Java Applets

1
SEN 910 CSS/HTML Programming Java Applets Overview

• Java Overview
• Design goals features
• platform independence, portable, secure, simple,
  object-oriented,
• Programming models
• applications vs. applets vs. servlets
• intro to applets
• libraries, comments, classes, inheritance
• applet tag in HTML
• applet parameters

2
Java

• Java was developed at Sun Microsystems, 1995
• originally designed for small, embedded systems
  in electronic appliances
• initial attempts used C, but frustration at
  limitations/pitfalls
• recall C C OOP features
• the desire for backward compatibility led to the
  retention of many bad features

desired features (from the Java white
paper) simple object-oriented robust platform
independent architecture neutral portable dynamic
interpreted high-performance distributed multi-th
readed secure note these are desirable
features for any modern language thus, Java has
become very popular, especially when Internet
related also, Sun distributes free compilers
(JDK) and open source
3
Language features

• simple
• syntax is based on C (familiarity ? easier
  transition for programmers)
• removed many confusing and/or rarely-used
  features
• e.g., explicit pointers, operator overloading,
  automatic coercions
• added memory management (reference count/garbage
  collection hybrid)

• object-oriented
• OOP facilities similar C, all methods are
  dynamically bound
• pure OOP everything is a class, no independent
  functions

• robust
• lack of pointers and memory management avoids
  many headaches/errors
• libraries of useful, tested classes increases
  level of abstraction
• arrays strings are ADTs, well-defined interfaces

4
Language features (cont.)

• platform independence
• want to be able to run Java code on multiple
  platforms
• neutrality is achieved by mixing compilation
  interpretation
• Java programs are translated into byte code by a
  Java compiler
• byte code is a generic machine code
• byte code is then executed by an interpreter
  (Java Virtual Machine)
• must have a byte code interpreter for each
  hardware platform
• an Applet is a special form of Java application
• byte code is downloaded with page, JVM is
  embedded in browser

• architecture-neutral
• no implementation dependent features
• e.g., sizes of primitive types is set (unlike C)

• portable
• byte code will run on any version of the Java
  Virtual Machine (JVM)

5
Language features (cont.)

• dynamic
• JVM links classes at run-time as they are needed
• if supporting class is recompiled, dont have to
  recompile entire project

• interpreted
• needed for platform independence
• interpreted ? faster code-test-debug cycle,
  better run-time error checking

• high-performance
• faster than traditional interpretation since byte
  code is "close" to native code
• still somewhat slower than a compiled language
  (e.g., C)

6
Language features (cont.)

• distributed
• extensive libraries for coping with TCP/IP
  protocols like HTTP FTP
• Java applications can access remote URL's the
  same as local files

• multi-threaded
• a thread is like a separate program, executing
  concurrently
• can write Java programs that deal with many tasks
  at once by defining multiple threads (same shared
  memory, but semi-independent execution)
• threads are important for multi-media, Web
  applications

• secure
• Java applications do not have direct access to
  memory locations
• memory accesses are virtual, mapped by JVM to
  physical locations
• downloaded applets cannot open, read, or write
  local files
• JVM also verifies authenticity of classes as they
  are loaded
• Sun claim execution model enables virus-free,
  tamper-free systems

7
Java programming models

• Java applications are stand-alone programs
• must be compiled into Java byte code by Java
  compiler, then distributed
• executed by an interpreter (Java Virtual Machine)
• Java applets provide for client-side programming
• compiled into Java byte code, then downloaded as
  part of a Web page
• executed by the JVM embedded within the Web
  browser
• unlike JavaScript, Java is full-featured with
  extensive library support
• Java and its APIs have become industry standards
• the language definition is controlled by Sun,
  ensures compatibility
• Applications Programming Interfaces standardize
  the behavior of useful classes and libraries of
  routines
• Java servlets provide similar capabilities on the
  server-side
• alternative to CGI programs, more fully
  integrated into Web server

8
Java applets

• important point Java applets applications look
  different!
• if you want to define a stand-alone application,
  make an application
• requires public static void main function,
  similar to C main
• if you want to embed the code in a Web page, make
  an applet
• requires public void paint, public void init,
• can define dual-purpose programs, but tricky

