Applets 
Peter Komisar  ©   Conestoga College   latest version   5.7 / 2010

Technologies like Macromedia's Flash have won the
battle for certain niches that Java applets once might
have played a significant role, such as for visual display
ads and animations.

Java applets, though, remain a very powerful technology,
able to download to the client's machine an application
that supplies the full power of the Java programming
language.

Today's compilers and runtime environments, coupled
with fast PCs have eliminated the lack of performance
that 'dogged' the earliest years of Applet use. 

Lately, new tags have been provided for use in HTML
pages that pop up invitations to download the latest
Java Plug-Ins if Java is not supported by the user's
browser.

A reference section on the Java Plug-In is appended at
the end of the note. 

 

Characterizing the Applet

Standalone Applications vs. Container Managed Objects

To this point our discussions have been focused on
running stand-alone Java programs from the command
line. These are characterized by the presence of a main( )
method and the use of command, ' java ' which invokes
the java interpreter.  Stand-alone applications supply an
'in house' Java Virtual Machine locally on the same
computer on which the application is run.

// applets run in a 'context',  supplied by a web browser

Java Program Variations

There are a variety Java programs that can run within
a Java context provided by another program. Servlets
for instance, run in a JVM supplied by a web server.
The runtime environment in this case is referred to as
a 'Servlet Engine'. ( Java Server Pages are a variation
of the servlet and use the same context.)  Enterprise
JavaBeans also run in the context of another program
that has been generically named, a 'Container'.

Applets are a third example of this style of Java program. 
They run in an JVM supplied by the browser. The Applet
API refers to this shell as the 'AppletContext'. The Applet
style of programming was developed early in Java's history,
being debuted on Sun workstation at Sun One in 1995. A
more recent version of this style of programming is the
'Midlet' that runs in  very small devices such as cell
phones.

Java Objects That Run in the Context of a Container JVM

// KVM a Reduced Java Virtual Machine & Library designed
// to run in the constrained environment of cell phones

Because Applets are run in a Java Virtual Machine supplied
by a browser, it follows that applets will use a different form
of programming than a standalone Java application. These
differences are relatively minor and mainly involve not needing
to use the main( ) method.

Applets are able to take full advantage of all the capabilities you
have learned to use in standalone applications and as such can
provide a very powerful way distributing applications to multiple
clients via the browser.

A further benefit of learning to use applets, is they introduce
the developer to the general approach which is taken by all
the container managed components and their APIs.

Applet Sandbox

Because powerful code is being downloaded to a host machine
there is an opportunity for sinister code to do a lot of damage
on the host machine. To prevent this, applets by default are
contained in a security domain called the 'sandbox'. The
sandbox is like a firewall that is built within a system around
an application.

Restrictions are based on what the inner application can do
to the surrounding environment, for instance, by default, applets
cannot access the local file system of the host machine that it
is downloaded to.

Following is a list of functions that are not available to
downloaded applets.

Default Access of an Applet Downloaded Over the Net

• cannot read or write files
• cannot open a socket // except to it's home server
• cannot start a program on host system
• cannot call non-Java code

Signed applets (signed as in digitally signed using encryption
keys)  can be assigned privileges using Java's security system.
This supplies an alternative to the sandbox limitations on
standard applets where a trusted applet may be then assigned
special privileges such as the ability to read files from the local
machine.

Local Applets are Privileged

The HTML Connection

The applet is even more unique in that it is a 'card carrying' member
of the HTML language in that it has it's own HTML tag. To use applets
the programmer has to do some HTML coding in order to locate an
applet in a host web page that a browser may  display. It is from an
applet's tag in an HTML page that the browser is able to find and run
the applet class file. The tag also supplies the browser with the
dimensions the applet has been set to so the applet can be sized
correctly in the page.

// applets are referenced from special tags inside HTML documents

Applets Are AWT and Swing Visual Components

Applets are full fledged Java visual component. They need to be
painted on the screen just like any other visual component. The
Applet class is also a container component which means it can
be populated with other visual components.

Applets are 'HeavyWeight' Containers

AWT's Applet class and Swing's JApplet are both base container
classes. JApplet in Swing is one of the few heavyweight or peer-
based components that can host other lightweight Swing widgets.

// Applets are heavyweight, container-type visual components.

Java's Early History and the Applet

In Java's early years, applets were often used to create visual
novelties on web pages. 

