Advanced Java Lecture-6

8/10/2019 Advanced Java Lecture-6

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Applets are small java programs that are primarily used in

internet computing

They can be transported over the internet from one

computer to another and run usingApplet Viewer or any

web browser that supports java


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When to use applets??

When we need something dynamic to be

included in the display of a web page

When we require some flash outputs

When we want to create a program and make it

available on internet for use by others on their

computers


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Types

Local applet

Remote applet


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Local Applet

Internet

Remote appletLocal Computer

(client)

Remote Computer

(server)

Local Computer


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We can embed applets into web pages in 2 ways:

We can write our own applets and embed them

into web pages

We can download an applet from a remote

computer system and then embed it into a web

page


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Applet Security

For security reasons the applets that are loaded

over the network have several restrictions:

An applet cannot ordinarily read or write fileson the computer that its executing on

An applet cannot make network connections

except to the host that it came from


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Java.lang.Object

Java.awt.Component

Java.awt.Panel

Java.awt.Container

Java.applet.AppletChain of classes inherited by

Applet class


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The Applet CLASS: Every applet is an extension of thejava.applet.Applet class.

The base Applet class provides methods that a derived Applet

class may call to obtain information and services from the

browser context. These include methods that do the

following:

Get applet parameters

Get the network location of the HTML file that contains the

applet

Get the network location of the applet class directory

Print a status message in the browser

Fetch an image

Fetch an audio clip

Play an audio clip

Resize the applet


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12

Useful Applet Methods

getCodeBase, getDocumentBase

The URL of the:

Applet file- getCodeBase

HTML file- getDocumentBase

getParameter Retrieves the value from the associated HTML PARAMelement

getSize

Returns the Dimension(width, height) of the applet

showStatus

Displays a string in the status line at the bottom of the browser


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13

Useful Applet Methods,

(Continued)

getBackground, setBackground

Gets/sets the background color of the applet

SystemColorclass provides access to desktop colors

getForeground, setForeground

Gets/sets foreground color of applet (default color of drawing

operations)


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Useful Graphics Methods

drawString(string, left, bottom)

Draws a string in the current font and color with the bottom leftcornerof the string at the specified location

One of the few methods where the y coordinate refers to the bottomof shape, not the top. But y values are still with respect to the top left

corner of the applet window

drawRect(left, top, width, height)

Draws the outline of a rectangle (1-pixel border) in the current color

fillRect(left, top, width, height)

Draws a solid rectangle in the current color

drawLine(x1, y1, x2, y2)

Draws a 1-pixel-thick line from (x1, y1) to (x2, y2)


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Useful Graphics Methods,

continued drawOval, fillOval

Draws an outlined and solid oval, where the arguments describe a

rectangle that bounds the oval

drawPolygon, fillPolygon

Draws an outlined and solid polygon whose points are defined by

arrays or a Polygon(a class that stores a series of points)

By default, polygon is closed; to make an open polygon use the

drawPolyline method

drawImage Draws an image

Images can be in JPEG or GIF (including GIF89A) format


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Graphics Color

setColor, getColor

Specifies the foreground colorprior to drawing operation

By default, the graphics object receives the foregroundcolor of the window

AWT has 16 predefined colors (Color.red,Color.blue, etc.) or create your own color,new Color(r, g, b)

Changing the color of the Graphicsobject affects only

the drawing that explicitly uses that Graphicsobject To make permanent changes, call the appletssetForeground

method.


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Graphics Font

setFont, getFont

Specifies the font to be used for drawing text

Set the font of the window (I.e., call the appletssetFontmethod)

for permanent changes to the Graphicsobject

In JDK 1.1, only 5 fontsare available: Serif (akaTimesRoman),SansSerif (aka Helvetica), Monospaced (akaCourier),

Dialog, and DialogInput


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Applet life cycle

Born

IdleRunning

Dead

stop()

start()

destroy()

destroyedEnd

Exit of browser

start()

InitializationBegin

(Load Applet)

paint()

Display

stopped


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Initialization state

Applet enters this state when it is first loaded.We may do following at this stage:

create objects needed by applet

set up initial valuesload images or fonts

set up colors

public void init()

{

(action)

}


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Running state

Applet enters this state when the system calls start()

method of Appletclass

This method is automatically called after the browser

calls the init method. It is also called whenever the user

returns to the page containing the applet after havinggone off to other pages

public void start()

{ ....

