27javascript

1 © 2001-2003 Marty Hall, Larry Brown http://www.corewebprogramming.com
core
programming
JavaScript
Adding Dynamic Content
to Web Pages

JavaScript2 www.corewebprogramming.com
Agenda
• Generating HTML Dynamically
• Monitoring User Events
• Basic JavaScript Syntax
• Applications
– Using JavaScript to customize Web pages
– Using JavaScript to make pages more dynamic
– Using JavaScript to validate CGI forms
– Using JavaScript to manipulate HTTP cookies
– Using JavaScript to interact with and control frames
– Controlling applets and calling Java from JavaScript
– Accessing JavaScript from Java

Generating HTML Dynamically
• Idea
– Script is interpreted as page is loaded, and uses
document.write or document.writeln to insert
HTML at the location the script occurs
• Template
...
<BODY>
Regular HTML
<SCRIPT TYPE="text/javascript">

</SCRIPT>
More Regular HTML
</BODY>

A Simple Script
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>First JavaScript Page</TITLE>
</HEAD>
<BODY>
<H1>First JavaScript Page</H1>

</SCRIPT>
</BODY>
</HTML>

Simple Script, Result

Extracting Document Info with
JavaScript, Example
<HTML>
<HEAD>
<TITLE>Extracting Document Info with JavaScript</TITLE>
</HEAD>
<BODY BGCOLOR="WHITE">
<H1>Extracting Document Info with JavaScript</H1>
<HR>
<!--
function referringPage() {
if (document.referrer.length == 0) {
return("<I>none</I>");
} else {
return(document.referrer);
}
}

JavaScript, Example, cont.
...
document.writeln
("Document Info:n" +
"<UL>n" +
" <LI><B>URL:</B> " + document.location + "n" +
" <LI><B>Modification Date:</B> " + "n" +
document.lastModified + "n" +
" <LI><B>Title:</B> " + document.title + "n" +
" <LI><B>Referring page:</B> " + referringPage() + "n" +
"</UL>");
document.writeln
("Browser Info:" + "n" +
"<UL>" + "n" +
" <LI><B>Name:</B> " + navigator.appName + "n" +
" <LI><B>Version:</B> " + navigator.appVersion + "n" +
"</UL>");
// -->
</SCRIPT>
<HR>
</BODY>
</HTML>

JavaScript, Result

Multi-Browser Compatibility
1. Use Language Attribute
<SCRIPT LANGUAGE="JavaScript">

</SCRIPT>
<SCRIPT LANGUAGE="JavaScript1.1">

</SCRIPT>
...
<SCRIPT LANGUAGE="JavaScript1.5">

</SCRIPT>
Note: Don’t include that attribute TYPE="text/javascript"

Multi-Browser Compatibility,
cont.
2. Use Vendor/Version Info
– navigator.appName
– navigator.appVersion

Monitoring User Events
• Use Various onXxx Attributes
– onClick
– onLoad
– onMouseOver
– onFocus
– etc.

User Events, Example
<HTML>
<HEAD>
<TITLE>Simple JavaScript Button</TITLE>

</SCRIPT>
</HEAD>
<H1>Simple JavaScript Button</H1>
<FORM>
<INPUT TYPE="BUTTON"
VALUE="Don't Click Me"
onClick="dontClick()">
</FORM>
</BODY>
</HTML>

User Events, Result

JavaScript Syntax: Dynamic
Typing
• Idea
– Like Lisp, values are typed, not variables
– A value is only checked for proper type when it is
operated upon
• Example
var x = 5; // int
x = 5.5; // float
x = "five point five"; // String

JavaScript Syntax: Function
Declarations
1. Declaration Syntax
– Functions are declared using the function reserved word
– The return value is not declared, nor are the types of the
arguments
– Examples:
function square(x) {
return(x * x);
}
function factorial(n) {
if (n <= 0) {
return(1);
} else {
return(n * factorial(n - 1));
}
}

JavaScript Syntax: Function
Declarations, cont.
2. First Class Functions
• Functions can be passed and assigned to variables
• Example
var fun = Math.sin;
alert("sin(pi/2)=" + fun(Math.PI/2));

