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Applets and HTML
Chapter 13
Chapter 13
1
Introduction
• Applets are simply Java programs designed to
run from a document (page) on the World
Wide Web.
• HyperText Markup Language (HTML) is the
language used to create Web documents.
Chapter 13
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Introduction to Applets
• An applet is a “small application” or a “little
Java program.”
• Applets are Java programs that are typically
displayed on a Website and viewed over the
Internet.
• An applet can also be run as a stand-alone
program on a computer which is not
connected to the Internet.
Chapter 13
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Applet Basics
• An applet is a derived class of class JApplet
which is a class in the Swing library.
• When writing an applet, it is a good idea to
include all of the following:
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
Chapter 13
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Applet Basics, cont.
Chapter 13
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Applet Basics, cont.
• A JApplet is a Container, which permits you to
add components to it in the same way you
can add components to a JFrame.
• But, applets do not use the setVisible method
or the setTitle method, nor do they need any
size instructions.
• Applets are displayed automatically, and do
not need to have a main method.
Chapter 13
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Applet Basics, cont.
• Applets typically do not use constructors.
Instead they use method init which serves
the same purpose.
• Setting colors, adding components, etc. is
done in method init.
• Method init has no parameters and is not
overloaded.
• Applets do not need to be closed using
listeners.
Chapter 13
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Applet Basics, cont.
• class HelloApplet
Chapter 13
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Applet Basics, cont.
Chapter 13
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Running an Applet
• Applets are compiled the same way other
Java classes are compiled.
• However, the normal way to run an applet is
as part of a Web document.
– The applet then is viewed using a Web
browser.
• An applet can also be viewed using an applet
viewer which is a program designed to run an
applet as a stand-alone program.
Chapter 13
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Programming Example: An
Adder Applet
• class AdderApplet
Chapter 13
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Programming Example: An
Adder Applet, cont.
Chapter 13
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Converting a Swing
Application to an Applet
• Derive the class from JApplet rather than from
JFrame.
• Remove method main.
• Replace the constructor(s) with a method
named init.
Chapter 13
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Converting a Swing
Application to an Applet
• Delete any invocation of addWindowListener.
• Delete any invocation of setTitle.
• Delete any invocation of setSize.
Chapter 13
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Adding Icons to an Applet
• An icon typically is a small picture.
• By placing the icon in a JLabel, the icon is
displayed.
• A JLabel can consist of text, an icon, or both.
• A JButton or JMenuItem can also have an icon.
Chapter 13
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Adding Icons to an Applet,
cont.
• class DukeApplet
Chapter 13
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Adding Icons to an Applet,
cont.
Chapter 13
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Adding Icons to an Applet,
cont.
• ImageIcon
is a class in the Swing library.
• syntax
ImageIcon Name_of_ImageIcon =
new ImgeIcon(Picture_File_Name);
is a string giving either a
relative or absolute path name to the
picture file.
– Picture_File_Name
Chapter 13
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Introduction to HTML
• Documents to be read on the Web or using a
Web browser typically are expressed in a
language called HTML.
• HTML stands for HyperText Markup
Language.
• Hypertext contains links (or hyperlinks) which
permit you to go to other documents.
Chapter 13
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Introduction to HTML, cont.
• These documents are called pages.
• HTML is not a general-purpose programming
language like Java.
• Instead, it is a collection of simple (markup)
commands that can produce something that
can be viewed using a Web browser.
Chapter 13
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Introduction to HTML, cont.
• The commands allow you to include pictures
and hyperlinks.
• The commands also allow you to specify
headings, subheadings, paragraph
beginnings, etc.
• In short, HTML is mostly a language for
formatting a manuscript so that it can be
viewed on the Web.
Chapter 13
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HTML Basics
• Most HTML commands are of the form
<command>
Some text
</command>
• example
<h1>
My Home Page
</h1>
Chapter 13
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HTML Basics, cont.
• Anything between <center> and </center> is
centered on the page when it is displayed.
• Some commands do not need a closing
command. For example
<br> begins a new line and
<p> begins a new paragraph.
• The browser inserts breaks where necessary
to fit the text appropriately on the screen.
Chapter 13
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HTML Basics, cont.
• HTML is not case sensitive. <table>, <Table>,
<TABLE>, <tAbLe> are all the same…
• An HTML file is a regular text file that you
create and edit with a text editor.
• HTML files should end with .html or .htm
• Commands such as <table> and </table>
form a “container” (in this case a table
container).
Chapter 13
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HTML Basics, cont.
Chapter 13
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HTML Basics, cont.