Many of the first Java textbooks were written with Java applets
serving as the base component over which Java's abilities were
demonstrated. These particular early books often emphasized
the use of Graphics and animations.

There is a large legacy of Java applets that are still being run
on the net. If an applet needs the additional power of the Java
programming language it is still a popular choice for creating a
portable web 'codelet' that can run in a client's browser.

Applets in HTML & the Object Tag

It is necessary to become familiar with the HTML page from
which an applet is referenced by the browser. This page is
coded in HTML. Hypertext Markup Language is a page
description language that provides instructions in the form
of Tags.

The browser interprets these tags to format it's presentation
which might include multimedia content such as sound and
images. Java-enabled browsers support applets which are
set in an HTML page using the APPLET tag.

HTML tags are not case sensitive.

The following example uses the convention where HTML
tags are set to upper case. The APPLET tag must appear
somewhere within the BODY tags of an HTML document.

// hTmL is not case sensitive, APPLET tags go in the
// BODY of the page. 

Applet vs Object Tag

The HTML 4 specification deprecates the APPLET tag
in favor of the OBJECT tag. The OBJECT tag is really
not any harder to use than the APPLET tag and for the
most part is simply a matter of substituting one for the
other. The Applet tag attributes become a subset of the
larger and more generic attribute list supported by the
OBJECT tag.

Because of the legacy of applets on the web, the APPLET
will be supported for a very long time to come and probably
indefinitely. Already there are trends that seek to support
legacy aspects of the Internet. The applet tag will live
probably as long as we are using HTML in browsers. 

Following is an example of a basic Java APPLET tag
followed by an Object tag that may be used to replace it.
You'll notice there is an extra attribute called 'codetype'
in the Object tag. The Object tag is designed to be a
generic container for a variety of code types of which
Java applets are just one. Also notice the 'code' attribute
is replaced with the 'classid' attribute.

Comparing APPLET and OBJECT Tags

<APPLET code="Bubbles.class" width="500" height="500">
    Bubbles!
</APPLET>

  // This example may be rewritten with OBJECT as follows:

<OBJECT codetype="application/java" classid="java:Bubbles.class"
   width="500" height="500">
   Bubbles!
</OBJECT>
 
 

HTML Tags of a Typical Page

The following HTML code is the skeleton of a simple HTML
page. First the page is identified as an HTML document by the
<HTML> tag. The <HEAD> tag contains special information
about the page. For example the HEAD can be used to supply
tips for search engines to find the page. The <TITLE> tag is
used to name the document. This name appears in the browsers
button for the page but not in the document. The BODY is where
all the page content goes in a traditional HTML page. The
APPLET tag should go in the BODY. In the following example
the Applet class resides in the same directory as the Web page.

Key HTML Tags

• <HTML>
• <HEAD>
• <TITLE>
  
• <BODY>
  

Basic Form of an HTML Page With Applet

< HTML >
  < HEAD >
   < TITLE ><!- appears in pages  title bar   --> </ TITLE >
  < /HEAD >

<BODY> 
<!- applet tags must appear somewhere between body tags  -->
    < H1 >
     Here's a little text about this applet
  < /H1 >  < BR >
 < APPLET CODE = Apt.class WIDTH = 200 HEIGHT = 150 >
 </ APPLET >
 </ BODY >
</ HTML >

<!- the Applet class file in this example would be
      in the same directory as the Web page. -->

Here is an example that shows of a very minimal page
you can construct to use to develop and test your applets.
Note the BODY and TITLE tag have been left out all
together.

Minimal HTML Page With Applet

<HTML>
  <BODY>
      <APPLET CODE=Zapplet.class WIDTH=100 HEIGHT=100>
              </APPLET>
  </BODY>
</HTML>

A lot of the general functionality of HTML is available
for use within the APPLET tag as shown in the detailed
example below.

The Applet Tag in Detail

Following is a more detailed description of the APPLET tag
with all it's attributes listed. Notice most of these attributes are
located within the angle braces of the opening APPLET tag.
Only the PARAM tags are located between the two Applet
Tags. We carry forward a brief comparison of Applet and
Object tags in comments below each tag or in the regular text. 