JavaScript Syntax: Objects and
Classes
1. Fields Can Be Added On-the-Fly
– Adding a new property (field) is a simple matter of
assigning a value to one
– If the field doesn’t already exist when you try to assign
to it, JavaScript will create it automatically.
– For instance:
var test = new Object();
test.field1 = "Value 1"; // Create field1 property
test.field2 = 7; // Create field2 property

Classes, cont.
2. You Can Use Literal Notation
– You can create objects using a shorthand “literal”
notation of the form
{ field1:val1, field2:val2, ... , fieldN:valN }
– For example, the following gives equivalent values to
object1 and object2
var object1 = new Object();
object1.x = 3;
object1.y = 4;
object1.z = 5;
object2 = { x:3, y:4, z:5 };

Classes, cont.
3. The "for/in" Statement Iterates Over
Properties
• JavaScript, unlike Java or C++, has a construct that lets
you easily retrieve all of the fields of an object
• The basic format is as follows:
for(fieldName in object) {
doSomethingWith(fieldName);
}
• Also, given a field name, you can access the field via
object["field"] as well as via object.field

Field Iteration, Example
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0
Transitional//EN">
<HTML>
<HEAD>
<TITLE>For/In Loops</TITLE>

</SCRIPT>

Field Iteration, Example
...
</HEAD>
<H1>For/In Loops</H1>

</SCRIPT>
</BODY>
</HTML>

Field Iteration, Result
The for/in statement iterates over
object properties

Classes, cont.
4. A “Constructor” is Just a Function that
Assigns to “this”
• JavaScript does not have an exact equivalent to Java’s
class definition
• The closest you get is when you define a function that
assigns values to properties in the this reference
• Calling this function using new binds this to a new
Object
• For example, following is a simple constructor for a
Ship class
function Ship(x, y, speed, direction) {
this.x = x;
this.y = y;
this.speed = speed;
this.direction = direction;
}

Constructor, Example
var ship1 =
new Ship(0, 0, 1, 90);
makeObjectTable("ship1", ship1);

Classes, cont.
5. Methods Are Function-Valued Properties
• No special syntax for defining methods of objects
• Instead, you simply assign a function to a property

Class Methods, Example
• Consider a version of the Ship class that
includes a move method
function degreesToRadians(degrees) {
return(degrees * Math.PI / 180.0);
}
function move() {
var angle = degreesToRadians(this.direction);
this.x = this.x + this.speed * Math.cos(angle);
this.y = this.y + this.speed * Math.sin(angle);
}
function Ship(x, y, speed, direction) {
this.x = x;
this.y = y;
this.speed = speed;
this.direction = direction;
this.move = move;
}

Class Methods, Result
var ship1 = new Ship(0, 0, 1, 90);
makeObjectTable("ship1 (originally)", ship1);
ship1.move();
makeObjectTable("ship1 (after move)", ship1);

Classes, cont.
5. Arrays
• For the most part, you can use arrays in JavaScript a lot like Java
arrays.
• Here are a few examples:
var squares = new Array(5);
for(var i=0; i<squares.length; i++) {
vals[i] = i * i;
}
// Or, in one fell swoop:
var squares = new Array(0, 1, 4, 9, 16);
var array1 = new Array("fee", "fie", "fo", "fum");
// Literal Array notation for creating an array.
var array2 = [ "fee", "fie", "fo", "fum" ];
• Behind the scenes, however, JavaScript simply represents arrays
as objects with numbered fields
• You can access named fields using either object.field or
object["field"], but numbered fields only via
object[fieldNumber]

Array, Example
var arrayObj = new Object();
arrayObj[0] = "Index zero";
arrayObj[10] = "Index ten";
arrayObj.field1 = "Field One";
arrayObj["field2"] = "Field Two";
makeObjectTable("arrayObj",
arrayObj);

Application: Adjusting to the
Browser Window Size
• Netscape 4.0 introduced the
window.innerWidth and
window.innerHeight properties
– Lets you determine the usable size of the current browser
window

Determining Browser Size,
Example
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE>Strawberries</TITLE>

</SCRIPT>
</HEAD>

Example, cont.
...

