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Chapter 14
Applets and HTML
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<APPLET> tag specifies applet information in a HTML page
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It must be enclosed between <BODY> and </BODY>
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</APPLET> tag is mandatory to close the <APPLET> tag
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CODE, WIDTH and HEIGHT are mandatory attributes within <APPLET> tag.
Their order is not significant. Instead of the class file, applet can specified
via a serialized file using OBJECT attribute. If we specify OBJECT attribute
CODE attribute is not specified and vice versa.
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In HTML 4.0, OBJECT tag can be specified instead of APPLET tag. With
OBJECT tag, we specify the applet with CLASSID attribute.
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The following are other optional tags
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Tag Name
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Purpose
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CODEBASE
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Directory for the applet’s class
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ALT
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Alternate text for browsers with no support for applets
but can understand <APPLET> tag
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HSPACE
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Left/Right boundaries b/w other HTML elements on a page
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VSPACE
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Top/Bottom boundaries b/w other HTML elements on a page
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ALIGN
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Alignment with respect to other HTML elements
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NAME
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Name of the applet for inter-applet communication
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ARCHIVE
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Name of the JAR file (Lot of files can be downloaded in
a single download to reduce the time needed) Even multiple jar files can
be specified, separated by commas.
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OBJECT
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Specified if CODE attribute is not present and vice
versa. Applet is read in from the specified serialized file.
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Between <APPLET> and </APPLET>, PARAM tags can be specified.
These are used to pass parameters from HTML page to the applet.
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<PARAM NAME = ”name” VALUE = ”value”>
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Applets call getParameter(name) to get the parameter. The name is not case
sensitive here.
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The value returned by getParameter is case sensitive, it is returned as
defined in the HTML page.
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If not defined, getParameter returns null.
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Text specified between <APPLET> and </APPLET> is displayed by
completely applet ignorant browsers, who cannot understand even the
<APPLET> tag.
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If the applet class has only non-default constructors, applet viewer
throws runtime errors while loading the applet since the default constructor is
not provided by the JVM. But IE doesn’t have this problem. But with applets
always do the initialization in the init method. That’s the normal practice.
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Methods involved in applet’s lifecycle.
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Method
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Description
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void init()
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This method is called only once by the applet context
to inform the applet that it has been loaded into the system. Always
followed by calls to start() and paint() methods. Same purpose as a
constructor. Use this method to perform any initialization.
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void start()
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Applet context calls this method for the first time
after calling init(), and thereafter every time the applet page is made
visible.
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void stop()
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Applet context calls this method when it wants the
applet to stop the execution. This method is called when the applet page
is no longer visible.
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void destroy()
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This method is called to inform the applet that it
should relinquish any system resources that it had allocated. Stop()
method is called prior to this method.
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void paint(Graphics g)
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Applets normally put all the rendering operations in
this method.
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Limitations for Applets:
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Reading, writing or deleting files on local host is not allowed.
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Running other applications from within the applet is prohibited.
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Calling System.exit() to terminate the applet is not allowed.
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Accessing user, file and system information, other than locale-specific
information like Java version, OS name and version, text-encoding standard,
file-path and line separators, is prohibited.
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Connecting to hosts other than the one from which the applet was loaded is
not permitted.
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Top-level windows that an applet creates have a warning message for
applets loaded over the net.
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Some other methods of Applet class
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Method
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Description
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URL getDocumentBase()
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Returns the document URL, i.e. the URL of the HTML file
in which the applet is embedded.
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URL getCodeBase()
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Returns the base URL, i.e. the URL of the applet class
file that contains the applet.
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void showStatus(String msg)
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Applet can request the applet context to display
messages in its “status window”.
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