<APPLET

      CODE = class file (file or URL)
      HEIGHT = pixels
      WIDTH = pixels

      <!--  [ ] square brackets indicate optional tags  -->
 
      [ CODEBASE = URL directory ]             
      [ ALT = alternate text message ]
      [ ARCHIVE = names of JAR files ]
      [ ALIGN = alignment ]
      [ NAME = name for other applets on page ]
      [ HSPACE = pixel ]
      [ VSPACE = pixel ]

 >  <!--  note above instructions are enclosed withing the first applet tag -->

      [ < PARAM NAME = identifier VALUE = value > ]
     [ < PARAM NAME = identifier VALUE = value > ]
                                   . . . . . . . . ..
      [alternate HTML ]

 </ APPLET >   

<!-- CODE WIDTH and HEIGHT tags are mandatory -->
 

WIDTH & HEIGHT Attributes

The width and height tags are obligatory tags. HTML requires
that they be included in the applet tag. They specify the width
and height of the applet in pixels These sizes reflect the
dimensions the applet will attempt to attain. Other page items
may constrain the size of the applet is actually allowed to be.

The getSize( ) method Applet inherits from Component
class can be used to report the actual size the browser
has given to the applet. This method returns a Dimension
object from which the height and width can be referenced.

The following code sample shows an overridden paint( )
method extracted from a sample applet. We explain the
details of this method later.

// Width and height attributes are the same in both
// Applet & Object Tags
 

Example  

public void paint( Graphics g){
      Dimension d=getSize( );
      g.setFont(new Font("Monospaced", Font.BOLD,16));
      g.drawString
     ("getSize reports width and height " + d.width +" "+ d.height, 50, 60 );
          }
 

The CODE Attribute

The applet class that will run in an HTML page when
that page is visited is specified by the CODE attribute.
The code attribute should be specified in quotes despite
the fact that browsers may forgive their omission.

( HTML veterans say that leaving quotes out might effect
the operation of older browsers. Also new XML and
XHTML standards require attribute values be specified
inside double quotes so it is a good general practice. )

Notice it is the class file that is specified to the code attribute.

Example  

code="X.class"

In HTML specifications, the CODE attribute is described
as specifiable by a path relative to the calling  HTML or
URL. More than one text have been printed stating this.

In practice, with many of today's browsers this isn't the case.
(Try for yourself! It should be tested periodically in any case. )
Van der  Linden in his book  'Just Java' gets it right and
states that CODE must be provided as a single file name
with no part of a longer path prefixed to it. 

// The Object tag uses the combination of codetype and classid
// ie.  codetype="application/java"  classid="java:Bubbles.class" 

The CODEBASE Attribute

The 'codebase' attribute is very important, especially in
light of the way the 'code' tag is currently implemented.
The 'codebase' attribute can be used to specify

• an absolute path
•  a URL or
  
• a relative path 

(The relative path is relative to where the calling HTML
file is located.)

The 'codebase' attribute may undergo semantic changes
which are described in the W3 HTML 4 specification. The
'codebase' attribute will be restricted to sub-directories of
the directory containing the current document for security
reasons. Note this restriction doesn't exist at this time as
will be evident when you look at the following example.

// memo: retest

Examples Using the CODEBASE Attribute

In this example a HTML page is put in H2 directory, in
H directory in P directory. The Applet code is put in C2
directory in C directory in P directory as per Fig.1
Directory Tree.

 Fig.1 DirectoryTree for Codebase Examples

     P
    _|_
   |   |
   H   C
   |   |
  H2   C2
 

Example 1

// accessing by an absolute path
<applet code="T.class" codebase="C:/P/C/C2"  width="300" height="300">
</applet>

Example 2

// accessing by a path relative to the location of the calling HTML page
<applet code="T.class" codebase="../../C/C2"  width="300" height="300">
</applet>

Example 3

// accessing the applet using an absolute path via a File URL form on a local machine
<applet code="T.class" codebase="file:///c:/newcode/P/C/C2"  width="300" height="300">
</applet>

Example 4

// using an absolute path via an HTTP URL for an applet located on an Internet machine
<applet code="T.class" codebase="http://www.sentex.net/C/A/T" width="90" height="90">
</applet>  

// If HTML 4 is fully implemented in the browser you are using, accessing a
// directory as in example 2 may not be allowed. 

The ALT Attribute

The ALT attribute provides an alternate text for browsers that
know the tag but do not support applets. This tag is useful for
really old browser, that don't support Java,  and may be useful
for really new browsers that may leave downloading the Java
plug-in up to the user.  It may now also be useful for new
lightweight browsers used on hand-held devices that eliminate
items from their displays.

The ALT tag is the same in APPLET and OBJECT tags.

