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CIS 5930-04 - Spring 2001
Part 2: Overview of Java
http://aspen.csit.fsu.edu/it1spring01/
Instructors: Geoffrey Fox, Bryan Carpenter
Computational Science and Information Technology
Florida State University
Acknowledgements: Nancy McCracken
Syracuse University
[email protected]
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Plan of Java Tutorial

Part 1:
– Overview including History of Java Development
– Overall Java Philosophy and Features including security etc.

Part 2:
– Java Programming Language
– Object Oriented and Class Structure
– Exceptions
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2
What is Java, in a Nutshell?

What is Java?
– A simple, object oriented, distributed, interpreted, robust, safe,
architecture neutral, portable, high performance, multithreaded,
dynamic, programming language.

Java is interesting because
– It is both a general purpose object-oriented language along the
lines of C++, and
– It is particularly designed to interface with Web pages and to
enable distributed applications over the Internet.

The Web is becoming the dominant software
development arena; this will drive Java as the best
supported, most widely taught language.
– Particularly good as a language for K-12 teaching.
– Even outside the Web, e.g. in scientific computing, Java is as
good and in some respects better than other languages.
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Java is an important language in the World





The Java language has several good design features
– secure, safe (w.r.t. bugs), object-oriented, familiar (to
C, C++ and even Fortran programmers)
Java has a very good set of libraries covering
everything from commerce, multimedia, images to math
functions (see http://math.nist.gov/javanumerics)
Java has best available electronic and paper training
and support resources, growing labor force trained in
Java.
Java is rapidly getting best integrated program
development environments.
Java naturally integrated with network and universal
machine supports powerful “write once-run anywhere”
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model.
Java is also important in computer
science

Increasingly, Java is the language used for important
computing paradigms that support applications:
– object-oriented computing, event-driven computing, distributed
objects, linkage to databases, visual/component computing,
client/servers, networking, multimedia computing . . .

So Java is an important language to include at an
advanced computer science level, along with other
languages like C++ that help students to get jobs.

But the good design features of Java also make it
suitable for teaching basic computer science concepts:
algorithms, data structures, software design, . . .
– See the (old, but still relevant) discussion by Doug Lea at
http://gee.cs.oswego.edu/dl/html/javaInCS.html
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Architecture of Java Applications

Java applications are
compiled and run on a
machine just like any
other general
programming
language such as
C/C++. No web server
or network are
required although Java
applications may also
use network
connections for
distributed computing.
Java code
Java code
is compiled
to produce
is compiled
to produce
byte code
native code
run by Java
interpreter
to produce
results
run directly
on machine
for better
performance
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Java Applications in a Nutshell

Java programs written in a file with extension “.java”.

Applications are .java files with a main() method. This
is called by the Java system.

Compile and run a Java application (using bytecodes):
– Run the compiler on a .java file:
javac MyProgram.java
producing a file of Java byte code,
MyProgram.class
– Run the interpreter on a .class file:
java MyProgram
which executes the byte code

The tools javac and java
are part of JDK.
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7
The Simplest Java Application: Hello,World!

Since Java is object-oriented, programs are organized
into modules called classes, which may have data in
variables called fields, and subroutines called methods.
Each program is enclosed in a class definition.
main() is the first method that is run.
class HelloWorld {
public static void main (String[] args) {
System.out.println(“Hello World!”);
}
}
The notation class.method or
Syntax is similar to C - braces for
package.class.method is how to
blocks, semicolon after each
refer to a public method (with
statement.
some exceptions).
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8
Java Applets

Java applets are classes written in Java that are not
intended to run as stand-alone programs (like
applications) but as subprograms of a browser that is
already managing a window.

