Applets

1
Applets

• The Applet Class
• The ltappletgt HTML Tag
• Passing Parameters to Applets

2
HelloWorld Applet

• // This application program prints Hello World!
• public class HelloWorld
• public static void main(String args)
• System.out.println(Hello World )
• // This applet prints Hello World!
• import java.awt.
• import java.applet.
• public class HelloWorldApplet extends Applet
• public void paint(Graphics g)
• g.drawString(Hello, World!)

3
Applications vs. Applets

• Similarities
• Since they both are subclasses of the Container
  class, all the user interface components, layout
  managers, and event-handling features are the
  same for both classes.
• Differences
• Applications are invoked by the Java interpreter,
  and applets are invoked by the Web browser.
• Applets have security restrictions
• Web browser creates graphical environment for
  applets, GUI applications are placed in a frame.

4
Security Restrictions on Applets

• Applets are not allowed to read from, or write
  to, the file system of the computer viewing the
  applets.
• Applets are not allowed to run any programs on
  the browsers computer.
• Applets are not allowed to establish connections
  between the users computer and another computer
  except with the server wherethe applets are
  stored.

5
Conversions Between Applications and Applets

• Conversions between applications and applets are
  simple and easy.
• You can convert an applet into an application.
• You can convert an application to an applet as
  long as security restrictions are not violated.

6
The Applet Class

• public void init()
• public void start()
• public void stop()
• public void destroy()
• public void paint(Graphics g)
• public void update(Graphics g)

7
Browser Calling Applet Methods
8
The init() Method

• Invoked when the applet is first loaded and
  again if the applet is reloaded.
• Common functions implemented in this method
  include creating threads, loading images, setting
  up user-interface components, and getting
  parameters from the ltappletgt tag in theHTML
  page.

9
The start() Method

• Invoked after the init() method is executed
  also called whenever the applet becomes active
  again after a period of inactivity (for example,
  when the user returns to the page containing the
  applet after surfing other Web pages).
• Functionality might include restarting
  threads(for example, to resume an animation) or
  simply telling the applet to run again.

10
The stop() Method

• The opposite of the start() method, which is
  called when the user moves back to the page
  containing the applet the stop() method is
  invoked when the user moves off the page.
• When the user leaves the page, any threads the
  applet has startedbut not completedwill
  continue to run.

11
The destroy() Method

• Invoked when the browser exits normally to
  inform the applet that it is no longer needed and
  that it should release any resources it has
  allocated.
• Usually, you will not need to override this
  method unless you need to release specific
  resources, such as threads that the applet
  created.

12
The paint() Method

• public void paint(Graphics g)
• is invoked to redraw the graphical representation
  of the applet.

13
The update() Method

• Override update(Graphics g) method to avoid
  flickering effect.
• public void update(Graphics g)
• // do something
• public void paint(Graphics g)
• update(g)

14
Example Applets

• Geocoordinates conversion
• http//www.cs.joensuu.fi/koles/utm/utm.html
  
• Vector map visualization
• http//www.cs.joensuu.fi/koles/svf/svf.htm
  l
• Electronic Documents Management (EDM)
• http//www.cs.joensuu.fi/koles/edm/edm2.ht
  ml

15
Writing Applets

• Always extends the Applet class or extends the
  JApplet class, which is a subclass of Applet for
  Swing components.
• Override init(), start(), stop(), and destroy()
  if necessary. By default, these methods are
  empty.
• Add your own methods and data if necessary.
• Applets are always embedded in an HTML page.

16
The ltappletgt HTML Tag

• ltapplet
• codeclassfilename.class
• widthapplet_viewing_width_in_pixels
• heightapplet_viewing_height_in_pixels
• archivearchivefile
• codebaseapplet_url
• vspacevertical_margin
• hspacehorizontal_margin
• alignapplet_alignment
• altalternative_text
• gt
• ltparam nameparam_name1 valueparam_value1gt
• lt/appletgt

17
Passing Parameters to Applets

• ltapplet
• code MyApplet.class"
• width 200
• height 50gt
• alt"You must have a Java-enabled
• browser to view the applet"
• ltparam nameMESSAGE valueHello world!"gt
• ltparam nameX value20gt
• ltparam nameY value20gt
• lt/appletgt

18
Parsing Parameters in Applet

• public class MyApplet extends Applet
• private String message default message
• private int x 10
• private int y 10
• public void init()
• message getParameter(MESSAGE)
• try
• x Integer.parseInt(getParameter(X))
• y Integer.parseInt(getParameter(Y))
• catch(Exception e)

19
Examples of Applets

• See code and applet on web
• http//www.cs.armstrong.edu/liang/introjb4.h
  tml
• See more examples of applets on web
• http//www.dgp.toronto.edu/mjmcguff/learn/ja
  va/