Example

< APPLET 
  CODE = "Arizona.class" WIDTH ="400" HEIGHT ="250"
  ALT = "Picture of the Grand Canyon" >
  < /APPLET >
 

HSPACE, VSPACE & ALIGN Attributes

These variables control the applet position on the HTML page
and the space between them, and adjacent items on the page.
HSPACE specifies the distance in pixels between the applet's
left and right boundaries and any adjacent HTML elements.
VSPACE controls the distance in pixels between the applet's
top and bottom boundaries and any adjacent HTML elements.

ALIGN values may be assigned any of the following HTML
constants,

• LEFT
• RIGHT
• BOTTOM
• TOP
• TEXTTOP
• ABSBOTTOM
• ABSMIDDLE
• MIDDLE
• BASELINE

These tags are the same in both Object and Applet tags.

Example  

ALIGN="MIDDLE" 

The ARCHIVE Attribute

A JAR or Java 'ARchive' file is a 'zipped' collection of files.
The ARCHIVE tag identifies JAR files on the server that the
browser needs to download. Using JAR files speeds remote
class loading. Bundling files in JARs reduces what might be
several download transactions to a single archive download.

The CODE tag still needed to identify the class inside the jar
that needs to be executed. The Object tag uses the ARCHIVE
attribute in the same way as does the Applet tag.

Example

<APPLET  CODE=X.class WIDTH=90 HEIGHT=60 ARCHIVE="Zip_In.jar" >
< /APPLET >

Multiple JARs can be downloaded using a comma-separated
list as is shown in the next example.

Example

<APPLET  CODE=A.class
WIDTH=100 HEIGHT=100 ARCHIVE="A.jar, B.jar, C.jar">
< /APPLET >

The NAME & VALUE Attributes

<applet code=clock.class name=clock1>
</applet>
<applet code=clock.class name=clock2>
</applet>
<applet code=setter.class>
  <param name=clockToSet value=clock1>
</applet>
<applet code=setter.class>
  <param name=clockToSet value=clock2>
</applet>

The PARAM Tag 
// used to pass values to applet via HTML

PARAM tags are the only nested elements defined for the
APPLET tag. Unlike the other APPLET attributes, PARAM
appears between the two applet tags. The PARAM tag
allows the HTML developer to supply parameters to a Java
applet without having to rework the Java code itself.

Whether a multimedia player, a weather applet or a news
reel, the HTML developer can pass information to the applet
via the PARAM Tags without having to recompile the applet. 

The PARAM tag has two attributes, NAME and VALUE
which can be used to supply name value pairs to the applet.

Example   // lower case html for a change

<applet code="ParTake.class" width =300 height=300>
      <param name="artisto" value="Pablo Picasso">
      <param name=title value="Bather With Beach Ball">
      <param name=image value="Bather.jpg">
      <param name=imageNo value="32">
</applet>

The Applet Class    

Applet & JApplet

When we talk about applets, we must consider both the
classic Applet class found in the java.awt package and
the newer JApplet which resides in the javax.swing
package.

Applet JApplet  Differences

• Layouts
  

Applet and JApplet are different in a couple of ways.
Applet class has a flow layout manager by default  while
the JApplet class has a border layout as the default
layout manager. Recall the flow layout allows components
to remain their preferred size and adds them left to right,
top to bottom, centering as addition are made.

The border layout on the other hand has five fixed regions,
east, west south, north and center and has a hybrid policy
where components are allowed to be their preferred size
on one axis but are controlled by the layout on the other
axis. Only one component can occupy each of the five
regions of the border layout at a time.

// JApplet's default layout is BorderLayout, Applet's is FlowLayout

• Frame Layering
  

JApplet also has the more complex layered behavior
that has been incorporated into JFrame, where an area
to add a menu is provided and layered panes are used
to show pop up menus and tooltips. JApplet like other
Swing components can have icons added.

Classic Applets Were the Safest Bet For Widespread Support
// no longer so relevant with the Java Plug-in and high speed connections

In the past browsers did not provide uniform support for newer
versions of Java. Microsoft browsers for a long time supported
a dated version of Java (JDK1.7). ) Accordingly, classic Applet
enjoyed widespread support and were a 'safe bet'. .

Today there is considerable support in browsers for downloading
the Java Plug-In, similar to how you might be invited to download
Macromedia's Flash plug-in if it isn't on your PC.

With the prevalence of high speed connections, the complication
to get a recent version of Java with Swing support is no longer as
much an issue as it was in the past.  This makes using the newer
Swing JApplet much more viable.

