Applets

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Applets

• Life Cycle
• Drawing and Event Handling
• Methods for UI Components
• Applet Capabilities
• Example

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Life Cycle

• Loading the applet
• instance of applet subclass is created
• applet initializes itself init() (like
  constructor)
• applet starts running start() (execution,
  except user input)
• Leave and Come Back
• stopping stop()
• restarting start()
• iconification minimize

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• Reloading the Applet
• Quitting the Browser
• stop()
• final cleanup destroy()
• init, start, stop, destroy methods

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Drawing and Event Handling

• paint() Basic display method.
• update() method used with paint() to improve
  performance
• Objects notified of registered events (listen for
  them)
• Register event by implementing appropriate
  interface (mouseListener)

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Pre-Made AWT UI Components

• Buttons (java.awt.Button)
• Checkboxes (java.awt.Checkbox)
• Single-line text fields (java.awt.TextField)
• Larger text display and editing areas
  (java.awt.TextArea)
• Labels (java.awt.Label)
• Lists (java.awt.List)
• Pop-up lists of choices (java.awt.Choice)
• Sliders and scrollbars (java.awt.Scrollbar)
• Drawing areas (java.awt.Canvas)
• Menus (java.awt.Menu, java.awt.MenuItem,
  java.awt.CheckboxMenuItem)
• Containers (java.awt.Panel, java.awt.Window and
  its subclasses)

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Adding UI Components

• add adds the component to the applet
• remove removes the component
• setLayout sets the applets layout manager which
  controls the position and size of the applet
  components

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Working with Applets

• Cannot load libraries or define native methods
• Cannot read or write files on local host
• Cannot make network connections
• Cannot start a program on local host
• Cannot read system properties on local host
• Implemented with SecurityManager which throws
  SecurityExceptions

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Applets Can...

• Display html
• If local, restrictions off
• make network connections back to host they came
  from
• Dont have to stop when you leave

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ltAPPLET CODE AppletSubclass.class WIDTHanInt
HEIGHTanIntgt lt/APPLETgt
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Finding and Loading Data Files
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• Applet.getCodeBase() returns URL of directory
  from which applet classes load
• Applet.getDocumentBase() returns URL of directory
  from which applets html came
• Image imagegetImage(getCodeBase(),"imgDir/a.gif")
  

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Showing Status

• Applet.showStatus(String s)
• showStatus("MyApplet Loading image file "
  file)

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Displaying Documents

• AppletContext class allows applet to interact
  with context (i.e. browser) in which it runs.
• Display in browser (wont work with appletviewer)
• public void showDocument(java.net.URL url)
• public void showDocument(java.net.URL url, String
  targetWindow)

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"_blank" Display the document in a
new, nameless window. "windowName"
Display the document in a window named
windowName. This window is created if
necessary. "_self" Display the
document in the window and frame that contain
the applet. "_parent" Display
the document in the applet's window but in the
parent frame of the applet's frame. If the applet
frame has no parent frame, this acts the
same as "_self". "_top"
Display the document in the applet's window but
in the top-level frame. If the applet's frame
is the top-level frame, this acts the same as
"_self".
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...//In an Applet subclass
urlWindow new URLWindow(getApplet
Context()) . . .
class URLWindow extends Frame . .
. public URLWindow(AppletContext
appletContext) . . .
this.appletContext appletContext
. . .
. . . public boolean action(Event
event, Object o) . . .
String urlString / user-entered
string / URL url null
try
url new URL(urlString)
catch (MalformedURLException e)
...//Inform the user and return...
if
(url ! null) if (/
user doesn't want to specify the window /)
appletContext.showDocume
nt(url) else
appletContext.showDocument
(url,
/ user-specified window /)

. . .
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Sending Messages to Other Applets

• Same host, same directory, same page
• Find by name on same page
• Two ways to name applet

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1) By specifying a NAME attribute within the
applet's ltAPPLETgt tag. For example
ltAPPLET CODEBASEexample/ CODESender.class
WIDTH450
HEIGHT200 NAME"buddy"gt
. . . lt/appletgt 2) By
specifying a NAME parameter with a ltPARAMgt tag.
For example ltAPPLET
CODEBASEexample/ CODEReceiver.class
WIDTH450 HEIGHT35gt
ltPARAM NAME"name" value"old pal"gt
. . . lt/appletgt
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Applet receiver null String receiverName
nameField.getText() //Get name to search
for. receiver getAppletContext().getApplet(recei
verName)
if (receiver ! null) //Use the
instanceof operator to make sure the applet
//we found is a Receiver object.
if (!(receiver instanceof Receiver))
status.appendText("Found applet named
"
receiverName ", "
"but it's not a Receiver object.\n")
else
status.appendText("Found applet named "
receiverName ".\n"
" Sending
message to it.\n") //Cast the
receiver to be a Receiver object
//(instead of just an Applet object) so that
the //compiler will let us call
a Receiver method.
((Receiver)receiver).processRequestFrom(myName)
. . .
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Playing Sound
AudioClip onceClip, loopClip onceClip
applet.getAudioClip(getCodeBase(), "bark.au")
loopClip applet.getAudioClip(getCodeBas
e(), "train.au") onceClip.play()
//Play it once. loopClip.loop()
//Start the sound loop.
loopClip.stop() //Stop the sound loop.
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Using the Applet Tag

• Like command line arguments for applets
• Customize applet operation
• User Configures Names
• Provide Default Values
• Parameter values are all strings
• Let applet interpret string

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ltAPPLET CODEAppletButton.class CODEBASEexample
WIDTH350 HEIGHT60gt
ltPARAM NAMEwindowClass VALUEBorderWindowgt
ltPARAM NAMEwindowTitle VALUE"BorderLayout"
gt ltPARAM NAMEbuttonText
VALUE"Click here to see a BorderLayout in
action"gt lt/APPLETgt
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int requestedWidth 0 . . .
String windowWidthString getParameter("WINDOWWID
TH") if (windowWidthString ! null)
try
requestedWidth Integer.parseInt(windowWidthStrin
g) catch (NumberFormatException
e) //Use default width.

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String windowClass String buttonText
String windowTitle int
requestedWidth 0 int requestedHeight
0 . . . public void init()
windowClass getParameter("WINDOW
CLASS") if (windowClass null)
windowClass "TestWindow"
buttonText
getParameter("BUTTONTEXT") if
(buttonText null)
buttonText "Click here to bring up a "
windowClass
windowTitle getParameter("WINDOWTITLE")
if (windowTitle null)
windowTitle windowClass
String windowWidthString
getParameter("WINDOWWIDTH") if
(windowWidthString ! null)
try requestedWidth
Integer.parseInt(windowWidthString)
catch (NumberFormatException e)
//Use default width.
...
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Combining Applet Files

• If applet has more than one file, combine into
  zip archive
• jar tool and JAR format from java
• jar cvf file.zip com/mycompany/myproject/.class
  .gif (Solaris)
• jar cvf file.zip com\mycompany\myproject\.class
  .gif (Windows 95/NT)

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ltAPPLET CODE"AppletSubclass.class"
ARCHIVE"file1, file2"
WIDTHanInt HEIGHTanIntgt lt/APPLETgt