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Java Applets A lab course by Dr. Junaid Ahmed Zubairi SUNY Fredonia
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/in
dex.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 tagsTable 9.1 Common tags
MeaningMeaning----------------------------documentdocument headdocument bodydocument titledifferent header levelsboldfaceItalicunderlinedsubscriptsuperscriptcenteredline breakordered listunordered listan item in the listan imagean address (hyperlink)
Beginning TagBeginning Tag----------------
<HTML><HEAD><BODY><TITLE>
<H1 or H2…><B><I><U>
<SUB><SUP>
<CENTER><BR><OL><UL><LI>
<IMG><A>
Ending TagEnding Tag----------------</HTML></HEAD></BODY></TITLE>
</H1 or /H2…></B></I></U>
</SUB></SUP>
</CENTER>
</OL></UL></LI>
</A>
<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>
Program 9.4Program 9.4 HTML ProgramHTML Program
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 colors and run again
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 with colors as desired