Applets are not trusted by default, so they have several
restrictions in running on the client machine

Applets should NOT have a main() method. Instead
they have init(), start(), paint(), etc. for displaying on
the browser window
– no printing or file I/O
– cannot connect through the network to any machine but its own
server
– any new windows created by the applet have a warning label
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9
Architecture of Java Applets

Browsers (HotJava, Netscape 2.0/3.0/4.0, Microsoft IE ...)
supporting Java allow arbitrarily sophisticated dynamic
multimedia applications inserts called Applets, written in
Java, to be embedded in the regular HTML pages and
activated on each exposure of a given page.
web server
web client, running browser
such as Netscape or IE
Java code
Internet
executes (restricted)
applet code to display
in browser window
is compiled
to produce
applet codes,
part of web
document
collection
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10
The Simplest Java Applet: Hello, World!

Java applets can call methods to display on a screen
(within the browser window). One way is to call the method
drawString() from the standard method paint().
The import statement (similar to an include) allows
the use of methods from the Graphics class .
import java.awt.Graphics;
Makes this a subclass of Applet.
public class HelloApplet extends java.applet.Applet {
public void paint (Graphics g) {
g.drawString(“Hello World!”, 5, 25);
}
}
The paint() method displays a graphics object on
the screen - one of the standard methods that takes
the place of main() for applets. [email protected]
11
Displaying your applet from a Web page.

Name the file HelloWorldApplet.java. Run the
compiler, javac, to get a byte code file
HelloWorldApplet.class. Put this in a web directory.
<html><head>
<title>Simple Hello Page</title>
</head>
<body>
Name of your applet class.
My Java applet says:
<applet code=“HelloWorldApplet.class” width=150 height=25>
</applet>
</body></html>
The browser will use a rectangle of width 150 pixels and
height 25 pixels to display the applet within the other html.
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12
Java vs. JavaScript

Despite the name, JavaScript is a different language from
Java, albeit with some similarities.

A JavaScript program is written directly in the HTML page,
and executed by the JavaScript interpreter, so also allows
dynamic web page content in the browser window.

JavaScript is special purpose - it is an object-based
language that deals directly with browser entities like
windows, text fields, forms, frames and documents.

JavaScript can respond to browser events like mouse
clicks and user-typed text.

JavaScript is fast to write, but not as powerful as Java.
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13
Multi-tier Architecture


Distributed applications on the web naturally have a multi-tier
architecture.
Java plays a role at all three levels:
– Graphical User Interface and client side analysis systems,
including visualization
– Middle layer servers and software integration, including web
servers, distributed object servers and other application servers.
– Less important for back end client software, which may be legacy
code.
Internet
Client user
interface running
through browser
Middle level
servers
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Internet or
proprietary
network
Backend
computing or
databases14
Overview and History of Java
Development
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15
History of Java Language and Team





Starts in 1991 as Project Green—semi-autonomous task
force in Sun focusing on operating software for
consumer electronic devices, e.g. smart set-top boxes.
Gosling (creator of Sun NeWS) considers C++ too
complex and initiates development of Oak, later
renamed to Java. Similarities to UCSD P-system.
A PDA (Personal Digital Assistant), codename “*7”,
based on Oak/Java ready in 1993. Green Team
becomes FirstPerson, Inc.
*7 proposal to Time-Warner rejected in 1993. 3DO deal
falls through in 1994. FirstPerson, Inc. dissolves.
Small group (~30 people, now the Java Team) continues
and decides to adapt Oak as a Web technology.
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History of Java Language and Team to Dec. 95

Experimental Web browser written in Java, called
WebRunner, later renamed HotJava, ready in 1994.

Alpha release of Java and HotJava April '95.

Netscape licences Java in May '95 and builds Java into
Netscape 2.0

Beta Java Development Kit (JDK) and first Java books
Summer/Fall '95.

Dec 4, 1995 Business Week cover story: "Software
Revolution—The Web Changes Everything" presents
Java as a breakthrough force in the expanding
Web/Internet.