• as with JavaScript, security is central
• when a Java applet is downloaded, the bytecode
  verifier of the JVM verifies to see if it
  contains bytecodes that open, read, write to
  local disk
• a Java applet can open a new window but they have
  Java logo to prevent them from being disguised as
  system window (for stealing password)
• a Java applet is not allowed to connect back to
  other servers except the host
• this secure execution environment is called sand
  box model

9
First Java applet

• import java.awt.
• import java.applet.
• /
• This class displays "Hello world!" on the
  applet window.
• /
• public class HelloWorld extends Applet
• public void paint(Graphics g)
• g.drawString("Hello world!", 10, 10) //
  writes starting 10 pixels over down

• libraries
• Java provides extensive library support in the
  form of classes
• libraries are loaded using import (similar to
  include in C)
• java.awt contains Abstract Window Toolkit (for
  GUI classes routines)
• java.applet contains the applet class definition

10
First Java applet

• import java.awt.
• import java.applet.
• /
• This class displays "Hello world!" on the
  applet window.
• /
• public class HelloWorld extends Applet
• public void paint(Graphics g)
• g.drawString("Hello world!", 10, 10) //
  writes starting 10 pixels over down

• comments in Java
• // and / . . . / work the same as in C
• / . . . / designate documentation comments
• can be used to automatically generate HTML
  documentation (javadoc)

11
First Java applet

• import java.awt.
• import java.applet.
• /
• This class displays "Hello world!" on the
  applet window.
• /
• public class HelloWorld extends Applet
• public void paint(Graphics g)
• g.drawString("Hello world!", 10, 10) //
  writes starting 10 pixels over down

• class definitions in Java
• similar to C (but no semi-colon at end)
• can contain instance variables (data fields)
  methods(member functions)
• precede class method definitions with public to
  make available to all programs
• there are no stand-alone functions in Java
• must be stored in a file of same name with .java
  extension
• e.g., HelloWorld.java

12
First Java applet

• import java.awt.
• import java.applet.
• /
• This class displays "Hello world!" on the
  applet window.
• /
• public class HelloWorld extends Applet
• public void paint(Graphics g)
• g.drawString("Hello world!", 10, 10) //
  writes starting 10 pixels over down

• all applets inherit from the Applet class (in
  java.applet)
• default methods include
• init() called when page is loaded to
  create/initialize variables
• by default, does nothing
• paint(Graphics g) called to draw (after init) or
  redraw (after being obscured)
• here, the paint method is overridden to display
  text on the applet window

13
Embedding an applet in HTML

• to include an applet in a Web page, use either
• APPLET tag (deprecated)
• CODE specifies applet name, HEIGHT and WIDTH
  specify window size
• text between the APPLET tags is displayed if
  unable to execute (e.g., Java not enabled)
• OBJECT tag
• preferred for HTML 4, but not universally
  supported

• lthtmlgt
• lt!-- Hello1.html --gt
• ltheadgt
• lttitlegtHello World Pagelt/titlegt
• lt/headgt
• ltbodygt
• ltpgt
• ltapplet code"HelloWorld.class" height100
  width100gt
• You must use a Java-enabled browser to view
  this applet.
• lt/appletgt
• lt/pgt
• lt/bodygt
• lt/htmlgt

view page in browser
14
HTML applets

• lthtmlgt
• lt!-- Hello2.html --gt
• ltheadgt
• lttitlegtHello World Pagelt/titlegt
• lt/headgt
• ltbodygt
• ltpgt
• ltdiv align"center"gt
• lttable border1gt
• lttrgtlttdgt
• ltapplet code"HelloWorld.class" height200
  width200gt
• You must use a Java-enabled browser to view
  this applet.
• lt/appletgt
• lt/tdgtlt/trgt

an applet can be embedded within HTML elements
just like any other element useful for
formatting and layout
view page in browser
15
Parameters in HTML

• lthtmlgt
• lt!-- Hello3.html --gt
• ltheadgt
• lttitlegtHello World Pagelt/titlegt
• lt/headgt
• ltbodygt
• ltpgt
• ltdiv align"center"gt
• lttable border1gt
• lttrgtlttdgt
• ltapplet code"HelloWorld1.class" height35
  width300gt
• ltparam name"name" value"Chris"gt
• ltparam name"age" value20gt
• You must use a Java-enabled browser to view
  this applet.
• lt/appletgt

