Appletsbjhbjiibibibikbibibjibjbibbjb.ppt

Java Applets
Section 3.3 (JIA’s)
Appendix A (JIA’s)

Introduction to Java and
Java Applets
 Java applications

Run in stand-alone mode
 No additional software required (such as a Web browser)
 Java applets

Compiled Java class files

Run within a Web browser (or an appletviewer)
 Loaded from anywhere on the Internet

security restrictions!

Java Basic Concepts
 Source Code converted to Byte code
 Byte code -machine code of JVM (Java Virtual
Machine)
 Each real machine must have own JVM

Interpretation

JIT compilation

Direct Execution
 Java Byte Code consists of

1 Byte opcode

1 or more operands

Capabilities and Limitations
of Applets
 Build full-featured graphical user interfaces
(suitable for the Web)
 Communicate over the Internet to a host
server (support Client-Server architecture)
 Communicate with other applets on a form
 Environment-neutral (any platform)
 Limitations on Java applets to ensure client security

Capabilities and Limitations
of Applets
 Bytecode verification

Forces loaded Java applets to undergo a rigorous set of checks in order
to run on the local system

The verifier checks each bytecode before it is executed to make sure that
it is not going to perform an illegal operation
 Client-side precautions

Most Web browsers preclude Java applets from doing file access or
communicating with any computer on the Internet other than the
computer that the applet was loaded from

Enforced by the client Web browser (or other applet loader) but done by
a part of the Java runtime engine known as the class loader

First Java Applet
import java.awt.*; //Contains all of the classes for creating user interfaces
//and for painting graphics and images
import java.applet.Applet;
public class HelloFromVenus extends Applet {
public void paint(Graphics g) {
Dimension d = getSize();
g.setColor(Color.orange);
g.fillRect(0,0,d.width,d.height);
g.setFont(new Font("Sans-serif",Font.BOLD,24));
g.setColor(new Color(255, 10, 0));
g.drawString("Hello From Venus, a Mars Colony!",
40, 25);
g.drawImage(getImage(getCodeBase(),"venus.jpg"),
20, 60, this);
}

HTML Source
<html>
<head>
<title> Hello From Venus Applet </title>
</head>
<body bgcolor=black text=white>
<h2>Here is the <em>Hello From Venus</em>
Applet</h2>
<center>
<applet code="HelloFromVenus.class" width=700
height=500>
</applet>
</center>
<hr>
<a href="HelloFromVenus.java">The source.</a>
</body>
</html>

Elements of Java Applets
 Superclass: java.applet.Applet
 extend javax.swing.JApplet if you have swing
components
 Swing: Sun's set of GUI components that give much
fancier screen displays than the raw AWT
 No main() method
 paint() method paints the picture
 Applet tags:
code width height

Compile and Run an Applet
To compile: javac HelloFromVenus.java  Generates
HelloFromVenus.class
To run:
a) Use the appletviewer from JDK
appletviewer Venus.html
b) Open page from browser:
Venus.html

Applet’s Life
 Each applet has four major events in its lifetime:

Initialization --- init()
 Starting --- start()

Painting --- paint(Graphics)

Stopping --- stop()

Destroying --- destroy()
 The methods

defined Applet class

Except for paint()  in class java.awt.Container

do nothing--they are stubs

You make the applet do something by overriding these
methods

Applet’s Life
 When an applet begins the following sequence of methods
is called

init()

informs applet that it has been loaded into the system

Called only once

an ideal place to initialize variables and create UI objects

start()

informs applet that it should start its execution

Right after init()

Each time the page is loaded and restarted

paint(Graphics)
 When an applet dies (or is terminated), the following
sequence of method calls takes place:

stop()

informs applet that it should stop its execution

When a web browser leaves the HTML document

Applet’s Life
 destroy()

informs applet that it is being reclaimed and that it
should destroy any resources that it has allocated

Use destroy() to explicitly release system resources
(like threads)
 Usually released automatically (Auto garbage collection)

Called only once
 when the environment determines that your applet needs to
be removed completely from memory
 The stop() method is always called before destroy()
 no guarantee that this method will be completely executed