In next week, SGI, IBM, Adobe, Macromedia and finally
Microsoft adopt/license Java. Java is adopted by Web
community.
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More Recent Java History

1996 JavaOne Developer’s Conference (now held
annually in June) focused on applets. Attendance of
5000 people.

1997 JavaOne focused on JavaBeans and new version
of the language JDK1.1, both designed to put Java in
line with other developments in component and secure
distributed web object computing and use of databases.

1998 JavaOne featured attendance of 14,000+ and
focused on Enterprise JavaBeans and other Enterprise
classes, developing more on server-side applications.

1999 JavaOne had over 20,000 attendees. Continued
development of Jini for dynamic networking, embedded
devices such as Palm Pilots.
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18
Overall Java Philosophy and
Features
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Some Key Java Features

First we discuss original Java base language features
as discussed in Java: A White Paper by Sun
Microsystems—October 1995 draft by James Gosling
and Henry McGilton—enumerates the original design
goals of Java:
Object-oriented
Architecture-neutral
Portable
Somewhat Interpreted
Simple and Familiar
Distributed

Robust
Secure
High performance
Multi Threaded
Dynamic
Finally we mention additional features added to Java in
more recent versions.
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20
Java Features—It's Simple and Familiar

Familiar as it looks like C++, but simpler to program.
– omits several confusing features of C++ including
operator overloading, multiple inheritance, pointers
and automatic type coercions

Adds automatic garbage collection to make dynamic
memory management much easier than in C or C++.
– No more frees or deletes. No more memory leaks.

Adds Interface construct, similar to Objective C concept,
to compensate for the lack of multiple inheritance.

Small kernel is suitable for Java ports to consumer
electronic devices.
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21
Java Features—It's Object-oriented

Java model is sometimes viewed as a C++ subset, with
some elements imported from other languages.
– This is arguable. In many ways Java and C++ are very
different, and many of the similarities that do exist are at a fairly
superficial syntactic level.

Structures, Unions and Functions are absorbed into
data and methods of Java classes—Java is simple.

The strength of Java object-oriented model is in
simplicity and the extensive class library associated with
the system.
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22
Java Features—It's Architecture-Neutral

C/C++ programming in a heterogeneous network
environment demands compatibility across several
vendor platforms and their compilers.

Solved in Java by designing platform-independent
binary representation called Java bytecode—
comparable to P-code in UCSD Pascal.

Java compiler reads Java source and generates Java
bytecode, which is shipped to user—e.g. on browser
request, Jini lookup, etc.

Each client must have a Java Virtual Machine program,
which interprets (“runs”) Java bytecodes.
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Java Features—It's Portable



Java Virtual Machine model is identical for all platforms.
Sun “owns” the Java Virtual Machine specification—while
classes can be added by any user, JVM is Universal.
In C/C++ various integer types match the architecture of
machine at hand. Java byte, char, short, int and long
are always 8, 16 (unicode), 16, 32 and 64 bits,
respectively.
– No header files, preprocessors, #define etc.
– floating point is always IEEE 754
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Java Features—It's Somewhat Interpreted

Java represents a compromise between fully compiled
(e.g. C/C++) and fully interpreted (e.g. typical scripting
languages) models.

Java “compiler” produces a binary bytecode output
which is portable and typically smaller than the real
binary for a specific machine. (Typical bytecode size is
of order of the original source code, within a factor of 2).

Java “interpreter”—the JVM—executes this bytecode.
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25
Java Features—It's Robust

Java enforces compile-time type checking and this
eliminates some error prone constructs of C/C++.

Pointer arithmetic is eliminated which allows for, e.g.,
runtime checking of array subscripts, and enforces
security of the Java model.

Explicit declarations are always required; argument
types of methods are always checked (unlike C). This
allows the Java complier to perform early error
detection.
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26
Java Features—It's (Hopefully)
Secure

Java bytecodes may be shipped across the network and
executed on client machines. Security is therefore a
critical issue and strongly enforced in Java.

