Advanced GUI Applications Part II

Contents
Introduction to Applets
Applet Limitations
Introduction to HTML
Document Structure Tags
Text Formatting Tags
Breaks in Text
HTML Links
Creating Applets With Swing
Running an Applet
Using appletviewer
Java Applet Tutorial

Introduction to Applets

Java Applets
- Applets are Java programs designed to work within a Web page.
- If a user opens the Web page with a Java-enabled browser, the applet is displayed inside the browser window.
- It appears to the user that the applet is part of the Web page.
- Applets are stored on a Web server along with the site’s Web pages.
- Applets associated with a viewed Web page are transmitted to the user’s system.
- Once the applets are transmitted, the user’s system executes them.
- Applets can be used to extend the capabilities of the Web page.
  
  Your browser does not support Applets.
Hypertext Markup Language (HTML)
- Web pages are transmitted to the user’s system as HTML.
- HTML is static content, whereas Applets are dynamic.
- They are stored and executed on the local computer.

Applet Limitations

For security purposes, by default, Applets cannot:
- access the local computer file system.
- run any other program on the user’s system.
- retrieve certain information about the users or their systems.
- make network connections with any system except the server from which the Applet was transmitted.

Introduction to HTML

HTML is the language of Web pages.
- Hypertext can contain a link to other content on the Web page or another Web page.
- A markup language allows designers to “mark up” a text file by inserting special instructions.
- These instructions tell the browser how to format the text and create any hypertext links.
A Web page is often created as a text file.
- It contains HTML instructions (known as tags).
- It contains the text that should be displayed on the Web page.
- It typically has a .html file extension.
The tags instruct the browser...
- how to format the text
- where to place images
- what to do when the user clicks on a link.
Most HTML tags have an opening tag and a closing tag.
- The tags are enclosed in angle brackets.
- The closing tag is preceded by a forward slash.
- For example:
```
<div>Some Text</div>
```

Document Structure Tags

The html tag marks the beginning and the ending of an HTML document.
```
<html>The entire HTML document go here.</html>
```
The head tag marks the document header, a section containing information about the document.
The document header contains the title tag, which defines the title of the page.
Book example
- BasicWebPage1.html

Example code:

<html>
<head>
    <title>Web Page Demo</title>
</head>
</html>

After the document header comes the body tag.
The document body contains tags and text that produce output in the browser.
Book example
- BasicWebPage2.html

Example code:

<html>
<head>
    <title>Web Page Demo</title>
</head>
<body>
    <p>This is the first paragraph of this page.</p>
    <p>This is the second paragraph of this page.</p>
</body>
</html>

Text Formatting Tags

There are many HTML tags that you can use to change the appearance of text.

For example, there are six different heading tags.

<html>
<head>
    <title>Web Page Demo</title>
</head>
<body>
    <h1>Header One</h1>
    <h2>Header Two</h2>
    <h3>Header Three</h3>
    <h4>Header Four</h4>
    <h5>Header Five</h5>
    <h6>Header Six</h6>
</body>
</html>

A level one heading appears in boldface and is much larger than regular text.
A level two heading also appears in boldface but is smaller than a level one heading.
This pattern continues with the other heading tags.

Many tags allow an align attribute to modify where the text shows on the web page:

<h1 align="center">Header One</h1>
<h1 align="left">Header One</h1>
<h1 align="right">Header One</h1>

An old way of centering text is to use the center tag to center a line of text.
You can display text in boldface with tag b and italics with tag i.
Book example:
- BasicWebPage3.html

Example:

<html>
<head>
    <title>Web Page Demo</title>
</head>
<body>
    <h1 align="center">Header One</h1>
    <p>This is a paragraph under header one.  <b>This line of text is bolded</b></p>
    <h2 align="center">Header Two</h2>
    <p>This is a paragraph under header two.  <i>This line of text is italicsed</i></p>
</body>
</html>

Breaks in Text

The hr tag causes a horizontal ruler to appear at the point in the text where it is inserted.
A horizontal ruler is a thin, horizontal line that is drawn across the web page.
Book Example:
- BasicWebPage4.html

Example:

<html>
<head>
    <title>Web Page Demo</title>
</head>
<body>
    <h1>Lorem Ipsum</h1>
    <p>Lorem ipsum dolor sit amet, anim id est laborum.</p>
    <hr/>
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit.</p>
</body>
</html>

HTML Links

A link is some element in a Web page that can be clicked by the user.
The tag that is used to insert a link has the following general format:
```
<a href="address">Text</a>
```
The Text that appears between the opening and the closing tags is the text that will be displayed in the Web page.
The Web resource that is located at the address will be displayed in the browser.
This address is a uniform resource locator (URL).

The address is enclosed in quotation marks.

<a href="http://www.jade-cheng.com/">Click here to go to Jade's website</a>

Book Example:
- LinkDemo.html

Creating Applets With Swing

Applets are very similar to the Swing applications.
Instead of displaying its own window, an Applet appears in the browser’s window.
The differences between GUI application code and applet code are:
- A Swing application window is commonly derived from JFrame.
- An Applet class is derived from JApplet.
- The JApplet class is part of the javax.swing package.
- A Swing application commonly has a constructor that creates and initializes the GUI.
- An Applet class does not normally have a constructor. Instead, it has a method named init that performs the same operations as a constructor.
- The following methods, common to Swing applications, are not called in an Applet.
  - super
  - setSize
  - setDefaultCloseOperation
  - pack
  - setVisible
- No main method is needed to create an Applet object.
- The browser creates an instance of the class automatically.
Book Example:
- SimpleApplet.java
- SimpleApplet.html
Example code:
- Refer to Jade’s Applet Tutorial page.

Running an Applet

The process of running an Applet is different from that of running an application.

Applets are executed by creating an HTML document with an applet tag, which has the following general format:

<applet
    code="Filename.class"
    archive="Filename.jar"
    width="pixel-width"
    height="pixel-height">
</applet>

Attributes are enclosed in quotes.
Filename.class is the compiled byte-code of the class that extends JApplet.
The browser
- downloads the specified Applet archive.
- executes the Applet in an area that is the size specified by the pixel-width and pixel-height.
Example code:
- Refer to Jade’s Applet Tutorial page.

Using `appletviewer`

The appletviewer program loads and executes an Applet without the need for a Web browser.
When running the appletviewer program, the name of an HTML document is specified as a command-line argument.

$ appletviewer SimpleApplet.html
This command executes any Applet referenced by an applet tag in the file SimpleApplet.html.
If the document has more than one applet tag, it will execute each Applet in a separate window.

Java Applet Tutorial

Please visit my Java Applet Tutorial page.