</SCRIPT>
<HR>
Strawberries are my favorite garden crop; a fresh ...
</BODY>
</HTML>

Results

Application: Using JavaScript
to Make Pages Dynamic
• Modifying Images Dynamically
– The document.images property contains an array of
Image objects corresponding to each IMG element in
the current document
– To display a new image, simply set the SRC property of
an existing image to a string representing a different
image file

Modifying Images, Example
• The following function changes the first
image in a document
function changeImage() {
document.images[0].src = "images/new-image.gif";
}
• Referring to images by name is easier:
<IMG SRC="cool-image.jpg" NAME="cool"
WIDTH=75 HEIGHT=25>
function improveImage() {
document.images["cool"].src = "way-cool.jpg";
}

Modifying Images: A Clickable
Image Button, Example
<!--
imageFiles = new Array("images/Button1-Up.gif",
"images/Button1-Down.gif",
"images/Button2-Up.gif",
"images/Button2-Down.gif");
imageObjects = new Array(imageFiles.length);
for(var i=0; i<imageFiles.length; i++) {
imageObjects[i] = new Image(150, 25);
imageObjects[i].src = imageFiles[i];
}
function setImage(name, image) {
document.images[name].src = image;
}

Modifying Images: A Clickable
Image Button, Example
function clickButton(name, grayImage) {
var origImage = document.images[name].src;
setImage(name, grayImage);
var resetString =
"setImage('" + name + "', '" + origImage + "')";
setTimeout(resetString, 100);
}
// -->
</SCRIPT>
</HEAD>
...
<A HREF="location1.html"
onClick="clickButton('Button1', 'images/Button1-Down.gif')">
<IMG SRC="images/Button1-Up.gif" NAME="Button1"
WIDTH=150 HEIGHT=25></A>
<A HREF="location2.html"
onClick="clickButton('Button2', 'images/Button2-Down.gif')">
<IMG SRC="images/Button2-Up.gif" NAME="Button2"
WIDTH=150 HEIGHT=25></A>
...

Highlighting Images Under the
Mouse, Example
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0
Transitional//EN">
<HTML>
<HEAD>
<TITLE>High Peaks Navigation Bar</TITLE>
<!—
// Given "Foo", returns "images/Foo.gif".
function regularImageFile(imageName) {
return("images/" + imageName + ".gif");
}
// Given "Bar", returns "images/Bar-Negative.gif".
function negativeImageFile(imageName) {
return("images/" + imageName + "-Negative.gif");
}

Mouse, Example, cont.
// Cache image at specified index. E.g., given index 0,
// take imageNames[0] to get "Home". Then preload
// images/Home.gif and images/Home-Negative.gif.
function cacheImages(index) {
regularImageObjects[index] = new Image(150, 25);
regularImageObjects[index].src =
regularImageFile(imageNames[index]);
negativeImageObjects[index] = new Image(150, 25);
negativeImageObjects[index].src =
negativeImageFile(imageNames[index]);
}
imageNames = new Array("Home", "Tibet", "Nepal",
"Austria", "Switzerland");
regularImageObjects = new Array(imageNames.length);
negativeImageObjects = new Array(imageNames.length);
// Put images in cache for fast highlighting.
for(var i=0; i<imageNames.length; i++) {
cacheImages(i);
}

Mouse, Example, cont.
...
function highlight(imageName) {
document.images[imageName].src = negativeImageFile(imageName);
}
function unHighlight(imageName) {
document.images[imageName].src = regularImageFile(imageName);
}
// -->
</SCRIPT>
</HEAD>
<TABLE BORDER=0 WIDTH=150 BGCOLOR="WHITE"
CELLPADDING=0 CELLSPACING=0>
<TR><TD><A HREF="Tibet.html"
TARGET="Main"
onMouseOver="highlight('Tibet')"
onMouseOut="unHighlight('Tibet')">
<IMG SRC="images/Tibet.gif"
NAME="Tibet"
WIDTH=150 HEIGHT=25 BORDER=0>
</A>
...