The Sun Microsoft Truce Helped Bring Support the Swing Applets

Recently Sun and Microsoft buried the hatchet, a shed a long
running competitive feud. Perhaps old adversaries were allying
perceiving Linux and generally the Open Source paradigm as a
threat to the proprietary model.  

Now in 2010, Oracle has bought Sun and Google is the new
gorilla on the block and Oracle finds itself supporting Sun's
position of resisting splintering of the Java language into
incompatible dialects. .

All things considered, the scales seemed to have tipped in favor
of support for Swing applets, so classic AWT applets can be put
to pasture.

A word of caution, when developing applets, sometimes browsers
can be 'too effective' at caching an applet and won't load a revised
version instead reverting to the cached version. If you are making
changes to an applet's code, and they are not being reflected in
the browser it is reasonable to suspect you are seeing an earlier
cached version.

Though, refresh or reload should get rid of the applet, sometimes
it is necessary to reopen the browser on the applet containing page.
If this doesn't work you can close and reopen the browser itself! 

Clearing Cache

Clearing the cache is the best strategy.

On Mozilla go to:

Edit --> Preferences --> Advanced --> Cache --> Clear Cache

On Window's Explorer go to:

Tools --> Internet Options --> Delete Files

// get rid of cached pages by
1) Refreshing  2) Reopening on File 3) Clearing Cache // 4 ) Restart browser

You may still find you have to close a browser and reopen it
occasionally.

Methods of the Applet Life Cycle

When an applet is run in a browser a series of methods are
called which represent the applet's life cycle. The Java
developer overrides these methods in order to derive the
desired behavior. Following are the methods that are called
during normal applet service that can be overridden to control
the functionality of the applet.
 

Table Summarizing the Applet's Life Cycle Methods 
// info from JDK Documentation 
 

 public void 
  destroy( )

init( )  // initialize and load GUIs in init( )

When a Web page containing an applet has been accessed, the
applet class is loaded into the browser's runtime environment. The
first method called is init( ) which acts like a constructor in a regular
Java program to initialize values. GUI components will typically be
instantiated and added to the applet from inside the init( ) method.
 

start( )  // like main( )  

The start( ) method is called after init( ). This method is like the main( )
method in a stand-alone application. It is different in that what triggers
it is the page being visited in the browser. If the applet is going to spawn
threads when the applet is visited start( ) is where these threads are 
activated.
 

paint(Graphics g )

After start( ), the paint( ) method is called. The paint method is not
unique to applets. This is the same method that is used by all of
Java's visual components. As with other components paint( ) can
be overridden to create custom displays or to create custom
components. The paint( ) method supplies a handle to a graphics
context object that can be used to access the drawing methods
of the classic Graphics class as well as the methods of the new
2D graphics class. The paint( ) method is automatically called
whenever a component needs painting.
 

stop( )

When the browser leaves the page, stop( ) is called. Stop shuts
down the applet and closes any resources that may have been
associated with it. After the stop( ) method is called the applet
is down but not out! The applet still resides in memory cache in
case the user returns to the applet's page.

destroy( )

The destroy( ) method is the final method of the applet life cycle. It
is called to tell the applet that it is being reclaimed and  thus should
release any resources that it has allocated. The stop( ) method is
always called before destroy( ). The destroy( ) ,method supplies a
good place to kill any threads that were created in init( ) and started
in start( ). Applet's default version of destroy( ) does not do anything.
 

Other Applet Methods              

                                      
Following is a table summary of key Applet's methods. They
are supplied here mainly for reference and to show what sorts
of things are available for use.

Aside from Applet's life cycle methods, there are methods for
getting and playing audio files. The applet class also has several
methods for handling images. Notice the methods provided for
obtaining parameters from the HTML page that the applet resides
in. 

Applet class also has informational methods that tell which
directory the applet is located in and which directory the
HTML page that references the applet is located in. Applet
class has a method that returns a handle to the Java Runtime
Environment in which the applet is running. The method,
getAppletContext( ) returns an object of the AppletContext
class. AppletContext supplies methods to control the HTML 
environment in which the applet is resident.
 

Survey of Applet Methods
 

'HelloWorld' in an Applet

AWT Applet Imports

If an applet is built using the classic AWT package the applet
package must be imported along with the java.awt  package.

