The Fundamental Client Side Programming Language for the Web

8/3/2019 The Fundamental Client Side Programming Language for the Web

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* The fundamental client side programming

language for the web.

* "Java is just a small, simple, safe, object-oriented, interpreted or dynamically optimized, byte-

coded, architecture-neutral, garbage-collected,

multithreaded programming language with a

strongly typed exception-handling mechanism for

writing distributed dynamically extensible program."---- Bill Joy, cofounder of SUN Microsystems.

History

* It was originally called Oak, developed by SUN

Microsystems, in a project called FirstPerson for

smart appliances, such as set-top-box (interactive

TV) 1992, lead by James Gosling.

* It adopted C++ syntax but get rid of pointer,memory management, and multiple inheritance,

which are main causes of programming errors.


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* It failed to be adopted by Timer Warner in

Spring 93's ITV trial.

* 1993 Marc Andreesen developed Mosaic WebBrowser, started the web revolution.

* Oak found its reason for existence. A Web

browser called WebRunner was developed for

demonstration.

* January 1995, Sun adopted Java name due to

trademark issue.

* Marc Andreesen saw the demo and license it for

use in its browser.

* Nov. 1995, 1st Beta release with developer kitand source code.

What is Java Applet

* Java is a general purpose programming

language.


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* A Java program is created as a text file with

.java extension.

* It is compiled into one or more files ofbytecodes with .class extension.

* Bytecodes are a set of instructions similar to

machine code of a specific machine but it can be run

on any machine that has a Java Virtual Machine

(JVM). JVM interprets bytecodes.

* JVM was first implemented on Sparc/Solaris

and PC/Win32. Now ported to many other

platforms.

* Java applets are Java programs that are retrieved

and executed by web browsers through the JVM

under tight secure environment.

* Web browsers retrieve Java applets according to

following tags in the web pages:

deprecated or the new more general,


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are used to specifyparameters for the applet. For example,

-- the height (in

pixels) of the tallest frame

-- the directory

that has the animation frames (gif, jpg) All other

types of java programs are called Java applications.

* Examples:

* Network Restoration Applet: User specifies twoend node of a failed link. The applet retrieves the

simulation results of two network restoration

algorithms (twoprong and rreact) in two


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corresponding files at original web server site, and

plot the results.

* SUN Java demos:

o MoleculeViewer

o SimpleGraph

o SortDemo

o BarChart

o Animator

o JumpingBox

Java is safe

* 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.* Java applet can open new window but they have

Java log to prevent them from being disguised as

system window (for stealing password).


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* Java applet is not allowed to connect back to

other servers except that hosts itself.

* This secure execution environment is calledsand box model.

* JDK 1.1 extends this tight security with digitally

signed applet using jar file.

* More detailed fine grained security levels are

planned for future release.

Java Development Kit

* The current release is JDK 1.1.7A. It is free for

download, containing documents, demos, library,

bin/utilities, and Java API packages.

* Java Application Programming Interface (API)

consists of classes for programming:o java.applet.*: for applet context and

interface to browsers


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o java.lang.*: variable, string manipulation,

exception handing, thread.

o java.io.*: File I/O through [buffered]input/outputStream,

o java.awt.*: Abstract Window Toolkit

(AWT) for window/GUI design, image creation and

manipulation

o java.util.*: hashtable, stringTokenizer

(simple split), date, random, bitset, vector.

o java.net.*: url, urlconnection for connecting

to original web servers, socket

o java.math.*: math functions.

o java.sql: for Java-Database interface

* Extended APIs:

o Media API for sound, video, 3D, VRML

o Commerce API

o Servlet API

* JDK 1.2 will soon be released.


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Java Applet Development Environment

* Make sure the CLASSPATH in your

autoexec.bat does not contain local directory, or just

comment it out. They triggers the security alarm

saying that the applet trying access to local drive.

* Use Jbuilder2 (in our PC lab). It contains

o java source code editor

o class browser

o debugger

o visual GUI design tool similar to VBasics.

o wizard for guiding the java application, java

applet, java class development.

o extensive java beans and libraries for

Database access and enhanced GUI design.

* Create new Java Applet by

o first create a project folder by


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+ select the File/New Project, the project

wizard window appears

+ replace the untitle string in the filetextfield with your project name, say cs301hw6.

+ replace the title of the project, click

finish, a project template will appear.

o then create the Java applet template by

+ select File/New

+ click on the Applet icon

+ fill in the information in the applet

creation dialog box (choose the core Java and Swing

only option)

+ write java code in the created template.

+ control-s to save file the myprojects\cs301hw6 directory, where

is the directory setup for jbuilder2in your system.

+ select build/make

"CompareCookieSales.java", it will compile the


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code, show the error if any, and save the class byte

code in

myclasses\CompareCookieSales.class

+ ftp the .java code and .class code (note

there may be more than one .class generated) back to

the web server directory.