Mouse, Result

Making Pages Dynamic:
Moving Layers
• Netscape 4 introduced “layers” – regions
that can overlap and be positioned
arbitrarily
• JavaScript 1.2 lets you access layers via the
document.layers array, each element of
which is a Layer object with properties
corresponding to the attributes of the LAYER
element
• A named layer can be accessed via
document.layers["layer name"] rather
than by using an index, or simply by using
document.layerName

Moving Layers, Example
• Descriptive overlays slowly “drift” to final spot
when button clicked
<HTML>
<HEAD>
<TITLE>Camps on K-3</TITLE>
<!--
function hideCamps() {
// Netscape 4 document model.
document.layers["baseCamp"].visibility = "hidden";
document.layers["highCamp"].visibility = "hidden";
// Or document.baseCamp.visibility = "hidden";
}
function moveBaseCamp() {
baseCamp.moveBy(1, 3);
if (baseCamp.pageX < 130) {
setTimeout("moveBaseCamp()", 10);
}
}

Moving Layers, Example, cont.
function showBaseCamp() {
hideCamps();
baseCamp = document.layers["baseCamp"];
baseCamp.moveToAbsolute(0, 20);
baseCamp.visibility = "show";
moveBaseCamp();
}
function moveHighCamp() {
highCamp.moveBy(2, 1);
if (highCamp.pageX < 110) {
setTimeout("moveHighCamp()", 10);
}
}
function showHighCamp() {
hideCamps();
highCamp = document.layers["highCamp"];
highCamp.moveToAbsolute(0, 65);
highCamp.visibility = "show";
moveHighCamp();
}
// -->
</SCRIPT>

Moving Layers, Example, cont.
<LAYER ID="highCamp" PAGEX=50 PAGEY=100 VISIBILITY="hidden">
<TABLE>
<TR><TH BGCOLOR="WHITE" WIDTH=50>
<FONT SIZE="+2">High Camp</FONT>
<TD><IMG SRC="images/Arrow-Right.gif">
</TABLE>
</LAYER>
<LAYER ID="baseCamp" PAGEX=50 PAGEY=100 VISIBILITY="hidden">
<TABLE>
<TR><TH BGCOLOR="WHITE" WIDTH=50>
<FONT SIZE="+2">Base Camp</FONT>
<TD><IMG SRC="images/Arrow-Right.gif">
</TABLE>
</LAYER>
<FORM>
<INPUT TYPE="Button" VALUE="Show Base Camp"
onClick="showBaseCamp()">
<INPUT TYPE="Button" VALUE="Show High Camp"
onClick="showHighCamp()">
<INPUT TYPE="Button" VALUE="Hide Camps"
onClick="hideCamps()">
</FORM>

Moving Layers, Result

to Validate CGI Forms
1. Accessing Forms
– The document.forms property contains an array of
Form entries contained in the document
– As usual in JavaScript, named entries can be accessed
via name instead of by number, plus named forms are
automatically inserted as properties in the document
object, so any of the following formats would be legal
to access forms
var firstForm = document.forms[0];
// Assumes <FORM NAME="orders" ...>
var orderForm = document.forms["orders"];
// Assumes <FORM NAME="register" ...>
var registrationForm = document.register;

to Validate CGI Forms, cont.
2. Accessing Elements within Forms
– The Form object contains an elements property that
holds an array of Element objects
– You can retrieve form elements by number, by name
from the array, or via the property name:
var firstElement = firstForm.elements[0];
// Assumes <INPUT ... NAME="quantity">
var quantityField = orderForm.elements["quantity"];
// Assumes <INPUT ... NAME="submitSchedule">
var submitButton = register.submitSchedule;

Checking Form Values
Individually, Example
<HTML>
<HEAD>
<TITLE>On-Line Training</TITLE>
<!--
...
// When the user changes and leaves textfield, check
// that a valid choice was entered. If not, alert
// user, clear field, and set focus back there.
function checkLanguage() {
// or document.forms["langForm"].elements["langField"]
var field = document.langForm.langField;
var lang = field.value;
var prefix = lang.substring(0, 4).toUpperCase();
if (prefix != "JAVA") {
alert("Sorry, '" + lang + "' is not valid.n" +
"Please try again.");
field.value = ""; // Erase old value
field.focus(); // Give keyboard focus
}
}