• can specify parameters to the APPLET when it is
  embedded in HTML
• each parameter must have its own PARAM tag inside
  the APPLET element
• specifies parameter name and value

view page in browser
16
Applet parameters

• import java.awt.
• import java.applet.
• /
• This class displays a message based on
  parameters.
• /
• public class HelloWorld1 extends Applet
• public void paint(Graphics g)
• String userName getParameter("name")
• int userAge Integer.parseInt(getParamete
  r("age"))
• String message1 "Hello " userName
  "."
• String message2 "On your next birthday,
  you will be "
• (userAge1) " years
  old."
• g.drawString(message1, 10, 10)
• g.drawString(message2, 10, 30)

• can access parameters passed in from the HTML
  document
• getParameter accesses the value of the parameter
  (must know its name)
• if the parameter represents a number, must
  parseInt or parseFloat

17
Java vs. C

• in Java, every variable method belongs to a
  class
• as in C, by default each object has its own
  copies of data fields
• thus, known as instance variables
• as in C, a variables declared static are shared
  by all class objects
• thus, known as class variables
• similarly, can have a static method (class
  method)
• can only operate on class variables, accessible
  from the class itself
• class Math
• public static final double PI 3.14159 //
  access as Math.PI
• public static double sqrt(double num) . . .
  // access as in Math.sqrt(9.0)
• . . .

18
Primitive vs. reference types

• primitive types are handled exactly as in C
• space for a primitive object is implicitly
  allocated
• variable refers to the actual data (stored on the
  stack)
• reference types (classes) are handled differently
• space for a reference object must be explicitly
  allocated using new
• variable refers to a pointer to the data (which
  is stored in the heap)
• Note unlike with C, programmer is not
  responsible for deleting dynamic objects
• JVM performs automatic garbage collection to
  reclaim unused memory

• Java only provides by-value parameter passing
• but reference objects are implemented as pointers
  to dynamic memory
• resulting behavior mimics by-reference
• public void Init(int nums)
• for (int i 0 i lt nums.length i)
• numsi 0
• _____________________________ _
• int nums new int10
• Init(nums)

19
Java libraries

• String class (automatically loaded from
  java.lang)
• int length()
• char charAt(index)
• int indexOf(substring)
• String substring(start, end)
• String toUpperCase()
• boolean equals(Object)
• ...
• Array class (automatically loaded from java.lang)
• int length instance variable
• Type (index) operator
• String toString()
• ...
• Java provides extensive libraries of data
  structures algorithms
• java.util ? Vector Stack LinkedList
• Dictionary HashTable Random
• Calendar

String str "foo" String str new String("foo")
int nums 1,2,3,4,5 int nums new
int10
20
Java inheritance

• Java does NOT have templates
• generic functions/classes are obtained via
  inheritance (e.g, Applet)
• with inheritance, can derive a new class from an
  existing class
• automatically inherit attributes methods of the
  parent class
• object of derived class can be used wherever
  parent object is expected
• every reference type is implicitly derived from
  the Object class
• by defining data structures that contain Objects,
  any reference type can be stored (and even mixed)
• Stack things new Stack()
• String str "foobar"
• things.push(str) // pushes String as an
  Object
• Calendar today Calendar.getDate()
• things.push(today) // pushes Calendar as an
  Object

21
Hello again

• import java.awt.
• import java.applet.
• import java.util.Random
• /
• This class displays lots of "Hello world!"s on
  the applet window.
• /
• public class HelloWorld1 extends Applet
• private static final int NUM_WORDS100
• private static Random randy
• private int randomInRange(int low, int high)
• return (Math.abs(randy.nextInt())
  (high-low1)) low
• public void init()

Random class provides methods for generating
random values
can override init method to allocate initialize
(similar to a constructor)
ltapplet code"HelloWorld1.class" height200
width200gt You must use a Java-enabled browser
to view this applet. lt/appletgt
22
Applet behavior

• recall
• the init method is called when the applet is
  first loaded
• useful for initializing variables objects
• the paint method is called immediately after
  init, and whenever the applet needs to be redrawn
  (e.g., after window resized or obscured)

• when paint is called, it is given the default
  Graphics object
• Graphics methods include
• void drawString(String msg, int x, int y)
• void setColor(Color color)
• Color class is predefined, constants include
• Color.red, Color.blue, Color.black, . . .

