Java I Lecture 1

8/10/2019 Java I Lecture 1

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Java I-- Copyright © 2000-2007 Tom Hunter




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

* After his stint at Sun, Mr. Naughton went on to work for Disney and, temporarily, for the FBI….




The Genesis of Java

Naughton announced to CEO

Scott McNealy that he was quittingSun.

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 goalwas 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.


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• 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 tomodify 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 outsidehis 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 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.

*

Technically, Java still has pointers but they are calledreferences

and cannot be used to do pointer arithmetic or any of the other shenanigans that make C programming such a joy.



• 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.

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 thatthe needs of the Web [ reliability, platform

independence, security ] exactly matched the

characteristics of Oak, which had just beenrenamed Java.



The Genesis of Java

• Java Catches Fire

23 Mar 1995

Although Java had not yet been

officially released , it was spreading likewildfire 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 onstage 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 7thversion, named Java 7.

• Java is Object-Oriented*--that means

everything in the language behaves like an

object.

• What exactly that means will be explainedin the coming lectures.

Java Architecture

* Purists will say the presence of primitive variables makes this only 99.9% true.



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 Application

Programming Interface, or

API.




Java Architecture

Hello.java

Java

Compiler

Hello.class

Compile-Time

Environment

Run-Time

Environment

Java

Virtual

Machine

Object.class String.class

Example Java API class files Combined, 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 iscompiled into a native machine language

module that consists of 1’s and 0’s.

What is the ?

Java Architecture

Java

VirtualMachine




C object module

compiled intomachine 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”, butthe 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 differentimplementation of the

Java Virtual Machine.

Java Architecture




• The JVM sets up its own world withinyour RAM.

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

• The bytecode talks to the JVM, and theJVM 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 BillGates isn’t in love with Java!

Thebytecode

is met

half-wayby the

JVM.




• The Virtual Machine interprets the

bytecode one instruction at a time, andtranslates it into native machine code.

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

time it runs.

• JIT [Just In Time] compilers can cache

the compiled machine code.

Java Architecture




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 memoryspace beyond.

• Hackers love that about C and C++.





• Another weakness of C/C++, that is a

favorite among Hackers, is the BufferOverflow.

• In this attack, the Hacker floods too muchdata 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 verifier 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 security

rules.5.) The JVM Interpreter reads bytecode

and makes platform native code.





• You see, preventing problems is a major

design consideration in Java. Other languagesdo this too but Java was built around it.

• 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 inthe 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, calledmembers, 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,

writeor 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 programsquickly.

• To master Java, you must master theseclass 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.




• The two largest varieties of Java programs:

Applications

Applets

First Java Program

i




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 ofan HTML browser.




The double slashes denote a “C++”-style comment.

Everything on the line after the double slashes is

ignored by the compiler.

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 static void main( String args[])

{

System.out.println( “Hello World!” );

}}




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 exact

same 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 guaranteedthat method main will run.



{

}



{


{


}}

A J A li ti




A Java Application

C:\>javac HelloWorld.java

C:\>

• A successful compile of your java

program will return to a bare cursor, asyou see here.

A J A li ti




C:\>javac HelloWorld.java

C:\>java HelloWorld

Hello 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 JDK. I recommend using th

most current one.Get the documentation,

change the class path and

write your first Java program.

In Class Activity

Java I Lecture 1

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