Individually, Example, cont.
// -->
</SCRIPT>
</HEAD>
<H1>On-Line Training</H1>
<FORM ACTION="cgi-bin/registerLanguage" NAME="langForm">
To see an introduction to any of our on-line training
courses, please enter the name of an important Web
programming language below.
<P>
<B>Language:</B>
<INPUT TYPE="TEXT" NAME="langField"
onFocus="describeLanguage()"
onBlur="clearStatus()"
onChange="checkLanguage()">
<P>
<INPUT TYPE="SUBMIT" VALUE="Show It To Me">
</FORM>
</BODY>
</HTML>

Individually, Results

Checking Values When Form is
Submitted, Example
<HTML>
<HEAD>
<TITLE>Camp Registration</TITLE>

</SCRIPT>

Submitted, Example, cont.
<H1>Camp Registration</H1>
<FORM ACTION="cgi-bin/register"
NAME="registerForm"
onSubmit="return(checkRegistration())">
Age: <INPUT TYPE="TEXT" NAME="ageField"
onFocus="promptAge()"
onBlur="clearStatus()">
<BR>
Rank: <INPUT TYPE="TEXT" NAME="rankField"
onFocus="promptRank()"
<BR>
Serial Number: <INPUT TYPE="TEXT" NAME="serialField"
onFocus="promptSerial()"
<P>
<INPUT TYPE="SUBMIT" VALUE="Submit Registration">
</FORM>
</BODY>
</HTML>

Submitted, Results

to Store and Examine Cookies
1. Using document.cookies
– Set it (one cookie at a time) to store values
document.cookie = "name1=val1";
document.cookie = "name2=val2; expires=" + someDate;
document.cookie = "name3=val3; path=/;
domain=test.com";
– Read it (all cookies in a single string) to access values

to Store and Examine Cookies
2. Parsing Cookies
function cookieVal(cookieName, cookieString) {
var startLoc = cookieString.indexOf(cookieName);
if (startLoc == -1) {
return(""); // No such cookie
}
var sepLoc = cookieString.indexOf("=", startLoc);
var endLoc = cookieString.indexOf(";", startLoc);
if (endLoc == -1) { // Last one has no ";"
endLoc = cookieString.length;
}
return(cookieString.substring(sepLoc+1, endLoc));
}

Cookie, Example
<HTML>
<HEAD>
<TITLE>Widgets "R" Us</TITLE>
<!--
function storeCookies() {
var expires = "; expires=Monday, 01-Dec-01 23:59:59 GMT";
var first = document.widgetForm.firstField.value;
var last = document.widgetForm.lastField.value;
var account = document.widgetForm.accountField.value;
document.cookie = "first=" + first + expires;
document.cookie = "last=" + last + expires;
document.cookie = "account=" + account + expires;
}
// Store cookies and give user confirmation.
function registerAccount() {
storeCookies();
alert("Registration Successful.");
}

Cookie, Example, cont.
function cookieVal(cookieName, cookieString) {
var startLoc = cookieString.indexOf(cookieName);
if (startLoc == -1) {
return(""); // No such cookie
}
var sepLoc = cookieString.indexOf("=", startLoc);
var endLoc = cookieString.indexOf(";", startLoc);
if (endLoc == -1) { // Last one has no ";"
endLoc = cookieString.length;
}
return(cookieString.substring(sepLoc+1, endLoc));
}
function presetValues() {
var firstField = document.widgetForm.firstField;
var lastField = document.widgetForm.lastField;
var accountField = document.widgetForm.accountField;
var cookies = document.cookie;
firstField.value = cookieVal("first", cookies);
lastField.value = cookieVal("last", cookies);
accountField.value = cookieVal("account", cookies);
}
// -->
</SCRIPT>