• The entire document is enclosed between
<html> and </html>.
• The head of the document is enclosed
between <head> and </head>.
– The head contains information used by a
browser, but typically not displayed.
– It might consist only of a title enclosed
between <title> and </title>, used to
name the document.
Chapter 13
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HTML Basics, cont.
• Two parts of the document are displayed on
the screen.
– The body is enclosed between <body> and
</body> and is the real content of the
document.
– The address is optional. It is enclosed
between <address> and </address>. It
includes the address of the document’s
owner and the last date the document was
modified.
Chapter 13
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HTML Basics, cont.
Chapter 13
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HTML Basics, cont.
Chapter 13
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Inserting Hyperlinks
• The key active element in an HTML
document is a link that the person viewing the
document can click to view another HTML
document.
• syntax
<a href=“Path_to_Document”>
Displayed_Text_to_Click
</a>
Chapter 13
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Inserting Hyperlinks, cont.
• example
<a href=“http://www-cse.uscd.edu/users/savitch”>
Walter Savitch
</a>
• The Path_to_Document can be either a full path
name or a relative path name to a HTML file or
a URL to any place on the Web.
Chapter 13
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URLs
• A URL is a kind of path name for the World
Wide Web.
• URL stands for Uniform Resource Locator.
• URLs are absolute path names to documents
anywhere in the world.
• Relative path names can be used for
documents on your own computer.
Chapter 13
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Displaying the Most Current
Version of a Document
• While you are developing an HTML page, you
can display the most recent version of the
page by clicking the button labeled Reload (or
perhaps Refresh).
• Otherwise, for efficiency, the browser may
access an earlier copy of the page.
Chapter 13
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Displaying a Picture
• A picture can be inserted into an HTML
document using
<img src=“File_with_Picture”>
• example
<img src=“images/mypicture.jpg”>
• The picture can be in any directory, but the
path name, either full or relative, leading to
the picture must be provided.
Chapter 13
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Placing an Applet in an HTML
Document
• To display the adder window created by
class AdderApplet, place the following
command in an HTML document:
<applet code=“AdderApplet.class” width=400
height=200>
</applet>
(Actually
“.class”
code=“AdderApplet”
is optional.
works just as well)
Chapter 13
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Placing an Applet in an HTML
Document, cont.
• This command assumes that the HTML file
and the file AdderApplet.class are in the same
directory.
– Otherwise, a relative or absolute path
name to AdderApplet.class is needed.
Chapter 13
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Placing an Applet in an HTML
Document, cont.
Chapter 13
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Placing an Applet in an HTML
Document, cont.
Chapter 13
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Using an Old Web Browser
• A Web browser must be set up to run applets.
• Web browsers do not use the same Java
interpreter used to run Java applications.
• Older Web browsers (yours or someone
else’s who may want to view your HTML
document) may not be able to run applets
from an HTML document.
Chapter 13
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Using an Old Web Browser,
cont.
• Furthermore, Java updates for browsers
typically lag core Java language updates.
• Using the older Applet class sometimes can
remedy the problem.
• These problems do not exist if you are
running applets from the applet viewer using
a recent version of Java.
Chapter 13
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The Older Applet Class
• To use the older Applet class instead of the
JApplet class
– remove the Js from JApplet, JButton, JLabel,
etc. (that is, use Applet, Button, Label)
– use the following import statements
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
Chapter 13
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The Older Applet Class,
cont.
– you do not need
import javax.swing.*;
– add components to the applet to itself
rather than using a content pane (whatever
was done to the content pane of a JApplet
should be done directly to the Applet).
Chapter 13
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The Older Applet Class,
cont.
– example:
substitute
add(friendlyLabel);
for
getContentPane().add(friendlyLabel);
• Furthermore, class Applet cannot
accommodate icons easily.
Chapter 13
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Applets and Security
• Your applet is a program that may be run on
someone else’s computer.
• Worse, someone else’s applet might be run
on your computer!
• Furthermore, you don’t know that an HTML
page contains an applet until you load it into
your browser, and then it is too late to reject
the applet; it is already stored on your
computer.
Chapter 13
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Applets and Security, cont.
• Someone else’s program running on your
computer creates serious security concerns.
– Will it leave a virus?
– Will it alter your files or read confidential
information?
– Will it corrupt your operating system?
• Applets cannot do any of these things (at
least not easily).
Chapter 13
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Applets and Security, cont.
• Applets cannot run your programs, nor can
they read from or write to files on your
computer (unless the applet originated on
your computer).
Chapter 13
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