Example  

import java.awt.*;
import java.applet.*;

Applet is Public

The second key aspect to creating applets is simply to extend
the Applet class. The compiler will tell you that the public keyword
must be included in the class signature. An applet not marked
public will compile with default access however will fail at runtime.

An IllegalAccessException is thrown, because a subclass
has been defined that is more private than it's parent class.

Example  

public ThisApplet extends Applet {   /*etc. */  }

Override Life Cycle Methods

Finally, in order for your applet to do anything, one of the
applet's life cycle methods  must be overridden. In most
HelloWorld examples written for applets, the paint( )  method
is overridden and a Graphic's class method,  drawString( ) is
used to draw the text HelloWorld over the component. Just
to make the code more interesting looking, we borrow another
method from the Graphics class to make the font bold.
 

Hello World in an Applet Code Sample

Adding the Applet Tag to Source Code

For convenience, the Java environment comes with a command
line tool called appletviewer that allows applets to be viewed
before they are set inside HTML pages.

Adding the APPLET tag to the top of the source code there is
enough information for 'appletviewer' to run the source code as
the document containing the applet.

The source code still needs to be compiled to a class file as the
tag only provides a reference to the class file.

Because appletviewer first looks for the applet tag, it is the Java
source code is supplied as an argument to the 'appletviewer' tool.

The example above as well as the following examples shows the
minimum Applet tag added to the source code. Note the tag is
commented out. This makes it a comment to the Java code but
in terms of HTML the comment symbols are just document character
content and as an HTML reader, 'appletviewer' only 'sees' the code
tagged inside angle brackets. Such a tag is exampled below.

Example

// <applet code=OK.class width =400 height =100></applet>

// Applet Tag Commented out & Added to Java Applet Source Code

The following example shows that the Java Source code
is supplied as an argument to the appletviewer command,
which views the source code first as an HTML document.

Example   

C:\>   appletviewer  OK.java

// appletviewer finds the applet reference then runs the class after finding it
 

Summary For Using appletviewer Top Develop Applets

Following is a summary of the steps involved in developing applets.
First the APPLET tag is put into the source code and commented
out. The applet is saved and compiled like any other Java file. Then,
'appletviewer' is run not on the class name but using the Java source
file containing the applet tag as an argument.

1) Put the applet tag into the source file
2) Comment it out     
// this leaves the tag visible as script but not as code
3) Save and compile as usual ( using the .java
   extension and 'javac' )
4) Run appletviewer on source file.

Example

> appletviewer applet_file_name.java 
 

Setting the Applet to the HTML Page

Once the applet is running like you want it to it can be moved
into an HTML file. Initially, even the HTML file can be tested with
appletviewer. The final test is to open the page in whichever
browsers you want the page and applet to work in.

1) Move your applet tag into the body of an HTML page
2) Assuming the Java source code has been compiled,
  run appletviewer on the HTML file.

Example

> appletviewer html_file_name.html 

3) The page is also ready to view in any Java-enabled
    browser.

'HelloWorld' in Swing

Swing Imports

Following is the Swing version of 'HelloWorld' using
Swing components. First the imports are different.

Example

import java.awt.*;
import javax.swing.*;

The second difference is the JApplet class is extended.
Notice too that now any components added to JApplet
will be added to a border layout and will need to call
getContentPane( ).

While In earlier years Swing applets would not run in Netscape
4.x or Internet Explorer 5.x browsers, the newer Java plug-in
has made Swing support nearly universal on the web.

Following is the Swing version of  'HelloWorld'. This example
includes the commented out applet tag.

Hello World using the Swing Class, JApplet

// <applet code=OK2.class width =400 height =100></applet>

import java.awt.*;
import javax.swing.*;

public class OK2 extends JApplet{
   public void paint( Graphics g){
     g.setFont(new Font("Monospaced", Font.BOLD, 22));
     g.drawString("OK, It's Hello World In Swing", 50, 60 );
   }
}

Overriding Applets Methods & Building GUIs in Applets

The following code example shows the approach one can take
to build a GUI in an applet. Basically, if the components need
to be accessed from other class members the components will
need to be declared in class scope. The instantiation will usually
be done in the init( ) method.

In this example all the applet life cycle methods are overridden.
The console output and the visual components are used to signal
the effects of each overridden method.

To see paint being called resize the window dramatically by
dragging and dropping the applet frame with the mouse. The
stop( ) method will be called when the applet is iconified or
closed. Try iconifying a few times first before closing out the
applet. Following is the code.

