What is an Applet?
According to Sun "An applet is a small program that is intended not
to be run on its own, but rather to be embedded inside another
application....The Applet class provides a standard interface between applets
and their environment."
Four definitions of applet:
·
A small application
·
A secure program that runs inside
a web browser
·
A subclass of java.applet.Applet
·
An instance of a subclass of
java.applet.Applet
public class Applet extends Panel
java.lang.Object
|
+----java.awt.Component
|
+----java.awt.Container
|
+----java.awt.Panel
|
+----java.applet.Applet
Hello World: The Applet
import java.applet.Applet; import java.awt.Graphics;
public class HelloWorldApplet extends Applet
{
public void paint(Graphics g) { g.drawString("Hello world!",
50, 25); }}
The applet version of HelloWorld is a little more complicated than the
HelloWorld application, and it will take a little more effort to run it as
well.
First type in the source code and save it into file called
HelloWorldApplet.java. Compile this file in the usual way. If all is well a
file called HelloWorldApplet.class will be created. Now you need to create an
HTML file that will include your applet. The following simple HTML file will
do.
<HTML>
<HEAD>
<TITLE> Hello World </TITLE>
</HEAD>
<BODY>
This is the applet:<P>
<applet code="HelloWorldApplet.class"
width="150" height="50"> </applet>
</BODY>
</HTML>
Save this file as HelloWorldApplet.html in the same directory as the
HelloWorldApplet.class file. When you've done that, load the HTML file into a
Java enabled Web Browser..
The APPLET HTML Tag
Applets are embedded in web pages using the <APPLET> and
</APPLET> tags. The <APPLET> tag is similar to the <IMG> tag.
Like <IMG> <APPLET> references a source file that is not part of
the HTML page on which it is embedded. IMG elements do this with the SRC
attribute. APPLET elements do this with the CODE attribute. The CODE attribute
tells the browser where to look for the compiled .class file. It is relative to
the location of the source document.
The CODEBASE attribute is a URL that points at the directory where the
.class file is. The CODE attribute is the name of the .class file itself. For
instance if on the HTML page of the previous section you had written
<APPLET
CODE="HelloWorldApplet.class"
CODEBASE="classes" WIDTH=200 HEIGHT=200>
</APPLET>
then the browser would have tried to find HelloWorldApplet.class in the
classes directory in the same directory as the HTML page that included the
applet.
The HEIGHT and WIDTH attributes work exactly as they do with IMG,
specifying how big a rectangle the browser should set aside for the applet.
These numbers are specified in pixels and are required.
Spacing Preferences
The <APPLET> tag has several attributes to define how it is
positioned on the page.
The ALIGN attribute defines how the applet's rectangle is placed on the
page relative to other elements. Possible values include LEFT, RIGHT, TOP,
TEXTTOP, MIDDLE, ABSMIDDLE, BASELINE, BOTTOM and ABSBOTTOM. This attribute is
optional.
You can specify an HSPACE and a VSPACE in pixels to set the amount of
blank space between an applet and the surrounding text. The HSPACE and VSPACE
attributes are optional.
<APPLET
code="HelloWorldApplet.class"
CODEBASE="http://www.foo.bar.com/classes"
width=200
height=200
ALIGN=RIGHT
HSPACE=5
VSPACE=10>
</APPLET>
The ALIGN, HSPACE, and VSPACE attributes are identical to the attributes
of the same name used by the <IMG> tag.
The <APPLET> has an ALT attribute. An ALT attribute is used by a
browser that understands the APPLET tag but for some reason cannot play the
applet. , then the browser should display the ALT text.
<applet
code="HelloWorldApplet.class"
CODEBASE="http://www.foo.bar.com/classes" width=200 height=200
ALIGN=RIGHT HSPACE=5 VSPACE=10
ALT="Hello World!">
</APPLET>
Naming Applets
You can give an applet a name by using the NAME attribute of the APPLET
tag. This allows communication between different applets on the same Web page.
