demo of SimpleKey.java// Demo of Keyboard Events

import java.awt.*;

import java.awt.event.*;


public class SimpleKey extends Appletimplements KeyListener{String msg = ;

int X = 10, Y = 20; // output coordinates

public void init() { addKeyListener(this);

requestFocus();} // request input focus

public void keyPressed(KeyEvent ke)

{ showStatus(Key Down); }

SimpleKey java


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SimpleKey.java ...public void keyReleased(KeyEvent ke) {

showStatus(Key UP);

}

public void keyTyped(KeyEvent ke) {

msg += ke.getKeyChar( ); repaint( );

}


g.drawString(msg, X, Y);

}

}

Creating animation using thread


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g g

Animation is an ideal use of threadsThe program given below creates an animation

and plays a sound as well in the background

import java.awt.*;

import java.applet.AudioClip;

public class ImageAnim extendsjava.applet.Applet implements Runnable {

Image pics[]=new Image[10];

Image currimage;AudioClip c1;

Thread timage;



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public void init() {String picsource[]={"T1.gif","T2.gif","T3.gif","T4.gif",

"T5.gif","T6.gif","T7.gif","T8.gif","T9.gif", "T10.gif"};

for (int i=0;i


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public void stop() {timage=null;

if (c1!=null) c1.stop();

}

public void run() {

setBackground(Color.pink);

int i=0;

Thread thisThread=Thread.currentThread();

while(timage == thisThread) {

currimage = pics[i];

repaint();



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if (c1!=null) c1.loop();try { Thread.sleep(1000);

} catch(InterruptedException e) {}

i++;

if (i==pics.length) i=0;

}

}


if (currimage != null)

g.drawImage(currimage,10,40,this);}

}

Reducing Animation Flickering


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While creating animation, a call

to repaint() method is made

The repaint() method in turn

calls update() method before

calling paint() method

The update() method clears the

screen which in turn causesanimation flickering



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Animation flickering can be

reduced if the update() method s

overridden in your applicationpublic void update(Graphics g) {

paint(g); }

Sometimes animation flickering

persists even after overridingupdate() method



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In such a case double-buffering

technique can be used that

eliminates flickering

In double-buffering, the entire

frame is first painted on a non-visible area called buffer

Then this frame is printed on thescreen

Double Buffering


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However, double buffering takesa lot of memory space and isnt

always the most efficient methodto reduce flickering

The following program showsthe movement of a ball along a

trajectory

Double Buffering


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import java.awt.*;public class BallWithBuff extends java.applet.Appletimplements Runnable {

Thread t;

int xPos=5, yPos=5;int xMove=2, yMove=4;

Double Buffering


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Image offscreenImage;Graphics offscreeng;

public void init() {

offscreenImage = createImage(getSize().width,

getSize().height );offscreeng=offscreenImage.getGraphics(); }

public void start() {

if (t==null) {

t=new Thread(this);t.start();

}

}

public void stop() {t=null;}

Double Buffering


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public void run() {Thread currThread =Thread.currentThread();

while(t == currThread) {

xPos+=xMove;yPos+=yMove;

if (yPos>=300) yMove*=-1;

if (yPos


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public void update(Graphics g) {paint(g);

}


offscreeng.setColor(Color.yellow);

offscreeng.fillRect(0,0,360,351);

offscreeng.setColor(Color.red);

offscreeng.fillOval(xPos,yPos,50,50);g.drawImage(offscreenImage,0,0,this);

}

public void destroy() {offscreeng.dispose();

}

}

Summary


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Participants are familiarized with the:

Java threads - the Thread class, thread states,

priorities, synchronization, the Runnableinterface and creating threads.

Applet - architecture, initialization &termination, restrictions, display methods,writing simple applets, the HTML APPLET Tag,and passing parameters to applets.

Event - classes, sources of events, event

listeners and interfaces, handling mouse andkeyboard events.


Applets and Event Handling in Java

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