Java i lecture_1_upd1

Chapter 1
Introduction to Computers,
the Internet and the Web


The Genesis of Java
• It all started with an angry programmer.
1990

Sun Microsystems Software
Engineer Patrick Naughton,
age 25, was disgusted with
his job at Sun. He had the impossible job of
making different software APIs--from
dozens of languages, platform OS’s and
vendors--all work as one. It was impossible.

The Genesis of Java
Naughton announced to CEO
Scott McNealy that he was quitting Sun.
Pat was going to join NeXT,
because Sun was a mess.

McNealy asked Pat to write a memo
outlining his gripes.
The CEO asked Pat to suggest a
solution, “As if you were God.”

The Genesis of Java
• Formation of the “Green Project”
Jan 1991

The array of standards spurred the
formation of the “Green Project.” Its goal
was making Consumer Electronics devices
talk to each other.
Since VCRs, Laser Disc Players and
Stereos were all made with different CPUs,
they all needed special programming.

The Genesis of Java
• James Gosling, then age 36, was asked to find a
programming language for the project.
Gosling, who had left
IBM in 1984 to join Sun,
first chose C++. But he
soon gave up on C++,
which was incapable of doing
what he wanted. So, he started to
modify C++, (which is a
direct descendant of the C programming
language).

The Genesis of Java
• Soon, Gosling was writing a new language,
which he named “Oak” after the tree outside
his window.
Oak to had to be:
• Small to work on Consumer electronics,
• Platform independent, to avoid hassles like the
ones Naughton encountered,
• an Interpreted language,
• Object Oriented,


The Genesis of Java
• Reliable--which made him remove aspects of C++:
i.) No Multiple Inheritance--he used interfaces
instead
ii.) No Operator Overloading
iii.) No Manual Memory allocation and dealloc
iv.) No Pointers--no pointer arithmetic
v.) No assignment in conditionals (== vs = )
and add things C++ lacked:
i.) Implicit Garbage Collection--no memory leaks
ii.) Data Structures only in Objects
iii.) Built in Security.


The Genesis of Java
• Demo of *7, Programmed in Oak
3 Sept 1992

This was the prototype of the
first device to use the Oak
programming language.
The “Star7” also featured the
debut of “Duke,” the Java
mascot. An early applet
showed Duke doing cartwheels
across the screen. Java I--Copyright © 2000-2004 Tom Hunter

The Genesis of Java
• Oak becomes Java.
Jan 1995

By this time, the Internet had taken off.
Bill Joy, one of Sun’s founders, realized that
the needs of the Web [ reliability, platform
independence, security ] exactly matched the
characteristics of Oak, which had just been
renamed Java.


The Genesis of Java
• Java Catches Fire
23 Mar 1995

Although Java had not yet been
officially released, it was spreading like
wildfire among developers.
Then, something very lucky happened...


The Genesis of Java
•Netscape Navigator 2.0
23 May 1995

Two months later, at the SunWorld
conference, Marc Andreessen stepped on
stage and announced that “Java is real, and it
will be incorporated into Netscape Navigator
2.0.”
At this moment, Sun’s entire Java team only
numbered 30 people.

Java’s
Major Advantage
over C & C++


Java’s Major Advantage over C & C++

• Because pointers were a major source of
bugs in C and C++, Gosling omitted pointers
entirely from Java.

• Actually, pointers are still an important part
of the language--all objects are referenced by
pointers--but the language handles them, not
the programmer.


Java’s Origins in C & C++

Thus, it has been said that...

“Java is C without
the Guns and
Knives.”

Java Architecture
• By now, Java itself has matured into its 3rd
version, named Java 2. This course is based
on Java 2. The most current is Java 2 (1.5.1)
• Java is Object-Oriented--that means
everything in the language behaves like an
object.
• What exactly that means will be explained
in the coming weeks.

Java Architecture

Java’s Architecture comes from four
separate but intertwined technologies:
• the Java Programming Language
• the Java class file format
• the Java API, or Application Programming Interface
• the Java Virtual Machine


Java Architecture
Source programs are written in the Java
Programming Language.

All procedural code falls within methods.

Programs are compiled into Java class files.

Classes run in the Java Virtual Machine.


Java Architecture

• When a Java program runs,
it is assisted by other classes
in the Java the Application
Programming Interface, or
API.


Java Architecture
Combined, the Java Example Java API class files
Virtual Machine and
Object.class String.class
the Java API form a
“Platform.”
Compile-Time
Environment
Hello.class
Java
Virtual
Java
Compiler Machine

Run-Time
Hello.java Environment
The Java PlatformJava I--Copyright © 2000-2004 Tom Hunter

Java Architecture

• The Java Platform is unique, because it can
work without modification on
any platform, on any
operating system, if that
platform has a “Java Virtual
Machine.”


Java Architecture
Java
What is the Virtual ?
Machine

Comparison of a typical Procedural
Program with a Java Program:
• In a typical C program, the source code is
compiled into a native machine language
module that consists of 1’s and 0’s.

C Source Code

C object module
compiled into
machine language

• The machine language is specifically tailored
to one OS, be it Wintel, Mac, UNIX or MVS.
• Therefore, it is impossible for one object
module to be portable between platforms.

Java Architecture

Java “bytecode”
In contrast to conventional programming
languages, a Java program is not compiled
into native machine language.
• Instead, Java makes bytecode.
• Bytecode is the result of a “compile”, but
the compile results in an intermediate form
that stops short of native machine-specific
code. Java I--Copyright © 2000-2004 Tom Hunter

Java Architecture

• Instead of making a machine language
native code for each particular OS, Java
makes a single, universal bytecode module
that feeds into any Java Virtual Machine
(JVM).