The Java Virtual Machine might exit before a long destroy
method has completed

Methods are called in this
order
 init and destroy are only
called once each
 start and stop are called
whenever the browser enters
and leaves the page
 do some work is code called by
your listeners
 paint is called again when the
applet needs to be repainted
init()
start()
stop()
destroy()
paint
do other work

public void paint(Graphics g)
 Needed if you do any drawing or painting other than just using
standard GUI Components
 Any painting you want to do should be done here, or in a method you
call from here
 For painting done in other methods

Never call paint(Graphics), always call repaint( )
 Life Cycle Applet via AppletViewer
 Automatically called when

when the applet begins execution

the window in which the applet is running may be overwritten by another
window and then uncovered

the applet window is resized

Other Applet Methods
 public void repaint()
 public void update (Graphics)
 public void showStatus(String)
 public String getParameter(String)
 http://download.oracle.com/javase/6/docs/
api/java/applet/Applet.html

repaint( )
 Call repaint() when you have changed something
and want your changes to show up on the screen

after drawing commands (drawRect(...),
fillRect(...), drawString(...), etc.)

Outside paint
 repaint( ) is a request

it might not happen!

When you call repaint( ), Java schedules a call to
update(Graphics g)

public void update(Graphics g) {
// Fills applet with background //
color, then
paint(g);
}

Sample Graphics methods
 A Graphics is something you can paint on
g.drawRect(x, y, width, height);
g.fillRect(x, y, width, height);
g.drawOval(x, y, width, height);
g.fillOval(x, y, width, height);
g.setColor(Color.red);
g.drawString(“Hello”, 20, 20); Hello

Drawing Strings
g.drawString("A Sample String", x, y)

The java.awt.Color Class
•Instances of the Color class represent colors
• new Color(r, g, b)
•where r, g, b are the values of the red, green, and
blue components, respectively
•Range of 0 to 255
•Set of constants defined in java.awt.Color

The java.awt.Font Class
 Fonts are specified with three attributes:
 font name: Serif Sans-serif Monospaced Dialog
DialogInput TimesRoman Helvetica Courier Dialog
 font style: PLAIN BOLD ITALIC
 Styles can be combined: Font.BOLD|Font.ITALIC
 font size: a positive integer
 A font can be created as follows:
 new Font(name, style, size)

The java.awt.Graphics
Class
Represent Graphics Context
A graphics context is an abstraction of various
drawing surfaces:
-screen
-printer
-off-screen image (an image stored in memory)
Provide a rich set of graphics methods
drawString() drawLine()
drawArc() fillArc()
drawOval() fillOval()
drawPolygon() fillPolygon()
drawRect() fillRect()
drawRoundRect() fillRoundRect()

The java.awt.Graphics Class
(cont'd)
setColor(color) set the current color
setFont(font) set the current font
setPaintMode() set the paint, or overwrite mode
setXORMode(color) set the XOR mode
getColor() get the current color
getFont() get the current font
getFontMetrics() get the font metrics of the current font
getFontMetrics(font) get the font metrics for the specified
font

showStatus(String s)
 showStatus(String s) displays the String
in the applet’s status line
 Each call overwrites the previous call
 You have to allow time to read the line!

Example Applet
 import java.awt.*;
 import java.applet.Applet;
 import javax.swing.JOptionPane;
 //try it in eclipse using AppletViewer
 public class LifeCycleApplet extends Applet
 {
 Font theFont = new Font("Helvetica", Font.BOLD, 20);
 String Status;
 public void init(){
 Status = "Initializing!";
 showStatus("The applet is initializing!");
 JOptionPane.showMessageDialog(this,Status);
 repaint();}
 public void start(){
 Status += "--Starting!";
 showStatus("The applet is starting!");
 repaint();}
 }

Example Applet

public void stop(){
 Status += "--Stopping!";
 showStatus("The applet is stopping!");
 repaint();}
 public void destroy(){
 Status += "--Destroyed!";
 showStatus("The applet is being destroyed!");
 //might cause freezing problems due to
 //unpredictability of when VM calls this method
 repaint();
 }