Cookie, Exmaple, cont.
</HEAD>
<BODY BGCOLOR="WHITE" onLoad="presetValues()">
<H1>Widgets "R" Us</H1>
<FORM ACTION="servlet/cwp.Widgets"
NAME="widgetForm"
onSubmit="storeCookies()">
First Name: <INPUT TYPE="TEXT" NAME="firstField">
<BR>
Last Name: <INPUT TYPE="TEXT" NAME="lastField">
<BR>
Account Number: <INPUT TYPE="TEXT" NAME="accountField">
<BR>
Widget Name: <INPUT TYPE="TEXT" NAME="widgetField">
<BR>
<INPUT TYPE="BUTTON" VALUE="Register Account"
onClick="registerAccount()">
<INPUT TYPE="SUBMIT" VALUE="Submit Order">
</FORM>
</BODY>
</HTML>

Cookie, Example, Result

to Interact with Frames
• Idea
– The default Window object contains a frames property
holding an array of frames (other Window objects)
contained by the current window or frame.
• It also has parent and top properties referring to
the directly enclosing frame or window and the top-
level window, respectively.
• All of the properties of Window can be applied to any
of these entries.

Displaying a URL in a Particular
Frame, Example
• ShowURL.html
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Frameset//EN">
<HTML>
<HEAD>
<TITLE>Show a URL</TITLE>
</HEAD>
<FRAMESET ROWS="150, *">
<FRAME SRC="GetURL.html" NAME="inputFrame">
<FRAME SRC="DisplayURL.html" NAME="displayFrame">
</FRAMESET>
</HTML>

Frame, Example, cont.
• GetURL.html
<HTML>
<HEAD>
<TITLE>Choose a URL</TITLE>
<!--
function showURL() {
var url = document.urlForm.urlField.value;
// or parent.frames["displayFrame"].location = url;
parent.displayFrame.location = url;
}
function preloadUrl() {
if (navigator.appName == "Netscape") {
document.urlForm.urlField.value =
"http://home.netscape.com/";
} else {
document.urlForm.urlField.value =
"http://www.microsoft.com/";
}
}
...

Frame, Example, cont.
• GetURL.html, cont.
<BODY BGCOLOR="WHITE" onLoad="preloadUrl()">
<H1 ALIGN="CENTER">Choose a URL</H1>
<CENTER>
<FORM NAME="urlForm">
URL: <INPUT TYPE="TEXT" NAME="urlField" SIZE=35>
<INPUT TYPE="BUTTON" VALUE="Show URL"
onClick="showURL()">
</FORM>
</CENTER>
</BODY>
</HTML>

Frame, Result

Frame, Result, cont.

Giving a Frame the Input Focus,
Example
• If JavaScript is manipulating the frames, the
fix is easy: just add a call to focus in
showUrl:
function showURL() {
var url = document.urlForm.urlField.value;
parent.displayFrame.location = url;
// Give frame the input focus
parent.displayFrame.focus();
}
• Fixing the problem in regular HTML
documents is a bit more tedious
– Requires adding onClick handlers that call focus to
each and every occurrence of A and AREA that includes a
TARGET, and a similar onSubmit handler to each
FORM that uses TARGET

Application: Accessing Java
from JavaScript
1. Idea
– Netscape 3.0 introduced a package called LiveConnect
that allows JavaScript to talk to Java and vice versa
– Applications:
• Calling Java methods directly.
– In particular, this section shows how to print
debugging messages to the Java console
• Using applets to perform operations for JavaScript
– In particular, this section shows how a hidden
applet can be used to obtain the client
hostname, information not otherwise available to
JavaScript
• Controlling applets from JavaScript
– In particular, this section shows how
LiveConnect allows user actions in the HTML
part of the page to trigger actions in the applet

Application: Accessing Java
from JavaScript
• Calling Java Methods Directly
– JavaScript can access Java variables and methods simply
by using the fully qualified name. For instance:
java.lang.System.out.println("Hello Console");
– Limitations:
• Can’t perform operations forbidden to applets
• No try/catch, so can’t call methods that throw
exceptions
• Cannot write methods or create subclasses