+ Make sure the case of the class file nameis correct after the file transfer.

* Download JDK1.1.7A (you can also try

JDK1.2RC2 a beta release) to your own PC.

* Use text editor to create .java source code.

* Reference document web page,

http://java.sun.com/products/jdk/1.1/docs/api/packag

es.html

* Use " javac ResPlot.java" to compile the java

code.

* Create a web page with the object or applet tag

to reference the Java Applet.


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Basics of Applet Programming

Operators and Their Precedence

The following table shows the precedence assigned

to Java's operators. The operators in this table are

listed in precedence order: the higher in the table an

operator appears, the higher its precedence.

Operators with higher precedence are evaluated

before operators with a relatively lower precedence.

Operators on the same line have equal precedence.

postfix operators [] . (params) expr++ expr--

unary operators ++expr --expr +expr -expr ~ !

creation or cast new (type)expr

multiplicative * / %

additive + -

shift > >>>

relational < > = instanceof

equality == !=


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bitwise AND &

bitwise exclusive OR ^

bitwise inclusive OR |

logical AND &&

logical OR ||

conditional ? :

assignment = += -= *= /= %= &= ^= |= =

>>>=

When operators of equal precendence appear in the

same expression, some rule must govern which isevaluated first. In Java, all binary operators except

for the assignment operators are evaluated in left to

right order. Assignment operators are evaluated right

to left.

Control Flow Statements

A statement such as the while statement is a control

flow statement, that is, it determines the order in


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which other statements are executed. Besides while,

similar to C, the Java language supports several

other control flow statements (except the exceptionhandling), including:

Statement Keyword

decision making if-else, switch-case

loop for, while, do-while

exception try-catch-finally, throw

miscellaneous break, continue, label: , return

Note: Although goto is a reserved word, currently

the Java language does not support the goto

statement.

Handling errors with Expression using Try, Catch,

and Throw

Basic Steps:


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* All Applet source code starts with

import java.applet.*;

and other import statements for packages used in

the applet such as

* import java.awt.*; import java.io.*; import

java.lang.*; import java.net.*; import java.util.*; Itthen starts with a class definition like:

public class Chart extends java.applet.Applet {

// followed by a list of variables or objects

declaration

//

Button b1 = new Button("play");

Button b2 = new Button("stop");

Button b3 = new Button("resume");

TextField tf = new TextField("Message here",

50);


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TextArea ta = new TextArea("please wait...",

10, 40);

* } The main class contains four basic Appletmethods:

public void init() {

// for any initialization code performed duringloading time of the applet

// such as loading graphics, initialize variables,

creating objects

// add() is used to insert the objects into the

area allocated for the applet

add(b1); add(b2); add(b3); add(tf); add(ta);

}

public void start() {

// implement the main action of the applet

behavior


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// it is also called by browser to restart after it

was stopped.

}

public void stop() {

// stop an applet's execution but leaves

resources intact so it can restart.

}

public void destroy() {

// release all its resources

// typically happens when the user leaves the

page containing the applet.

* } Many Applet only needs to implement init()

* For control and user input, Button, TextField,

TextArea, Canvas objects are created in init()

and special method action() is called when event

happens

public boolean action(Event e, Object o) {


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if (e.target instanceof Button) {

String s = (String) o;

if (s.equals("play") {

tf.setText("play button is pushed");

} else if (s.equal("stop") {

ta.setText("stop button is pushed");

} else {

ta.appendText("resume button is

pushed");

}

return true;

}

return false;

* } Try this basic example.

Abstract Window Toolkit (AWT)

Tutorial from Netscape


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Using AWT Components:

http://owl.uccs.edu/~cs301/java/awt/oldui/components/using.html

Layout the Components in a container:

http://owl.uccs.edu/~cs301/java/awt/oldui/layout/ind

ex.html

Note that there are new GUI packages such swing

and Java 2D graphics recommended by Netscape.

They are great for making Java applications withGUI, but there are still a lot of browsers do not

support that. They require JDK1.2 compatible

viewer or Java plug in.

"Java Plug-in software enables enterprise customers

to direct Java applets or JavaBeansTM components

on their intranet web pages to run using Sun's Java


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Runtime Environment (JRE), instead of the

browser's default Java runtime. This enables an

enterprise to deploy Java applets that take fulladvantage of the latest capabilities and features of

the Java platform and be assured that they will run

reliably and consistently. " --

http://java.sun.com/applets/index.html.

Using JFC/SWING to create GUI

The swing components in Java Foundation Classes

(JFC) is the new package in JDK1.1 and JDK1.2 for

supporting GUI design in Java applications and

applets.

It provides much more features to those of AWT.

For examples,

* allow images on buttons/labels,

* dynamically change of borders,

* component can be round,


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* specify look and feels (Java, Windows,

CDE/Motif).

