pravin yannawar, docs, nmu jalgaon basic java : applets 2 objectives of this session identify the...

Pravin Yannawar, DOCS, NMU Jalgaon

Basic Java : Applets2

Objectives of This Session

• Identify the need for Applets • Distinguish between Applets and Applications • State the restrictions on Applets • State Applet Life Cycle • Explain the Applet tag & its various attributes . • Explain process of passing parameters to Applet • State the concept of applet container and

AppletContext class • Explain InterApplet Communication


Applets

• Is an application designed to be transmitted over the internet & executed by a Java-compatible web browser.

• Are small applications that are accessed on a Internet server, transported over the Internet, automatically installed & run as part of a web document.


Applet Hierarchy

Object

Component

Container

Panel

Applet


Applet Vs Applications

• HTML file• Applet Context• No main()• Extend Applet class• init()• Flow Layout• No titles• Access Restrictions

• No HTML file• Operating System• main()• Not necessary• Constructor• Border Layout• Titles• No Restrictions


Security Restrictions

• Applets can’t read / write files on user’s file system.

• Can’t communicate with an internet site other than the one that served the web page that included the applet.

• Can never run any executable program.• All windows popped by an applet carry a

warning message.• Can never find any info about the local

computer.


Applet Demo

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

class MyApplet extends Applet{

public void paint(Graphics g){

g.drawString(“Hello world”,50,50);}

}


Html Demo File

<HTML><HEAD>

<TITLE> My first applet </TITLE></HEAD><BODY>

<APPLET CODE = “MyApplet.class” WIDTH = 200 HEIGHT = 200>

</APPLET></BODY>

</HTML>


Converting Applications to Applets

• Create web page with appropriate tags to load applet code.

• Eliminate main() method.• Replace Frame class with Applet class.• Remove call to setSize() and setTitle().• Remove call to addWindowListener.• Replace constructor with init() method.


Applet Life Cycle

• init()• start()• stop()• destroy()• paint(Graphics g)


public void init()

• Automatically invoked when java launches applet for the first time.

• Called only once.• Functionality performed commonly includes:

Creating the applet’s GUI. Reading values from <param> tags. Loading off-screen images & audio clips from

external files.


public void start()

• Automatically invoked after init().• Also invoked when browser browses back to

applet after following a link to a diff page.• Thus, code that needs to be executed

repeatedly must be put in start(). E.g. Restart thread to resume an animation.


public void stop()

• Automatically invoked whenever user moves off the page on which the applet resides.

• Works in partnership with start().


public void destroy()

• Automatically invoked when browser no longer needs the applet.

• Is an applets opportunity to release any non memory resources it may have, such as threads & network connections & graphic contexts.


<applet> attributes

• CODE• CODEBASE• WIDTH / HEIGHT• ALIGN• HSPACE / VSPACE• NAME• <PARAM> tag


CODEBASE attribute

• Tells browser to search for applet class files in the directory specified by the URL..

• If this attribute isn’t there, the codebase dir is assumed to be in the same dir that the HTML file resides


Applet(Parameter Passing)

HTML FILE

<APPLET CODE = “test.class ” WIDTH = 100 HEIGHT =100>

<PARAM NAME = font VALUE=“Times New Roman” >

<PARAM NAME = size VALUE=“24” >

</APPLET>


Applet

public void paint(Graphics g){

String fontName = getParameter(“font”);

int fontSize = Integer.parseInt(getParameter(“size”));

g.setFont(fontName,Font.BOLD,fontSize);

g.drawString(“Hey SEED !!!”,50,50);

}


The Applet Context

• Is the context (browser/ applet Viewer) in which the applet runs.

• The getAppletContext() methods returns an object that implements an interface of the type AppletContext.

• This enables your applet to access features of the browser that contains it.


Methods of AppletContext

• Applet getApplet(String name): returns a handle to a named applet.

• AudioClip getAudioClip(URL soundFileURL): returns an audio clip object.

• Image getImage(URL imageFile) : returns an image.

• void showDocument(URL docURL) : requests the browser to display the HTML page specified in parameter.


InterApplet Communication

• Get the reference of the AppletContext Interface using the getAppletContext() method of Applet.

• Call the getApplet(String name) on the AppletContext which returns a reference to a named Applet.

• Call any method of Applet using this interface.

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