.. (action)

}


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Idle or stopped state

This method is automatically called when the user

moves off the page on which the applet sits. It can,

therefore, be called repeatedly in the same applet.

public void stop()

{

(action)

}


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Dead state

An applet is said to be dead when it is removed frommemory.

This method is only called when the browser shuts downnormally. Because applets are meant to live on an HTMLpage, you should not normally leave resources behind

after a user leaves the page that contains the applet.

public void destroy()

{

(action)

}


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Display state

Applet moves to this state whenever it has to perform

some output operations on the screen

Invoked immediately after the start() method, and alsoany time the applet needs to repaint itself in the browser.The paint() method is actually inherited from the

java.awt.

public void paint (Graphics g)

{

....

.. (display statements)

}


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Graphics Behavior

Browser calls repaintmethod to request redrawing ofapplet

Called when applet first drawn or applet is hidden by anotherwindow and then re-exposed

repaint()

update(Graphics g)

paint(Graphics g)

sets flag

Clears screen, Calls paint


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Steps involved in developing and

testing an applet

Building an applet code (.java file)

Creating an executable applet (. Class file)

Designing a web page using HTML tags

Preparing tag

Incorporating tag into the web page

Creating HTML file

Testing the applet code


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1. Building applet code

Import java.awt.*;

Import java.applet.*;.

public class appletclassname extends Applet

{

..

..public void paint(Graphics g)

{

..

..

}

}


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Example

import java.awt.*;

import java.applet.*;

public class HelloJava extends Applet

{

public void paint(Graphics g)

{

g.drawString(hello world, 10,100);}

}


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2. Creating an executable applet

Move to the directory containing the source code

and type following command

javac HelloJava.java Compiled output file called HelloJava.class is placed

in same directory as source

If any error msg is received, then we must check for

errors, correct them and compile the applet again

3 D i i b


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3. Designing a web page

>

Title Tag

Applet Tag

Comment

section

Head

section

Body

section


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welcome to world of applets


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4. Applet tag

CODE=hellojava.class

WIDTH=400HEIGHT=200>

ddi l fil


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5. Adding applet to HTML file

welcome to java applets

welcome to world of applet

CODE=hellojav.class

WIDTH=400HEIGHT=200>


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More about Applet tag

Code= AppletFileName.class

Codebase=codebase_url

Width=pixels

Height=pixels

Name= applet_instance_name

Align=Alignment

Hspace=pixels

Vspace=pixels


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Passing parameters to applets

Eg.


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import java.awt.*;

import java.applet.*;//

public class HelloJavaParam extends Applet{

String str;

public void init()

{

str=getParameter(string); // receiving parameter value

if(str==null)

str=java;

str=hello + str; // using the value

}public void paint(Graphics g)

{

g.drawString(str,10,100);

}

}

Di l i i l l


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Displaying numerical valuesimport java.awt.*;

import java.applet.*;public class NumValues extends Applet

{

public void paint(Graphics g)

{ int value1=10;

int value2=20;

int sum=value1+value2;

String s=sum: + String.valueOf(sum);g.drawString(s,10,100);

}

}

Getting input from user


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Getting input from user

import java.awt.*;

import java.applet.*;public class UserIn extends Applet

{

TextField text1,text2;

public void init()

{

text1=new TextField(8);

text2=new TextField(8);

add(text1);

add(text2);text1.setText(0);

text2.setText(0);

}

Contd


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Contdpublic void paint(Graphics g)

{

int x=0,y=0,z=0;

String s1,s2,s;g.drawString ( input a no in each box , 10,50);

try

{

s1=text1.getText();

x=Integer.parseInt(s1);

s2=text2.getText();

y=Integer.parseInt(s2);

}

catch(Exception e) { }

z=x+y;

s=String.valueOf(z);g.drawString(the sum is: , 10,75);

g.drawString(s,100,75);

}

}


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Advanced Java Lecture-6

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