Introduction to Applets Overview What is an Applet? Steps for creating an applet What is HTML? Basic HTML tags Drawing Simple Graphical shapes » Rectangle » Ellipse » Lines Preview:
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Introduction to Applets
Overview
What is an Applet?
Steps for creating an applet
What is HTML?
Basic HTML tags
Drawing Simple Graphical shapes
» Rectangle
» Ellipse
» Lines
Preview: More on Applets
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What is an Applet?
An applet is a program that runs in a World
Wide Web browser when an HTML
document containing the applet is loaded
into the browser.
The code for an applet is enclosed in a class,
which is declared as public and as an
extension of a standard class called Applet.
You run applets within a page on
internet(browser) or within another
program called appletviewer.
Unlike java applications, applets do not have
main method.
The standard Applet class is contained in the
package called java.applet and must be
imported at the beginning of each applet
program
Applets can do most of the things that
applications can do but limited by some
security restrictions.
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Steps for creating and executing an applet
When you create an applet, you do the
following:
1.
Write the applet in the Java programming
language, and save it with a .java file extension,
just as when you write an application
Compile the applet into bytecode using the javac
command, just as when you do with an
application
Write an HTML document that includes a
statement to call your compiled java class
Load the HTML document into a Web
browser(such as Internet Explorer or, Netscape
Navigator)
2.
3.
4.
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What is HTML?
HTML or Hypertext Markup Language is a
simple language used to create Web pages
for the Internet.
It contains many commands that allow you
to format text on a Web page, import
graphic images, and link your page to other
Web pages.
These commands are case insensitive and
are usually called HTML Tags.
You can always identify a tag by the
brackets(< >) that enclose the tag name.
Example: The sentence:
Java is an <I> Object-oriented </I> language.
is displayed by the browser as:
Java is an Object-oriented language.
Obviously, the pair <I> and </I> are used to
indicate italics.
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Basics HTML tags
The following are some of the basic HTML
tags that we need load our applet:
<HTML> …</HTML>: These are used to
indicate the beginning and the end of a
HTML document. The entire content of a
HTML file should be enclosed in these pair
of codes.
<HEAD> …</HEAD>: A HTML file
consists of a head and a body and these tags
are used to indicate the beginning and end
of the head part.
<TITLE> …</TITLE>: This should appear
inside the head part of a HTML file and is
used to specify the information to be
displayed on the browser’s title bar. For
our case, this is the only tag that we need to
include in the head part.
<BODY> …</BODY>: These are used to
indicate the beginning and end of the body
part of a HTML file. Almost all the
contents of a html document appear within
these pair of tags.
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Basics HTML tags (Cont’d)
<APPLET CODE = “appletFile” WIDTH =
pixels HEIGHT = pixels> . . . </APPLET> :
This is used to instruct the browser to load an
applet.
Notice the additional specifications the applet
tag: CODE, WIDTH and HEIGHT. These are
called attributes of the tag. There are many
other attributes for the Applet tag but these are
the required ones.
The CODE is used to specify the applet file
(.class file), while WIDTH and HEIGHT are
used to specify the size of the window in which
the applet is displayed.
The following is a simple applet to display the
message “Hello World”
import java.applet.Applet;
import java.awt.Graphics;
public class HelloApplet extends Applet {
public void paint(Graphics g) {
g.drawString("Hello world!", 50, 25);
}
}
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Basics HTML tags (Cont’d)
The following is a sample HTML file, which may
be used to load the HelloApplet example.
<HTML>
<HEAD>
<TITLE> A Simple Program </TITLE>
</HEAD>
<BODY>
<APPLET CODE="HelloApplet.class"
WIDTH=150 HEIGHT=40>
</APPLET>
</BODY>
</HTML>
The following are sample output when the
applet is loaded in Windows explorer and Applet
viewer:
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Drawing simple graphical shapes
Java provides a package (java.awt) called the
Abstract Window Toolkit(AWT) to facilitate
graphics and GUI programming.