Applet GUI and Overriding Example

// <applet code=Cyclet.class width=500 height=200></applet>


import java.net.*;
import java.awt.*;
import java.applet.*;

public class Cyclet extends Applet{
    TextField init,start,paint,stop;
   

  public void init( ){
     System.out.println("Init has been called");
     setLayout(new GridLayout(5,1));

      init = new TextField("Init is called");
      start= new TextField();
      paint= new TextField();
      stop = new TextField();
      add(new Label("Use menu, (upper right). Resize to see paint() called. Restart to see stop() called"));
      add(init);
      add(start);
      add(paint);
      add(stop);

      }
  public void start( ){
      System.out.println("Start has been called");
       start.setText("Start has been called");
      }

  public void stop( ){
      System.out.println("Stop has been called");
      stop.setText("Stop is called");
      }

   public void paint( Graphics g){
     paint.setText("Paint is called");
     System.out.println("Paint has been called");
     }

   public void destroy( ){
     paint.setText("Destroy is called");
     System.out.println("Destroy has been called");
     }
}

Self Test                                                        Self Test With Answers

1) Which of the following does not run in the context of another program

a) stand-alone applications
b) an applet
c) a servlet
d) an enterprise java bean
 

2) Which of the following is not a mandatory applet tag?

a ) code
b) width
c) height
d) codebase
 

3)  Mark the method that is not one of the standard methods overridden in an applet.

a) init( )
b) start( )
c) run( )
d) stop( )
e) paint( )
 

4) Which of the following lines is not correct in the following applet?

a)  import java.awt.*;
b)  import java.awt.applet.*;
c) public ThisApplet extends Applet {
d) public void paint( Graphics g){ g.drawString("Hi!", 10, 10 );  }
}

5) Appletviewer

a) is a utility to facilitate quick viewing of applets during development
b) requires as an argument a class file just as is passed to the 'java' command
c) can run  an HTML file or a Java source file
d) to run an applet from source file the applet tag must be supplied and
   commented out.

6) True or False: Because the init( ) method of an applet acts like a
constructor, constructors cannot be used with applets.
 

7) In which of the following methods of the applet life cycle is the recommended
     location for building GUIs.
a) init( )
b) start( )
c) paint( )
d) stop( )

8) Which of the following statements is not correct. While accessing
parameters from the applet tag

a) The PARAM tag has a NAME and and a VALUE component
b) an applets getParameter( ) method argument must match case sensitive
    with the value assigned to the NAME attribute in the PARAM tag.
c) The PARAM tag is located between the two applet tags
d) The PARAM tag does not need a closing </ PARAM> tag

Exercise

1) In this exercise 4 AWT applets are created that will be used
in combination to print a statement to screen. Use Label objects
added to the overridden init( ) method in two of the applets to
add text. In the other two use the Graphics method, drawString( )
as exampled in the notes above.

Begin by using appletviewer together with a commented out applet
tag, to make sure each applet works.

Transfer the four applet code tags to an HTML page. You may use
ALIGN, HSPACE and VSPACE to locate the 4 applets on the page.

One approach successfully used is to use a  break tag, <BR> or
a paragraph tag, <P>, to pair applets on different rows.

If you are savvy at the use of HTML you might use spans or other
tags to do the job.

Use each applets height and width attributes so the four applets fill
the page. Include an ALT tag in applet tag so an alternative message
is provided in case the page is opened into a browser that doesn't
support Java. Once the applet and page are prepared, open the
page in Explorer and Netscape.

 2) Convert the AWT applets to Swing JApplets. Remember, JApplet
uses a BorderLayout, though if we are adding a single label to each
applet this shouldn't matter. Also you need to import the javax.swing
package. 

3) Create a second HTML page to support these applets. Instead of
transferring the Applet tags, convert them to the W3C recommended
Object tag. Observe in Netscape ( Mozilla ) and Explorer. Briefly report
what happened what results you obtained, using the different browsers.

Optional

There are some fancy Universal IDs that can be used to make the Object
tag work correctly in Explorer.

4 ) Find on the web the formulation of the Object tag that has ID and
attributes allowing the Object Tag to work  in Internet Explorer.

// so far we have found support in Mozilla but not in Explorer for the minimum forms
// of the object tag. If you seek out examples there are extra attributes you can add
// to get Object tag to work Explorer. Above, in Q.3 I am only asking to test the simple
// version of Object and report observations in two browsers.