23
A colorful hello

• import java.awt.
• import java.applet.
• import java.util.Random
• /
• This class displays lots of "Hello world!"s on
  the applet window.
• /
• public class HelloWorld2 extends Applet
• private static final int NUM_WORDS100
• private static final Color colors
• Color.black,Color.red,Color.blue,Color.green,Co
  lor.yellow
• private static Random randy
• private int RandomInRange(int low, int high)
• return (Math.abs(randy.nextInt())
  (high-low1)) low

can store possible colors in an array change
drawing color using the setColor method
view page in browser
ltapplet code"HelloWorld2.class" height200
width200gt You must use a Java-enabled browser to
view this applet. lt/appletgt
24
Parameters applet dimensions

• recall
• can specify parameters in the HTML document using
  ltPARAMgt tags
• access the parameter values (based on name) using
  getParameter method

• can also access the dimensions of an applet using
  a Dimension object
• Dimension dim getSize() // stores applet
  dimensions
• can then access applet height via dim.height
• can then access applet width via dim.width

25
Adaptive hello

• import java.awt.
• import java.applet.
• import java.util.Random
• /
• This class displays lots of "Hello world!"s on
  the applet window.
• /
• public class HelloWorld3 extends Applet
• private static final Color colors
• Color.black,Color.red,Color.blue,Color.green,Co
  lor.yellow
• private static Random randy
• private Dimension dim
• private int numReps
• private int RandomInRange(int low, int high)
• return (Math.abs(randy.nextInt())
  (high-low1)) low

getParameter accesses the values of the
parameters here, specify number of reps in Web
page
uses getSize to get dimensions, pick random
coords for text within the applet
ltapplet code"HelloWorld3.class" height300
width400gt ltparam name"reps" value200gt You
must use a Java-enabled browser to view this
applet. lt/appletgt
view page in browser
26
Applet graphics

• in addition to displaying text
• can also draw figures on a Graphics object
• void drawLine(int x1, int y1, int x2, int y2)
• void drawRect(int x, int y, int width, int
  height)
• void fillRect(int x, int y, int width, int
  height)
• void drawOval(int x, int y, int width, int
  height)
• void fillOval(int x, int y, int width, int
  height)
• EXAMPLE draw a red circle inscribed in a square,
  then draw random dots (dart pricks)
• by counting the number of dots inside vs. outside
  the circle, can estimate the value of p
• p 4 (area of circle/area of square)

27
Graphical applet

• public class Monte1 extends Applet
• private static Random randy
• private int NUM_POINTS
• private int SIZE
• private int RandomInRange(int low, int high)
  CODE OMITTED
• private double distance(int x1, int y1, int x2,
  int y2) CODE OMITTED
• public void init()
• randy new Random()
• NUM_POINTS Integer.parseInt(getParameter("p
  oints"))
• Dimension dim getSize()
• SIZE Math.min(dim.width, dim.height)
• public void paint(Graphics g)

init method creates random number generator
gets parameters paint method draws a circle
and a bunch of random points
ltapplet code"Monte1.class" height300
width300gt ltparam name"points" value20000gt
You must use a Java-enabled browser... lt/appletgt
view page in browser
28
Double buffering

• note paint is called every time the page is
  brought to the front
• in current version of Monte, this means new dots
  are drawn each time the page is obscured and then
  brought back to the front
• wastes time redrawing
• dots are different each time the applet is
  redrawn
• the double buffering approach works by keeping an
  off-screen image
• in the init method (which is called when the page
  loads)
• draw the figures on a separate, off-screen
  Graphics object
• in the paint method (which is called whenever the
  page is brought forward)
• simply display the off-screen image on the screen

