Java and its Evolution

Contents

Java Introduction Java Features How Java Differs from other OO

languages Java and the World Wide Web Java Environment Build your first Java Program Summary and Reference

Java - An Introduction

Java - The new programming language developed by Sun Microsystems in 1991.

Originally called Oak by James Gosling, one of the inventors of the Java Language.

Java Authors: James , Arthur Van , and others

Java is really “C++ -- ++ “

Java Introduction

Originally created for consumer electronics (TV, VCR, Freeze, Washing Machine, Mobile Phone).

Java - CPU Independent language Internet and Web was just

emerging, so Sun turned it into a language of Internet Programming.

It allows you to publish a webpage with Java code in it.

Sun white paper defines Java as:

Simple and Powerful Safe Object Oriented Robust Architecture Neutral and Portable Interpreted and High Performance Threaded Dynamic

Java is Compiled and Interpreted

Text Editor Compiler Interpreter

Programmer

Source Code

.java file

Byte Code

.class file

Hardware and Operating System

Notepad, emacs,vi

javac

javaappletviewernetscape

Total Platform Independence

JAVA COMPILERJAVA COMPILER

JAVA BYTE CODEJAVA BYTE CODE

JAVA INTERPRETERJAVA INTERPRETER

Windows 95 Macintosh Solaris Windows NT

(translator)

(same for all platforms)

(one for each different system)

Architecture Neutral & Portable

Java Compiler - Java source code (file with extension .java) to bytecode (file with extension .class)

Bytecode - an intermediate form, closer to machine representation

A interpreter (virtual machine) on any target platform interprets the bytecode.

Architecture Neutral & Portable

Porting the java system to any new platform involves writing an interpreter.

The interpreter will figure out what the equivalent machine dependent code to run

How Does Java Compare to C++ and Other OO Languages?

Overlap of C, C++, and Java

C

C++

Java

Java better than C++ ?

No Typedefs, Defines, or Preprocessor No Global Variables No Goto statements No Pointers No Unsafe Structures No Multiple Inheritance No Operator Overloading No Automatic Coercions No Fragile Data Types

?

Java Integrates Power of Compiled Languages

and Flexibility of Interpreted Languages

Java Applications

We can develop two types of Java programs: Stand-alone applications Web applications (applets)

Applications v/s Applets

Different ways to run a Java executable code are:Application- A stand-alone program that

can be invoked from command line . A program that has a “mainmain” method. It is executed by the Java Interpreter.

Applet- A program embedded in a web page , to be run when the page is browsed . A program that contains no “main” method. It is executed on a Java-enabled web browser.

Java Development Kit

javac - The Java Compiler java - The Java Interpreter jdb- The Java Debugger appletviewer -Tool to run the applets javap - to print the Java bytecodes javaprof - Java profiler javadoc - documentation generator javah - creates C header files

Process of Building and Running Java Programs

Text Editor

Java Source Code

javac

Java Class File

java

Output

javadoc

javah

jdb

HTML Files

Header Files

18

Java Program Structure

A Java program consists of: One or more classes A class contains one or more methods A method contains program statements

We will explore these terms in detail

19

Java Program Structure

public class MyProgram

{

}

// comments about the class

class header

class body

Comments can be placed almost anywhere

20

Java Program Structure

public class MyProgram

{

}

// comments about the class

public static void main (String[] args)

{

}

// comments about the method

method headermethod body

21

Hello World

public class HelloWorld

{

}

// HelloWorld.java

public static void main (String[] args){

}

System.out.println(“Hello World!”);

22

Hello World

public class HelloWorld

{

}

// HelloWorld.java

public static void main (String[] args){

}

System.out.println(“Hello World!”);

Creates a “class” called HelloWorld Compiled to HelloWorld.class Classes used to define objects… later

23

Hello World

public class HelloWorld

{

}

// HelloWorld.java

public static void main (String[] args){

}

System.out.println(“Hello World!”);

The “main” method is where it starts to run Ignore “public static void” and “String[]

args” for now

24

Hello World

public class HelloWorld

{

}

// HelloWorld.java

public static void main (String[] args){

}

System.out.println(“Hello World!”);

Contains one “statement” The System.out.println function comes

from the Java “class library” Ends with a semicolon (all statements do)

25

Compiling and Running

Create the file HelloWorld.java in a text editor

Compile: javac HelloWorld.java

Run: java HelloWorld

Output: Hello World!

26

Comments

Three kinds of comments:

To simplify: comments are good

// a one-line comment/* a multi-line comment */

/** a javadoc comment */

27

Reserved Words and Identifiers

Reserved words are specified by the language All Java reserved words are in the text

Identifiers are specified by a programmer Maybe you: e.g. HelloWorld Maybe someone else: e.g. println

28

Declaring Variables

Syntax:

Examples: int count1, int count 2; int count = 0; String course1 = “CMPT 126”;

<variable declaration> ::= <type> <declarator>, …. ;<declarator> ::= <identifier>

<declarator> ::= <identifier> = <expression>

29

Primitive Data Types in Java

Four integer types: byte, short, int, long

Two floating point types float, double

One of them is for characters char

One of them is for boolean values boolean

30

Expressions and Assignment

An expression is a combination of one or more operators and operands

Arithmetic operators: +, -, *, /, % Use the normal order of operationse.g. int exp = 2 * 5 +7; count = count + 1;

count++; Boolean operators: &&, ||

31

More Assignment Operators

x += y is equivalent to x = x + y

32

Example: String Objects

We have already seen one object type in Java: String

A String object is a list of characterse.g. “Hello world!” or “My name is Aaron”

Can be passed to print or println Can be concatenated using the (+)

operatore.g. “Hello world! ” + “My name is Aaron” “I can also append numbers, like “ + 2

33

Object Instances

We must create a new “instance” of an object to store something Each object type has a constructor (more later) Create instances using the reserved world new e.g. course = new String(“CMPT 126”);

This creates a new String in memory It stores the characters “CMPT 126” The assignment sets course to refer to

this instance

34

References and Instances

String course;

new String(“CMPT 126”)

course = new String(“CMPT 126”);

course:

CMPT 126

CMPT 126course:

Summary

Java has emerged as a general purpose OO language.

It supports both stand alone and Internet Applications.

Makes the Web Interactive and medium for application delivery.

Provides an excellent set of Tools for Application Development.

Java is ubiquitous!