java i lecture_1


Chapter 1

Introduction to Computers,

the Internet and the Web


• 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


The Genesis of JavaNaughton 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.


• 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 hesoon 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


The Genesis of Java

• Reliable--which made him remove aspects of C++:i.) No Multiple Inheritance--he used interfaces

insteadii.) No Operator Overloadingiii.) No Manual Memory allocation and deallociv.) No Pointers--no pointer arithmetic v.) No assignment in conditionals (== vs = )

and add things C++ lacked:i.) Implicit Garbage Collection--no memory leaksii.) Data Structures only in Objects

iii.) Built in Security.


• Demo of *7, Programmed in Oak3 Sept 1992

This was the prototype of thefirst device to use the Oakprogramming language.

The “Star7” also featured the debut of “Duke,” the Javamascot. An early applet showed Duke doing cartwheels

across the screen.

The Genesis of Java


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 Fire23 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 2.023 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++


• 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 Major Advantage over C & C++


“Java is C without the Guns and

Knives.”

Thus, it has been said that...

Java’s Origins in C & C++


• 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 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

Hello.java

Java Compiler

Hello.class

Compile-Time Environment

Run-Time Environment

JavaVirtual

Machine

Object.class String.class

Example Java API class filesCombined, the Java Virtual Machine and the Java API form a

“Platform.”

The Java Platform


• 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


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.

What is the ?

Java ArchitectureJava

Virtual Machi

ne


C object modulecompiled into

machine language

C Source Code

• 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.


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 “bytecode”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 Architecture


Java Source

Java Bytecode

Wintel Mac UNIX MVS

JVM-Win JVM-Mac JVM-Unix JVM-IBM

You can easily see why Bill Gates isn’t in love with

Java!

The bytecode is met half-way by the JVM.


• 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.

Java Architecture


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 boundariesJava halts Buffer OverflowsJava 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.”



JDK 1.2 Security Model

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

Trusted

SANDBOX

Security and the “Sandbox” Remote Code

Has Fine-Grain Access Control

All Code, both Local and Remote, Must Pass Security Policy

Local Code



• 5 Steps To Writing A Java Program:1.) Write it in a Text Editor2.) Compiler creates bytecode3.) The “Class loader” places the .class file in memory.4.) The “Bytecode Verifier” makes sure

the code adheres to Java’s securityrules.

5.) The JVM Interpreter reads bytecodeand 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 variablesand 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, andii.) 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 calledHello.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.


• The two largest varieties of Java programs:

Applications

Applets

First Java Program


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.


The double slashes denote a “C++”-style comment. Everything on the line after the double slashes is ignored by the compiler.

This is the class name.Every single bit of code in Java must sit in curly brackets.Class names are capitalized. Words within the name are also capitalized.This Java program must be saved in a file with the exactsame name--matching the upper case--as you see in blueabove: HelloWorld.java

Now our Application is complete. We have added the method “main”. All methods are lower case. main is a special method--it actually runs the program.

In any application, you are always guaranteed that method main will run.

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

public class HelloWorld{

}

// HelloWorld.java Our first Java Application

public class HelloWorld{

public static void main( String args[]){

System.out.println( “Hello World!” );}

}


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.


C:\>javac HelloWorld.java

C:\>java HelloWorldHello World!

A Java Application

• To run your compiled Application, you enter lowercase java HelloWorld on the command line.

• Notice, the “.class” extension is omitted.


Now load the JDK1.4.1,

the documentation,

change the class path and

write your first Java program.

In Class Activity

java i lecture_1

Technology

java api

basedon java

beenrenamed java

java programming language

java virtual machine

java class files

java class file format

suns entire java team