Chapter 14 Applets. 2 Knowledge Goals Understand the differing roles of applications and applets...

Chapter 14

Applets

2

Knowledge Goals

• Understand the differing roles of applications and applets

• Understand how a browser operates• Understand the role of HTML

3

Skill Goals

• Write an applet to perform a simple task• Embed Bytecode within a web page• Construct a simple HTML web page that

executes an applet

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What Is an Applet?

Applet

A mini-application Distributed along with web pages Run under a browser at the viewer’s site Run under an applet viewer Is distributed in Bytecode form

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What Is an Applet?

Applets differ from applications in several ways Applets don’t have a main method Applets are invoked differently Applets are subject to more security

constraints Applets are not in control of their own

destiny Applets do not have constructors;

initializations are done in method init

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What Is an Applet?

Inheritancehierarchy

forAWT and

Swing

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How Do We Write Applets?

// Applet Factorial computes the factorial of

// its input and stores it in a variable of

// type int, which is displayed on the

// screen.

import javax.swing; // JApplet class

import java.awt.*; // User interface classes

import java.awt.event.*; // Event classes

public class FactInt extends Applet implements ActionListener {…}

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public void actionPerformed(ActionEvent event)

{

int value;

value =

Integer.parseInt(inputField.getText());

inputField.setText("");

outLabel.setText(value + " factorial is ”

+ factorial(value));

}


Method call

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private int factorial(int n)

// Assumption: n is not negative.

{

if (n == 0)

return 1;

else return (n * factorial((n-1)));

}

Now, we have to set up a button, a label, and a text input field

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// Setting up a button, label, and input

// field

private static JTextField inputField;

private static JLabel label;

private static JLabel outLabel;

private static JButton button;

We need to write method init

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public void init(){ // Instantiate components label = new JLabel("Enter an integer; ” + “press Enter."); outLabel = new JLabel("Answer"); button = new JButton("Enter"); button.addActionListener(this); inputField = new JTextField("Value here");

Note

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// Add components

add(label);

add(inputField);

add(button);

add(outLabel);

// Specify a layout manager for the

// window object

setLayout(new GridLayout(4,1));

}


Why are the add s not applied to an object?

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How Do You Run an Applet?

The Bytecode version of an applet is run on the user’s browser

Yes, but how?

Some definitions are in order first

Computer network A collection of computing devices connected in order to communicate and share resources

Can you name some of the devices in a computer network?

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Local area network (LAN)

A network that connects a relatively small number of machines in a relatively close geographical area

Wide area network (WAN)

A network that connects local area networks over a potentially large geographic distance

Internet

A wide area network that spans the planet

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The Web An infrastructure of information combined and the network software used to access itUniform Resource Locator (URL)A standard way of specifying the location of a Web page, containing the hostname, "/", and a file

What is the relationship between theInternet and the Web?

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The World Wide Web

Why is the expression"visiting a website"

confusing?

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Hypertext Markup Language (HTML)The language used to create or build a Web pageMarkup language A language that uses tags to annotate the information in a documentTagsThe syntactic element in a markup language that indicates how information should be displayed

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HTML

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HTML Tags are enclosed in angle brackets

(<. . . >) Words such as HEAD, TITLE, and

BODY are called elements and specify the type of the tag

Tags are often used in pairs, with a start tag such as <BODY> and a corresponding end tag with a / before the element name, such as </BODY>

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The browser determines how the page should be displayed based on the tagsThe browser

Ignores the way we format the HTML document using carriage returns, extra spaces, and blank lines

Takes into account the width and height of the browser window

Reformats the contents to fit your browser window

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Tags

<HTML>…</HTML>

<HEAD>…</HEAD>

<TITLE>…</TITLE>

<BODY>…</BODY>

…

<HR>

…

…

<PRE>…</PRE>

Meaning

Enclose document

Enclose heading

Enclose title

Enclose body of document

Enclose paragraph

Insert horizontal rule

Enclosed should be boldface

Enclosed should be in italics

Preformatted; display text

as-is

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<APPLET code = “fileName.class” width=250height=150></APPLET>

<APPLET code = “FactInt.class” width=250height=150></APPLET>

Note FactInt.class is the file with the Bytecode version of the Applet FactInt

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Yes, But How Do You Run an Applet?

If you are in an integrated environment, your Java system will run the applet in the viewer

To run in a browser, compile the applet name the Bytecode file with a .class extension create the web page with the appropriate

<applet …> put the .class file in the same directory as the web

page access the web page

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Go tothiswebpageand

followthe

directions

Be sure FactInt.class is with this page

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Chapter 14 Applets. 2 Knowledge Goals Understand the differing roles of applications and applets...

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