<APPLET
code="HelloWorldApplet.class"
Name=Applet1
CODEBASE="http://www.foo.bar.com/classes"
width=200
height=200
ALIGN=RIGHT HSPACE=5 VSPACE=10 ALT="Hello
World!">
Hello World!<P>
</APPLET>
JAR Archives
HTTP 1.0 uses a separate connection for each request. When you're
downloading many small files, the time required to set up and tear down the
connections can be a significant fraction of the total amount of time needed to
load a page. It would be better if you could load all the HTML documents,
images, applets, and sounds a page needed in one connection.
One way to do this without changing the HTTP protocol, is to pack all
those different files into a single archive file, perhaps a zip archive, and
just download that.
You can pack all the images, sounds, and .class files an applet needs
into one JAR archive and load that instead of the individual files. Applet classes
do not have to be loaded directly. They can also be stored in JAR archives. To
do this you use the ARCHIVES attribute of the APPLET tag
<APPLET
CODE=HelloWorldApplet
WIDTH=200 HEIGHT=100
ARCHIVES="HelloWorld.jar">
<hr>
Hello World!
<hr>
</APPLET>
In this example, the applet class is still HelloWorldApplet. However,
there is no HelloWorldApplet.class file to be downloaded. Instead the class is
stored inside the archive file HelloWorld.jar.
The OBJECT Tag
HTML 4.0 deprecates the <APPLET> tag. Instead you are supposed to
use the <OBJECT> tag. For the purposes of ewbedding applets, the
<OBJECT> tag is used almost exactly like the <APPLET> tag except
that the class attribute becomes the classid attribute. For example,
<OBJECT
classid="MyApplet.class"
CODEBASE="http://www.foo.bar.com/classes" width=200 height=200
ALIGN=RIGHT HSPACE=5 VSPACE=10>
</OBJECT>
Finding an Applet's Size
When running inside a web browser the size of an applet is set by the
height and width attributes and cannot be changed by the applet. Many applets
need to know their own size. After all you don't want to draw outside the
lines. :-)
Retrieving the applet size is straightforward with the getSize() method.
java.applet.Applet inherits this method from java.awt.Component. getSize()
returns a java.awt.Dimension object. A Dimension object has two public int
fields, height and width. Below is a simple applet that prints its own
dimensions.
import java.applet.*; import java.awt.*;
public class SizeApplet extends Applet {
public void paint(Graphics g) { Dimension appletSize = this.getSize(); int appletHeight = appletSize.height; int appletWidth = appletSize.width;
g.drawString("This applet is " +
appletHeight +
" pixels high by " + appletWidth + "
pixels wide.", 15, appletHeight/2);
} }
Passing Parameters to Applets
Parameters are passed to applets in NAME=VALUE pairs in <PARAM>
tags between
the opening and closing APPLET tags. Inside the applet, you read the
values passed through the
PARAM tags with the getParameter() method of the java.applet.Applet
class.
The program below demonstrates this with a generic string drawing
applet. The applet parameter
"Message" is the string to be drawn.
import java.applet.*; import java.awt.*;
public class DrawStringApplet extends Applet
{
private String defaultMessage = "Hello!";
public
void paint(Graphics g) {
String inputFromPage = this.getParameter("Message");
if (inputFromPage == null)
inputFromPage = defaultMessage; g.drawString(inputFromPage,
50, 25);
}
}
You also need an HTML file that references your applet. The following
simple HTML file will
do:
<HTML>
<HEAD>
<TITLE> Draw String </TITLE>
</HEAD>
<BODY>
This is the applet:<P>
<APPLET
code="DrawStringApplet.class" width="300"
height="50"> <PARAM name="Message" value="Howdy,
there!">
</APPLET>
</BODY>
</HTML>
You pass getParameter() a string that names the parameter you want. This string should match the name of a <PARAM> tag in the HTML page. getParameter() returns the value of the parameter. All values are passed as strings.
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