Controlling Applets from
JavaScript, Example
• MoldSimulation.html, cont.
<BODY BGCOLOR="#C0C0C0">
<H1>Mold Propagation Simulation</H1>
<APPLET CODE="RandomCircles.class" WIDTH=100 HEIGHT=75>
</APPLET>
<P>
</APPLET>
<P>
</APPLET>
<FORM>
<INPUT TYPE="BUTTON" VALUE="Start Simulations"
onClick="startCircles()">
<INPUT TYPE="BUTTON" VALUE="Stop Simulations"
onClick="stopCircles()">
</FORM>
</BODY>
</HTML>

JavaScript, Example
• MoldSimulation.html
<HTML>
<HEAD>
<TITLE>Mold Propagation Simulation</TITLE>

</SCRIPT>
</HEAD>

JavaScript, Example
• RandomCircles.java
public class RandomCircles extends Applet
implements Runnable {
private boolean drawCircles = false;
public void startCircles() {
Thread t = new Thread(this);
t.start();
}
public void run() {
Color[] colors = { Color.lightGray, Color.gray,
Color.darkGray, Color.black };
int colorIndex = 0;
int x, y;
int width = getSize().width;
int height = getSize().height;
Graphics g = getGraphics();
drawCircles = true;
...

JavaScript, Example
• RandomCircles.java, cont.
while(drawCircles) {
x = (int)Math.round(width * Math.random());
y = (int)Math.round(height * Math.random());
g.setColor(colors[colorIndex]);
colorIndex = (colorIndex + 1) % colors.length;
g.fillOval(x, y, 10, 10);
pause(0.1);
}
}
public void stopCircles() {
drawCircles = false;
}
private void pause(double seconds) {
try {
Thread.sleep((int)(Math.round(seconds * 1000.0)));
} catch(InterruptedException ie) {}
}
}

JavaScript, Results

Accessing JavaScript from
Java
• Steps
1. Obtain and install the JSObject class
– Installed with Netscape 4 (javar40.jar)
– JDK 1.4 includes JSObject in jaws.jar
See Chapter 24 in
http://java.sun.com/j2se/1.4.1/docs/guide/plugin/
developer_guide/contents.html
2. Import it in your applet
import netscape.javascript.JSObject
3. From the applet, obtain a JavaScript reference to the
current window
JSObject window = JSObject.getWindow(this);

Accessing JavaScript from
Java, cont.
• Steps, cont.
4. Read the JavaScript properties of interest
– Use getMember to access properties of the
JSObject
JSObject someForm =
(JSObject)document.getMember("someFormName");
5. Set the JavaScript properties of interest
– Use setMember to set properties of the JSObject
document.setMember("bgColor", "red");
6. Call the JavaScript methods of interest
String[] message = { "An alert message" };
window.call("alert", message);
window.eval("alert(’An alert message’)");
7. Give the applet permission to access its Web page
<APPLET CODE=... WIDTH=... HEIGHT=... MAYSCRIPT>
...
</APPLET>

Matching Applet Background
with Web Page, Example
• MatchColor.java
import java.applet.Applet;
import java.awt.*;
import netscape.javascript.JSObject;
public class MatchColor extends Applet {
public void init() {
JSObject window = JSObject.getWindow(this);
JSObject document =
(JSObject)window.getMember("document");
// E.g., "#ff0000" for red
String pageColor = (String)document.getMember("bgColor");
// E.g., parseInt("ff0000", 16) --> 16711680
int bgColor =
Integer.parseInt(pageColor.substring(1, 7), 16);
setBackground(new Color(bgColor));
}
}

Matching Applet Background
with Web Page, Example, cont.
• MatchColor.html
<HTML>
<HEAD>
<TITLE>MatchColor</TITLE>
</HEAD>
<BODY BGCOLOR="RED">
<H1>MatchColor</H1>
<APPLET CODE="MatchColor.class"
WIDTH=300 HEIGHT=300 MAYSCRIPT>
</APPLET>
</BODY>
</HTML>

Applet That Controls HTML
Form Values, Example
• See on-line example for Everest.html

Summary
• JavaScript permits you to:
– Customize Web pages based on the situation
– Make pages more dynamic
– Validate HTML form input
– Manipulate cookies
– Control frames
– Integrate Java and JavaScript

83 © 2001-2003 Marty Hall, Larry Brown http://www.corewebprogramming.com
core
programming
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