* wider selection of components

* not using native code (more portable)

Unfortunately, Sun recommends that SWING and

AWT components not mix. Their drawing sequence

are basically different. "Heavyweight" AWT

components such as menu, scrollpane, canvas, and

panel always draw on top of "lightweight" SWING

components.

Tutorial on using GridBagLayout.

Note that the current tutorial on java.sun.com isgoing through major revision. Some web pages,

such as the layout manager tutorial, event hung the

whole browser. Use them with caution. 12/3/98.


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Example for using GridBagLayout and submit data

to a CGI script

http://owl.uccs.edu/~cs301/java/Register.html and

Register.java

Goal:

* Illustrate the use of the GridBagLayout manager

and AWT components.

* Learn how to exchange data between Applet and

CGI script using URL.

How we arrange the GridBagLayout using

GridBagConstraints:

* Here we create a title as a Label and set

gbc.anchor = GridBagConstraints.NORTH.


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* We have login and address Labels and set their

gbc.anchor = GridBagConstraints.WEST and

gbc.gridwidth = 1.* The login TextField and address TextArea are

set with gbc.grdiwidth =

GridBagConstraints.REMAINDER.

* The submit Button is set with gbc.anchor =

GridBagConstraints.CENTER and padded with 10pixels on each side using gbc.insets = new

Insets(10,10,10,10);

* Finally the return result TextArea is set with

gbc.anchor = GridBagConstraints.SOUTH and

gbc.grdiwidth = GridBagConstraints.REMAINDER.

Basic steps for using the applet:

* When the submit button is hit, the action

method first performs www-url-encode on the

content of address TextArea, then puts the fields of


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the login and address in the name-value pair format

attached at the end of the url.

* Create a URL with it, which in effect submit thedata the corresponding CGI script on the server side.

* The registerData.pl saves the data in a file and

return a web page that is then displayed in the result

TextArea.

Example for plotting the cookie sale of a particular

year.

http://owl.uccs.edu/~cs301/java/plotdemo/CookieSal

es.html and CookieSales.java.

Goal:

* Learn how to use the graphic drawing capability

of the AWT.* Learn how to retrieve data files from the web

server.


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Here are the basic steps for using the Applet:

* After the user selects the year in the choice

menu and hits the plot button, the action method

creates the url string by concatenating

http://owl.uccs.edu/~cs301/java/plotdemo/data/ with

CookieSale.

* It then creates an URL object with the string and

open a DataInputStream for reading the 12 month

sale data from the web server into an array.

* Each line of the data file contains the month andbox numbers of the cookie sale.

* The data file is displayed in the contents

TextArea.

* It then sets the array in the CookieSalesCanvas

class and asks it to plot the bar chart based on the

data.


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* drawRect() and fillRect() are used to draw the

bar of each month.

Reasons for Extending Canvas Class

* For AWT components, we do not have extend

classes unless there is a special need.

* For plots using Canvas class, we have a special

requirement for handling the redrawing when the

part or whole of the canvas area is destroyed due to

the resizing of the applet or window hierarchy

changes (other window pops up).

* Those destroyed areas need to be redrawn.

* The paint(Graphics g) method is designated for

handling the redrawing and the first drawing of the

area.

* It will be given the area that needs to be

redrawn.


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* It needs to know how to redraw the area -->

needs to have access to the original source data.

* Therefore, we extend the canvas class withadditional variables, such as Year and Sv[][] for

holding a copy of the original source data.

* Here we use a setValue() to copy the source data

from the applet to CookieSalesCanvas.

* In the paint(), we use the basic drawing

functions create the content.

Important Site

* http://java.sun.com

* Applet collections:

http://java.sun.com/applets/index.html

* Java Tutorial

* http://java.sun.com/products/jdk/1.1/docs.html:

main document web page for JDK


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* http://www.gamelan.com: depository for java

applets and links to related web sites.

* http://www.javaworld.com: magazine, article,source code, news.

*

http://www.yahoo.com/Computers_and_Internet/Pro

gramming_Languages/Java

*

Good References

* The Java Tutorial by Mary Campione and Kathy

Walrath, Addison Wesley, 1996 Note that there is a

new book "The Java Tutorial Continue" will appear

Dec 98.

* Java How to Program, by Deitel & Deitel,Prentice Hall, 1997.

* Java 1.1 Unleashed 3rd ed., Sam.net,


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* Graphic Java, Mastering the AWT, 2nd ed., by

David M, Geary, Prentice Hall, 1997.

*

Debug Java Applet

* Use showStatus() which shows string on

browser's status window.

* Create a TextArea or TextField for displaying

intermediate results.

* Use JDB that comes with JDK.

* Use tools such as Jbuilder, Visual cafe, MS J++.

Documents

The Fundamental Client Side Programming Language for the Web