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Java Applets
A lab course by
Dr. Junaid Ahmed Zubairi
SUNY Fredonia
Java Applets
Introduction to Applets
Applet Development
Graphics
Java Applications and Applets
In Java, you can develop applications and
applets
Applications can run by themselves
Applets are executed in a controlled
environment, usually within web browsers
Let us see some examples of applets
embedded in a web page
Some Applets
Hilo Game:
http://mainline.brynmawr.edu/Courses/cs11
0/fall2003/Applets/HiLo/Hi.html
Hangman Game:
http://www.bodo.com/Applets/Hangman/ind
ex.html
Applet Mechanism
Java applets are programs that are stored
with a web page
When a user requests that web page, the
applet embedded with it is sent from the
server to the user’s computer
Then the applet is executed in a “sandbox”
preventing it from corrupting the user’s
computer
Applet Development
There are two distinct phases in applet
development
The first phase is concerned with the
development of the actual Java applet
The second phase is about embedding the
applet in a web page
Let us try to change the programs developed
earlier to applets
Application Source Code
import javax.swing.JOptionPane;
public class myfirst {
public static void main(String[] args) {
String input =
JOptionPane.showInputDialog("What is your
name?");
JOptionPane.showMessageDialog(null,"Good
Morning "+input,"Greeting
Window",JOptionPane.ERROR_MESSAGE);
System.exit(0); }
}
Converting to Applet
Add the following line at the top:
import javax.swing.JApplet;
Add “extends JApplet” after the class name
Rename the main( ) to myfirst( ). There is
no main( ) method in an applet. Use public
myfirst( ) instead of the long list of
prequalifiers in the method title
Constructors
This new method “public myfirst( )” bears
the name of the class to which it belongs
Such methods are known as constructors.
They are very useful for initializing a
program
Constructors come in handy when there is
no main( ) method in a class
Embedding an applet inside a
web page
Web pages are written in a language called
HTML (HyperText Markup Language)
HTML provides tags and their attributes for
formatting the text
Following table shows the common tags
used in web pages
Table 9.1 Common tags
Beginning Tag
---------------<HTML>
<HEAD>
<BODY>
<TITLE>
<H1 or H2…>
<B>
<I>
<U>
<SUB>
<SUP>
<CENTER>
<BR>
<OL>
<UL>
<LI>
<IMG>
<A>
Ending Tag
---------------</HTML>
</HEAD>
</BODY>
</TITLE>
</H1 or /H2…>
</B>
</I>
</U>
</SUB>
</SUP>
</CENTER>
</OL>
</UL>
</LI>
</A>
Meaning
---------------------------document
document head
document body
document title
different header levels
boldface
Italic
underlined
subscript
superscript
centered
line break
ordered list
unordered list
an item in the list
an image
an address (hyperlink)
Program 9.4
HTML Program
<HTML>
<HEAD>
<TITLE> Sample Document </TITLE>
</HEAD>
<BODY>
This is the photo of a race:
<p align=“center”> <IMG SRC=“runner.jpg” ></p>
</BODY>
</HTML>
Tags and Attributes
In the HTML file (or the web page) shown on the
previous slide, a tag named img is used
The attributes of img are “src” and “align”
The “src” attribute points to the source of the
image
The “align” attribute lets us select left, center or
middle position for the image
Try to save the HTML file and load it in the web
browser. The image file is provided through a
separate URL on the course homepage
A Web Page that embeds our
applet
<applet
width=780
height=500
code="myfirst.class">
The description seen by inept
browsers</applet>
Explanation
In this web page, we use a tag named “applet”
The attributes of this tag include the width and
height of the applet window and code file name
Remember that the code file name used here is
“myfirst.class” instead of “myfirst.java”
This is because we need a compiled and ready to
run program as an applet
Please save this web page in a .html file
Compile the applet and then load the .html file to
see it run
Demo required
Graphics
We can draw various shapes inside an
applet window
A specific method named paint( ) is added
to the applet in order to draw shapes
Let us try to draw a rectangle
Java defines a class Rectangle that can be
used directly
Using Rectangle Class
Add the following import line at the top
import java.awt.Rectangle;
Define a rectangle in the program as
follows:
Rectangle book1 = new Rectangle(20,30,200,120)
The parameters are left top ‘x’ and ‘y’ followed by
‘width’ and ‘height’
In a Java graphics window, x increases left to right
and y increases top to bottom
Keeping it Simple
In order to keep it simple and manageable,
we replace the constructor method by paint
method
Replace the title line
– public myfirst( )
By this line
– public void paint(Graphics g)
Inside the Paint method
Inside the paint() method, simply add the
following line
– Graphics2D g2 = (Graphics2D)g;
The above line is required in order to use
advanced 2-D graphics features
Next insert the rectangle line as on slide 17
Call the draw method to draw the rectangle
– g2.draw(book1);
You may use fill method to fill the box with the
current color
Demo required for fill method
Changing Colors
The default fill color is black but we can change
the colors !!!
Add the following import line at the top
– Import java.awt.Color;
Define a new color with the following line inside
the program just before the fill call
– Color Fillcolor = new Color(1.0f, 0.2f, 0.3f);
Here the three floating point numbers represent
the strengths of red, green and blue in fillcolor
Now set the current color with the following line
– g2.setColor(fillcolor);
Demo: run the program, make at least two new
Drawing Lines
Add the following line at the top of the program:
– import java.awt.geom.Line2D;
Define a new line
– Line2D.Double bar = new
Line2D.Double(20,200,220,200);
Draw the line with the following statement
– g2.draw(bar);
Note how carefully we have placed the line below
the shape drawn earlier
Programming Exercise: Draw the front of a
colonial home with vaulted ceiling, front entrance
door and one window. Fill the door and window