29
Buffered applet

• public class Monte2 extends Applet
• . . .
• private Image offScreenImage
• private Graphics offScreenGraphics
• . . .
• public void init()
• randy new Random()
• NUM_POINTS Integer.parseInt(getParameter("p
  oints"))
• Dimension dim getSize()
• SIZE Math.min(dim.width, dim.height)
• offScreenImage createImage(SIZE, SIZE)
• offScreenGraphics offScreenImage.getGraphic
  s()
• offScreenGraphics.setColor(Color.red)
• offScreenGraphics.fillOval(0, 0, SIZE,
  SIZE)
• for (int i 0 i lt NUM_POINTS i)

init method is called when page is loaded does
drawing to a separate, off-screen Graphics
object paint is called after init and whenever
the applet is revisited Note dont see image in
progress
ltapplet code"Monte2.class" height300
width300gt ltparam name"points" value20000gt
You must use a Java-enabled browser... lt/appletgt
view page in browser
30
Better buffering

• public class Monte3 extends Applet
• . . .
• public void init()
• randy new Random()
• NUM_POINTS Integer.parseInt(getParameter("point
  s"))
• Dimension dim getSize()
• SIZE Math.min(dim.width, dim.height)
• public void paint(Graphics g)
• if (offScreenImage null)
• offScreenImage createImage(SIZE, SIZE)
• offScreenGraphics offScreenImage.getGraphic
  s()
• offScreenGraphics.setColor(Color.red)
• g.setColor(Color.red)
• offScreenGraphics.fillOval(0, 0, SIZE,
  SIZE)

if want to see image as it is drawn, must be done
in paint when first loaded, have paint draw on
the graphics screen and also to an off-screen
buffer on subsequent repaints, simply redraw the
contents of the off-screen buffer
ltapplet code"Monte3.class" height300
width300gt ltparam name"points"
value20000gt lt/appletgt
view page in browser
31
GUI elements in applets

• Java has extensive library support for GUIs
  (Graphical User Interfaces)
• has elements corresponding to HTML buttons, text
  boxes, text areas,
• each element must be created and explicitly added
  to the applet
• nameLabel new Label("User's name")
• add(nameLabel)
• nameField new TextField(20)
• nameField.setValue("Dave Reed")
• add(nameField)
• by default, GUI elements are placed in the order
  they were added, with elements moved to the next
  line as needed to fit

32
Text boxes

• public class Monte4 extends Applet
• . . .
• private Label insideLabel
• private TextField insideField
• private Label outsideLabel
• private TextField outsideField
• . . .
• public void init()
• randy new Random()
• NUM_POINTS
• Integer.parseInt(getParameter("points"))
• Dimension dim getSize()
• SIZE Math.min(dim.width, dim.height)
• insideLabel new Label("Inside")
• add(insideLabel)

public void paint(Graphics g) . . .
insideField.setText("0") outsideField.setTex
t("0") . . . if (distance(x, y, SIZE/2,
SIZE/2) lt SIZE/2) g.setColor(Color.white)
int value Integer.parseInt(insideF
ield.getText())1 insideField.setText(""val
ue) else g.setColor(Color.black)
int value Integer.parseInt(outsid
eField.getText())1 outsideField.setText(""
value) . . .
ltapplet code"Monte4.class" height300
width300gt ltparam name"points"
value20000gt lt/appletgt
view page in browser
33
GUI layout

• public class Monte5 extends Applet
• . . .
• public void init()
• randy new Random()
• NUM_POINTS Integer.parseInt(getParameter("p
  oints"))
• Dimension dim getSize()
• SIZE Math.min(dim.width, dim.height)
• setLayout(new BorderLayout())
• Panel p new Panel()
• insideLabel new Label("Inside")
• p.add(insideLabel)
• insideField new TextField(5)
• p.add(insideField)
• outsideLabel new Label("Outside")

Java provides several classes for controlling
layout FlowLayout is default BorderLayout
allows placement of elements around the borders
of the applet screen a Panel can contain
numerous elements
ltapplet code"Monte5.class" height300
width300gt ltparam name"points"
value20000gt lt/appletgt
view page in browser
34
Event handling

• in order to handle events (e.g., text changes,
  button clicks), can use the event delegation
  model
• must specify that the class implements the
  ActionListener interface
• public class Monte6 extends Applet implements
  ActionListener
• each source of events must be registered within
  the applet
• dotButton new Button("Click to generate dots")
• dotButton.addActionListener()
• must have an actionPerformed method to handle
  events
• public void actionPerformed(ActionEvent e)
• if (e.getSource() dotButton)
• drawDots()