The bytecodes sent across network are verified at the
client which prevents evil/corrupted classes from causing
problems
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27
Java Features—High Performance

Early Java interpreters performed on-the-fly execution of
the Java bytecodes, which gave “moderate” performance.
– Initial software was often 100 times slower than C



Performance is improved in newer “just-in-time” JVMs,
which compile methods after some number of executions,
and save machine code to give compiled-code efficiency
thereafter.
Support for generating native machine code out of Java
bytecodes also exists (e.g. TowerJ).
The performance of the machine code, generated from
Java bytecodes, may eventually be comparable to that
offered by typical C/C++ compilers on the same platform.
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28
Java Features—It's Multithreaded


Java model offers multithreading, implemented in terms
of the Thread class.
Thread methods offer a set of synchronization primitives
based on monitor and condition variable paradigm of
C.A.R. Hoare.
– One use of Java multithreading in applet programming, for
example, is having several independent but related simulations
running concurrently in an applet window. Multithreading is also
used internally by the browser to handle multiple document
dynamics.
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29
Java Features—It's Dynamic

Java model is more dynamic than C++, closer to
Smalltalk or Perl.

Classes (often) don’t need to to be recompiled after
implementation of a superclass (or other used class) is
updated—binary compatibility.

Classes have runtime representation (available through
the Class class) that allows one, e.g., to discover and
execute methods of a given object at runtime
– In C, can’t even distinguish at run-time whether a given pointer
references, say, an integer or a browser!
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30
Sun's Comparison of Language Features
Good
Fair
Java Smalltalk
Poor
TCL
Perl
Shells
C
Performance
Simple
Object-Oriented
Robust
Secure
Interpreted
Dynamic
Portable
Neutral
Threads
GarbageCollection
Exceptions
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31
C++
The Java 2 Platform
Sun are now offering 3 “editions”:

Java 2 platform, Standard Edition (J2SE)
– Develops earlier JDKs
– Available in V 1.3.

Java 2 platform, Enterprise Edition (J2EE)
– Incorporates multiple technologies for server-side and multi-tier
applications.

Java 2 platform, Micro Edition (J2ME)
– Optimized run-time environment for consumer products.
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32
Java Books—I


Core Java, by Gary Cornell and Cay S. Horstmann,
offers detailed coverage of the whole language and
packages for advanced programmers, including the
Swing Set. Also Volume 2 gives good coverage of
advanced topics such as JDBC, RMI, JavaBeans and
security.
Java, How to Program, by Deitel and Deitel, PrenticeHall, starts with beginning programming concepts and
progresses rapidly through Java language. It has the
most programming exercises and also has companion
teaching multimedia books. The third edition has Swing
Set and also the advanced API’s.
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33
Java Books—II


The Java Programming Language, by Ken Arnold and
James Gosling, David Holmes, 3rd Edition, AddisonWesley, 2000, is the classic on the language basics for
intermediate and advanced programmers. It covers
threads and I/O packages, but not applets or windowing
packages.
Java in a Nutshell, by David Flanagan, is the language
reference book in the familiar O'Reilly series. One of
the better references. Also Java Examples in a
Nutshell.
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34
Java Development Environments


These range from simple tools that give a windowing
interface to the edit/compile/run or view cycle, e.g.
JavaEdit from Dick Chase, on PC’s, . . .
. . . to the elaborate commercial development
environments that can also track projects and help
generate code for user interface components
–
–
–
–
–
Microsoft Visual J++
Symantec Visual Café
Java Workshop from Sun
Borland Jbuilder
Kawa from Tek-Tools
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35
Resources for the Java Programming
Language


The Java Language Specification, second edition. James
Gosling, Bill Joy, Guy Steele, Gilad Bracha, April 2000. The
ultimate reference for hardened computer scientists/compiler
writers.
http://www.javasoft.com web site has plenty of references
including
– Tutorial:
http://www.javasoft.com/books/Series/Tutorial/index.html
– Books:
http://www.javasoft.com/java.sun.com/aboutJavaSoft/booknews.html
– Collection of Applets:
http://www.gamelan.com
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