Example Applet

public void paint(Graphics g){
 Status += "--Painting!";

 Dimension d = getSize();
 g.setColor(Color.orange);
 g.fillRect(0,0,d.width,d.height);
 g.setFont(theFont);
 g.setColor(Color.blue);
 g.drawString("Author:"+getParameter("FName")+" "+getParameter("LName"),50,50);
 g.drawString("URL of the applet : " + getCodeBase(), 50, 100);
 g.drawString("URL of document : " + getDocumentBase(), 50, 150);
 g.drawString(Status, 50, 200);
 showStatus("The applet is painting!");
 //JOptionPane.showMessageDialog(this,Status);}

HTML Tags
<html>
<head>
<title> Hi World Applet </title>
</head>
<body>
<applet code="HiWorld.class" width=300
height=200>
<param name="arraysize" value="10">
</applet>
</body>
</html>

HTML Source

<html>
<head>
<title>Clock</title>
</head>
<body bgcolor=white>
<h1>The Digital Clock Applet</h1><p>
<applet code= DigitalClock.class
width=400 height=100>
</applet>
<p><hr>
<a href= LifeCycleApplet.java>The source</a>
</body>
</html>

The <APPLET> Tag
 The syntax for using the <APPLET> tag is the following:

<APPLET attributes>
<applet_parameter_tags>
alternate_content
</APPLET>
 The APPLET attributes are standard values that all applets
accept and are a standard part of HTML
 The applet_parameter_tags contain applet-specific
parameters that are read by the applet at runtime
 This is a handy way of passing arguments to an applet to
allow the applet to be more generic

The <APPLET> Tag
 <APPLET> Tag Attributes

ALT-Alternate text that can be displayed by text-only browsers

ALIGN-The ALIGN attribute designates the alignment of the applet
within the browser page

CODE-(Required) The CODE attribute is used to indicate the .class
file that loads the applet

CODEBASE-The CODEBASE attribute is used to indicate the location of
the .class file that loads the applet

HEIGHT-(Required) The HEIGHT attribute is used to set the applet's
bounding rectangle height

HSPACE-The HSPACE attribute sets the amount of horizontal space to
set off around the applet

NAME-The NAME attribute sets the symbolic name of the applet

VSPACE-The VSPACE attribute sets the amount of vertical space to set
off around the applet

WIDTH-(Required) The WIDTH attribute is used to set the applet's box
width

The <APPLET> Tag
 Passing Parameters to Java Applets

Parameters are an easy way to configure Java applets
without actually changing the source file

Background color based on preference (different HTML files)

In the previous applet example, the text drawn on the
screen was drawn using the blue color

This was "hardwired" into the applet's code

However, just as easily, we could have passed a
parameter to the applet specifying that it use the blue
tag

See next example

The <APPLET> Tag
 // Passing parameters to the applet using HTML parameters.
<HTML>
<HEAD>
<TITLE>This is the LifeCycle applet!</TITLE>
</HEAD>
<BODY>
<H1>Prepare to be amazed!</H1>
<BR>
<APPLET CODE="LifeCycleApplet.class" WIDTH=600
HEIGHT=50>
<PARAM NAME=color VALUE="blue">
If you can see this, your browser does not support Java
applets
</APPLET>
</BODY>
</HTML>
 The only question left to be answered is this: how does the Java applet
determine the value of the parameters?

The <APPLET> Tag
 The answer is that the applet has to call the getParameter() method
supplied by the java.applet.Applet parent class
 Calling getParameter("color") using the previous Java applet
example would return a String value containing the text "blue“
 It is then left up to the applet to take advantage of this information and
actually paint the text blue on the screen
 Here are three methods commonly used by applets:

String getParameter(String name): Returns the value for the specified
parameter string

URL getCodeBase(): Returns the URL of the applet

URL getDocumentBase(): Returns the URL of the document containing
the applet

Appletsbjhbjiibibibikbibibjibjbibbjb.ppt

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