Introduction to Java

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
 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:
 Hangman Game:
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
Applet Development
 There are two distinct phases in applet
 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
 JOptionPane.showMessageDialog(null,"Good
Morning "+input,"Greeting
 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
 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
 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
<H1 or H2…>
Ending Tag
</H1 or /H2…>
document head
document body
document title
different header levels
line break
ordered list
unordered list
an item in the list
an image
an address (hyperlink)
Program 9.4
HTML Program
<TITLE> Sample Document </TITLE>
This is the photo of a race:
<p align=“center”> <IMG SRC=“runner.jpg” ></p>
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
 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
 width=780
 height=500
 code="myfirst.class">
 The description seen by inept
 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 “”
 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
 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
 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
 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
 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

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