35
ActionListener

• import java.awt.
• import java.applet.
• import java.awt.event.
• import java.util.Random
• public class Monte6 extends Applet
• implements ActionListener
• . . .
• private Button dotButton
• public void init()
• randy new Random()
• NUM_POINTS
• Integer.parseInt(getParameter("points"))
• Dimension dim getSize()
• SIZE dim.width

public void drawCircle() CODE FOR DRAWING
CIRCLE public void drawDots()
drawCircle() Graphics g getGraphics()
for (int i 0 i lt NUM_POINTS i)
CODE FOR DRAWING DOTS public void
paint(Graphics g) g.drawImage(offScreenIma
ge, 0, 0, null) public void
actionPerformed(ActionEvent e) if
(e.getSource() dotButton) drawDots()

ltapplet code"Monte6.class" height300
width300gt ltparam name"points"
value20000gt lt/appletgt
view page in browser
36
Applet examples

• The Java Boutique has lots of sample applets with
  source code
• Graphing Calculator
• Mandlebrot Set
• Email front-end
• Web search front-end
• Java Tetris

37

• Combining Java JavaScript
• integrating Java with JavaScript
• calling Java routines from JavaScript
• controlling an applet from JavaScript
• accessing JavaScript HTML elements from an
  applet
• related topics
• JavaBeans, Java archives (JARs)

38
JavaScript vs. Java

• recall JavaScript is very good for simple tasks,
  GUI layout
• flexible data typing, primitive object types fine
  for quick development
• integration with HTML makes layout control of
  GUI elements easy
• not much library support, only primitive data
  structuring capabilities
• not well-suited to multi-file projects, OO
  approach

• recall Java is better at complex tasks,
  especially graphics
• full-featured, more robust, extensive libraries
  of classes/routines
• can support large projects, interacting objects
• GUI layout is difficult, integration with HTML
  not obvious

• IDEAL make use of the the strengths of each
  language
• include applets in a page when needed (e.g.,
  graphics)
• allow communication between applet JavaScript
  code

39
Calling Java routines from JavaScript

• Netscape Communicator allows direct calls to Java
  routines
• specify full package name of routine, then call
  as in Java
• useful for more esoteric routines that are not
  supported directly in JavaScript
• this feature is NOT supported by Internet Explorer

• lthtmlgt
• lt!-- Note works in Netscape only. --gt
• ltheadgt
• lttitlegtJavaJavaScript Demolt/titlegt
• lt/headgt
• ltbodygt
• ltscript language"JavaScript"gt
• document.write(java.lang.Math.random())
• lt/scriptgt
• lt/bodygt
• lt/htmlgt

40
Calling applet methods

• more commonly, want to include an applet in a
  page,
• control via HTML events JavaScript
• want to draw dots inside a square (with an
  inscribed circle)
• could build GUI interface into applet, but
  required tricky layout manager
• instead, leave graphics up to the applet,
  controlled via JavaScript

to call a Java applet method from
JavaScript document.appletName.methodCall()
41
MontePI revisited

• import java.awt.
• import java.applet.
• import java.util.Random
• public class Monte7 extends Applet
• private static Random randy
• private int SIZE
• private Image offScreenImage
• private Graphics offScreenGraphics
• private int randomInRange(int low, int high)
• private double distance(int x1, int y1, int x2,
  int y2)
• public void init()
• randy new Random()
• Dimension dim getSize()
• SIZE dim.width

init creates the random number generator gets
applet size drawDots draws the dots on the
screen and to the off-screen buffer paint
redraws the screen using the buffer
42
MontePI example (cont.)

• lthtmlgt
• lt!-- Monte7.html --gt
• ltheadgt
• lttitlegtMonte Carlo Darts Pagelt/titlegt
• lt/headgt
• ltbody bgcolor"gray"gt
• ltdiv style"text-aligncenter"gt
• ltapplet code"Monte7.class"
  name"MonteApplet"
• height300 width300gt
• You must use a Java-enabled browser to view
  this applet.
• lt/appletgt
• ltbr /gtltbr /gt
• ltform name"MonteForm"gt
• ltinput type"button" value"Generate
  points"
• onClick"document.MonteApplet.drawDots(10
  00)"gt

here, HTML button controls the applet (via
JavaScript)
view page in browser
43
Example (cont.)
better interface allow user to specify number
of dots in text box each click adds new dots,
have separate button to clear