• Each OS has its own different
implementation of the
Java Virtual Machine.

Java Architecture

• The JVM sets up its own world within
your RAM.

• The JVM creates an internal
software-only sub-computer within the OS.

• The bytecode talks to the JVM, and the
JVM talks to the Operating System.


Java Architecture

• Thus, you get the Holy Grail of software reuse:

“Write Once,
Run Anywhere”.


Java Source
You can easily see why Bill
The Gates isn’t in love with
bytecode Java Bytecode
Java!
is met
half-way
by the
JVM.

JVM-Win JVM-Mac JVM-Unix JVM-IBM
Wintel Mac UNIX MVS

Java Architecture

• The Virtual Machine interprets the
bytecode one instruction at a time,
and translates it into native machine
code.

• You compile your program once
into bytecode, but it is interpreted
anew every time it runs.


Security and the
“Sandbox”


Security and the “Sandbox”

C and C++ are famous for speed.

• One reason they are fast is because C
and C++ don’t do things like checking the
bounds of arrays.
• In C or C++, a program can walk off
the edge of an array and invade the memory
space beyond.
• Hackers love that about C and C++.

• Another weakness of C/C++, that is a
favorite among Hackers, is the Buffer
Overflow.

• In this attack, the Hacker floods too much
data into a buffer and whatever overflows it is
turned loose on the system.

• Java solves these problems



• How Java Combats malicious code:
Java checks array boundaries
Java halts Buffer Overflows
Java has Garbage collection to get rid
of objects that are no longer used.
Java’s compiler checks to make sure
the code is safe before it runs.
• Gosling built security into Java, using a
concept known as the “Sandbox.”

Security and the “Sandbox” Remote Code
Local Code
All Code, both Local and Remote, Must Pass Security
Policy
JDK 1.2 Security Model

SANDBOX

Vulnerable System Resources
(files, etc) Even Local Code is Not
Trusted

Has Fine-Grain Access Control Java I--Copyright © 2000-2004 Tom Hunter

• 5 Steps To Writing A Java Program:
1.) Write it in a Text Editor
2.) Compiler creates bytecode
3.) The “Class loader” places the .class
file in memory.
4.) The “Bytecode Verifier” makes sure
the code adheres to Java’s security
rules.
5.) The JVM Interpreter reads bytecode
and makes platform native code.

• You see, preventing problems is a major
design consideration in Java.

• This idea led to the most import aspect of
Java: Object Orientation.

• Object Orientation protects data and lets a
program do only what is explicitly permitted.

• You could say Java is pessimistic.

Objects in Java

• In Java, Object Orientation is so
pervasive that it’s nearly impossible
to write a strictly procedural program
in the language.


Objects in Java

• Objects are reusable components.

• In Java, everything must be run from a
“class” file. This “class” contains bytecode.

• Java source code has the extension
Xxx.java


Objects in Java

• If I write a Java program called:

Hello.java

then, when compiled, this program will
be called:

Hello.class


Objects in Java
• A class object is compiled Java code that
contains its own data variables, called
members, and sections of procedural code
called methods.

If you have programmed in COBOL, a
method is like a paragraph you perform.

If you have programmed in C or C++, a
method is like a function your program calls.

Objects in Java

• The combination of the data variables
and the methods
that are used to read,
write
or modify
those variables

is called a class.


Objects in Java

• Java has a rich collection of Class Libraries.

• These are also known as the Java API or
Application Programming Interface.

• To program in Java, you must

i.) Learn the Language, and
ii.) Learn the Class Libraries.

Objects in Java

• These class libraries greatly simplify your
job as a Java programmer.

• They help you to write complex programs
quickly.

• To master Java, you must master these
class libraries.


Compiling A Java Program
• You have created a Java program called
Hello.java
• To compile it, you run the JDK supplied
utility called:
javac
C:javac Hello.java
If this was successful, a file called:
Hello.class will be produced.

First Java Program
• The two largest varieties of Java
programs:

Applications

Applets


First Java Program

• A Java Application is a free-standing
program that is capable of running
directly in the Java Virtual Machine.

• A Java Applet is a mini-program that is
much more limited in its abilities. An
Applet can only run within the context of
an HTML browser.


A Java Application
// HelloWorld.java Our first Java Application

public class HelloWorld
{
public static void main( String args[])
{
System.out.println( “Hello World!” );
}
}

Nowdouble slashesname. a “C++”-style comment.
The our Application is complete. We have added the
This is the class denote
method “main”. All methodsJava lowerslashesmain is
Every single bit of code in are must case. is
Everything on the line after the double sit in curly brackets.
a specialby the compiler.
ignored names are capitalized. the program.
Class method--it actually runs
Words within the name are also capitalized.
In any application, you are always guaranteed
that method main will run.be saved in a file with the exact
This Java program must
same name--matching the upper case--as you see in blue
above: HelloWorld.java

A Java Application

C:>javac HelloWorld.java

C:>

• A successful compile of your java
program will return to a bare cursor, as
you see here.


A Java Application
C:>javac HelloWorld.java

C:>java HelloWorld
Hello World!

• To run your compiled Application,
you enter lowercase java HelloWorld
on the command line.
• Notice, the “.class” extension is
omitted.

In Class Activity

Now load the JDK1.4.1,
the documentation,
change the class path and
write your first Java program.


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