When you wish to draw graphics, you must
import the AWT this package.
An applet has a paint method, which is
executed by the browser each time the applet
is displayed. There are few other methods
that an applet may have which will be
discussed later.
The paint method receives an object of class
Graphics and is used to draw lines, ellipses
etc. on a web page. It is also use to set font
sizes and colors. Graphics is one of the
classes in the java.awt package.
However, recent versions of Java come with
another graphics class called Graphics2D,
which is better than the previous one. We
shall be using this new class instead.
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Drawing simple graphical shapes (cont’d)
To use the Graphics2D, we only need to convert
the object of class Graphics, which is passed to
the paint method into an object of class
Graphics2D using casting.
public void paint(Graphics g) {
Graphics2D g2 = (Graphics2D)g
...
In drawing graphics, the browser’s page is
considered as a drawing board with the top
right-hand corner as the origin (0, 0). The size
of the graphic shape that we wish to draw
should be specified relative to this origin
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Drawing a rectangle
To draw a rectangle, we need to create an
object of class Rectangle of the java.awt
package and then draw it using the draw
method of the Graphics2D object.
To create an object of class Rectangle, we
need to specify the following parameters:
» The top-left corner of the rectangle (x, y
co-ordinates)
» The width of the rectangle
» The height of the rectangle.
import java.applet.Applet;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
public class RectangleApplet extends Applet {
public void paint(Graphics g) {
Graphics2D g2 = (Graphics2D)g;
Rectangle rectangle = new
Rectangle(10,10,50,50);
g2.draw(rectangle);
}
}
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Drawing Ellipse
To draw an Ellipse, we need to import the
Ellipse2D class of the java.awt.geom. package.
The Ellipse2D class contained two Ellipse
classes, Ellipse2D.Float which uses float
coordinates and Ellipse2D.Double uses double
coordinates. We shall be using Ellipse2D.Double
Again, we need to create an object of class
Ellipse2D.Double and then use the draw method
of the Graphics2D object to draw it.
import java.applet.Applet;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.Ellipse2D;
public class CircleApplet extends Applet {
public void paint(Graphics g) {
Graphics2D g2 = (Graphics2D)g;
Ellipse2D.Double circle1 = new
Ellipse2D.Double(10,10,50,50);
Ellipse2D.Double circle2 = new
Ellipse2D.Double(20,20,30,30);
g2.draw(circle1);
g2.draw(circle2);
}
}
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Drawing lines
To draw a line, we need to create an object
of class Line2D.Double of the java.awt.geom.
package. To create such object, we need to
specify the two end-points the line. This can
be done in two ways:
By giving the x and y coordinates of both
points as in:
Line2D.Double line = new Line2D(x1, y1, x2, y2);
Or we can specify each point as an object as:
Point2D.Double from = new Point2D.Double(x1, y1);
Point2D.Double to = new Point2D.Double(x2, y2);
Line2D.Double line = new Line2D.Double(from, to);
The latter method is more object-oriented
and more useful particularly if the point
objects need to be re-used .
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Drawing lines (Cont’d)
import java.applet.Applet;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.Ellipse2D;
import java.awt.geom.Line2D;
import java.awt.geom.Point2D;
public class TriangleApplet extends Applet {
public void paint(Graphics g) {
Graphics2D g2 = (Graphics2D)g;
Point2D.Double pointA = new
Point2D.Double(30,10);
Point2D.Double pointB = new Point2D.Double(10,50);
Point2D.Double pointC = new Point2D.Double(50,50);
Line2D.Double lineAB = new Line2D.Double(pointA,
pointB);
Line2D.Double lineBC = new Line2D.Double(pointB,
pointC);
Line2D.Double lineCA = new Line2D.Double(pointC,
pointA);
g2.draw(lineAB);
g2.draw(lineBC);
g2.draw(lineCA);
}
}
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