• lthtmlgt
• lt!-- Monte7a.html --gt
• ltheadgt
• lttitlegtMonte Carlo Darts Pagelt/titlegt
• lt/headgt
• ltbody bgcolor"gray"gt
• ltdiv style"text-aligncenter"gt
• ltapplet code"Monte7.class"
  name"MonteApplet"
• height300 width300gt
• You must use a Java-enabled browser to view
  this applet.
• lt/appletgt
• ltbr /gtltbr /gt
• ltform name"MonteForm"gt ltinput
  type"button" value"Generate"
  onClick"numDots parseFloat(document.MonteForm.n
  umPoints.value)
  document.MonteApplet.drawDots(numDots)"gt
  ltinput type"text" name"numPoints" size6
  value100gt points ltbr /gtltbr /gt
  ltinput type"button" value"Clear the screen"
  onClick"document.MonteApplet.drawCircle(
  )"gt lt/formgt
• lt/divgt
• lt/bodygt

view page in browser
44
Dividing control

• where the control lies affects the
  efficiency/usability of an applet
• want the applet to be as self-contained as
  possible,
• take advantage of speed advantage, more advanced
  features
• but if GUI controls are in HTML, then JavaScript
  needs overall control

• consider adding counters for number of dots
  inside outside circle
• have the applet keep track of the dots in
  instance variables
• after drawing all dots, JavaScript could access
  counts display
• cant see counts in progress (in Netscape)
• could have applet update the HTML text boxes
  itself
• tricky (example later)
• could return more control to the page, applet
  draws one dot at a time
• repetition is handled by JavaScript, can update
  boxes after each dot
• ? slower, but more flexible

45
JavaScript in control

• import java.awt.
• import java.applet.
• import java.util.Random
• public class Monte8 extends Applet
• . . .
• public int numInside, numOutside
• . . .
• public void clear()
• numInside 0 numOutside 0
• . . .
• public void drawDot()

change applet so that method only draws a single
dot (repetition to be controlled by
JavaScript) have applet keep track of number
inside out (can access display with
JavaScript)
46
Example (cont.)

• lthtmlgt
• lt!-- Monte8.html --gt
• ltheadgt
• lttitlegtMonte Carlo Darts Pagelt/titlegt
• ltscript type"text/javascript"gt
• function doAll()
• var numDots parseFloat(document.MonteFor
  m.numPoints.value)
• for (var i 0 i lt numDots i)
• document.MonteApplet.drawDot()
• document.MonteForm.numIn.value
  document.MonteApplet.numInside
• document.MonteForm.numOut.value
  document.MonteApplet.numOutside
• function clearAll()
• document.MonteApplet.drawCircle()

Note can utilize HTML table to achieve desired
layout of elements
view page in browser
47
Accessing HTML/JavaScript from the applet

• it is possible for the applet to access elements
  in the page
• requires the JSObject class from the
  netscape.javascript package
• import netscape.javascript.JSObject
  

• use getWindow and getMember methods to access
  components
• JSObject jsWin JSObject.getWindow(this)
  // GETS WINDOW
• JSObject jsDoc (JSObject) jsWin.getMember("docum
  ent") // GETS DOCUMENT
• JSObject MonteForm (JSObject)
  jsDoc.getMember("MonteForm") // GETS FORM
• numInside (JSObject) MonteForm.getMember("numIn"
  ) // GETS TEXT BOX

• use getMember and setMember methods to access
  component attributes
• int num Integer.parseInt( (String)numInside.getM
  ember("value") )
• numInside.setMember("value", ""(num1))

48
Related topics

• JavaBeans
• reusable components (e.g., buttons, menus) that
  can be packaged and reused
• requires special tools for compiling and
  packaging (e.g., BDK)
• downloaded with an applet using the ARCHIVES
  attribute
• ltapplet code"javaApp.class" archives"jarfile.jar
  "gt

• JAR files
• for applets that are comprised of multiple
  classes, can bundle all necessary files into a
  Java Archive (JAR) file
• uses the popular ZIP file format
• download using ARCHIVES attribute, automatically
  unzipped by browser

49
End of Java Applets