Dr. Rajeshree Khande :Intoduction to java

Java Evolution
 Java Introduction
 Java Features
 How Java Differs from other OO languages
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Java - An Introduction
 Java - The new programming language developed by
Sun Microsystems (James Gosling) in 1991.
 Originally called Oak.
 Java Authors: James Gosling, Arthur Van Hoff, Andy
Bechtolsheim
 A general-purpose object-oriented language.
 Designed for easy Web/Internet applications
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Java - An 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.
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Year Development
1990 Sun decided to developed special software that could be used for
electronic devices. A project called Green Project created and
head by James Gosling.
1991 Explored possibility of using C++, with some updates announced
a new language named “Oak”
1992 The team demonstrated the application of their new language to
control a list of home appliances using a hand held device.
1993 The World Wide Web appeared on the Internet and transformed
the text-based interface to a graphical rich environment. The team
developed Web applets (time programs) that could run on all
types of computers connected to the Internet.
Java Milestones
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Java Milestones
Year Development
1994 The team developed a new Web browsed called
“Hot Java” to locate and run Applets. HotJava
gained instance success.
1995 Oak was renamed to Java, as it did not survive
“legal” registration. Many companies such as
Netscape and Microsoft announced their support for
Java
1996 Java established itself as both
1. “the language for Internet programming”
2. a general purpose OO language.
1997 A class libraries, Community effort and
standardization, Enterprise Java, Clustering, etc..
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Java Features
 Familiar, Simple, Small
 Object-Oriented
 Platform-Independent and Portable
 Compiled and Interpreted
 Robust and Secure
 Distributed
 Multithreaded
 High Performance
 Dynamic and Extensible
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Cont…
 Familiar, Simple & Small
 Java does not use Preprocessor header files,
goto statement and many others
 no pointers
 automatic garbage collection
 rich pre-defined class library
http://java.sun.com/j2se/1.4.2/docs/api/
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Cont…
 Object-Oriented
 Focus on the data (objects) and methods
manipulating the data
 Almost every thing in Java is Object.
 All functions are associated with objects
 Almost all datatypes are objects (files, strings, etc.)
 Potentially better code organization and reuse
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Cont…
 Platform-Independent and Portable
 Java is portable.
1) Java Complier generates bytecode that can be
implemented on any machine.
2) The sizes of the primitive data types are always
the same
 Application runs on all platforms.
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Cont…
JAVA COMPILER
JAVA BYTE CODE
JAVA INTERPRETER
Windows 95 Macintosh Solaris Windows NT
(translator)
(same for all platforms)
(one for each different system)
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Java Development Environment
Java Compiler
(Pentium)
Java Compiler
(PowerPC)
Java Compiler
(SPARC)
Java
Interpreter
Pentium
PowerPC
SPARC
Java
Interpreter
Java
Interpreter
Java
Code
Java
ByteCode
(Independent on
Platform)
javac Hello.java
Hello.class created
java Hello
Byte CodeRun JVM 11RAJESHREE KHANDE

JVM : Java Virtual Machine
 CPU are capable of executing only the machine
instruction.
 Different type of CPU hava different set of machine
instruction.
 To enable java program to be executed on different
CPUs without modification, Java program are
complied into machine instruction that can be
understood and executed by idealized CPU.
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JVM : Java Virtual Machine (Cont..)
 Such CPU is called as Java virtual Machine (JVM)
 Actually JVM is simulated program which is
executed on actual CPU.
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Cont…
 Compiled and Interpreted
 Java compiler generate byte-codes, not native
machine code.
 The compiled byte-codes are platform-
independent.
 Java bytecodes are translated to machine
readable instructions at runtime (Java Virtual
Machine)
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Compiled Languages: C Execution
Text Editor Compiler linker
Programmer
Source Code
.c file
Object
Code
.o file
Notepad,
emacs,vi
gcc
Executable
Code
a.out file
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Compiled Languages: Java Execution
Text Editor Compiler Interpreter
Programmer
Source Code
.java file
Byte Code
.class file
Hardware and
Operating System
Notepad, vi javac java
appletviewer
netscape
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Cont..
 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.
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Cont..
 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.
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Cont…
 Robust and Secure
 Usage in networked environments requires more
security
 Provides many safeguard to ensure reliable code.
 Strict compile time & run time type checking.
 Memory allocation model is a major defense
 Access restrictions are forced (private, public)
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Cont…
 Distributed
 Designed as distributed lang. For creating
application on network.
 Java program can access remote access remote
objects on internet
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Cont…
 Multithreaded
 Handles multiple task simultaneously.
 Java tools support multiprocessing
synchronization.
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How Does Java Compares to C++
and Other OO Languages
C
C++
Java
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Java better than C ?
 No Preprocessor
 No Global Variables
 No data types struct, union and enum
 No Goto ,size of and typedef
 No type modifier keywords auto, extern, register,
signed and unsigned.
 No Pointers
 Add labeled break and continue
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Rich Class Environment
 Core Classes
 Language support Package :
Collection of classes For Implementing Basic
features of Java (java.lang.*)
 Utilities Package :
provide utility function such as date & time
(java.util.*)
 Input/Output package:
Collection of class required for input/output (java.io.*)
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Rich Class Environment
 Networking package:
Collection of classes communication with other
computers via internet. (java.net.*)
 Abstract Window Tool Kit (AWT) :
Collection of Packages that implement platform
independent GUI (java.awt.*)
 Applet Package :
Set of classes allows to create applet program
(java.applet.*)
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Java Applications
 We can develop two types of Java programs:
 Stand-alone applications
 Web applications (applets)
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Applications v/s Applets
 Different ways to run a Java executable are:
Application-
A stand-alone program that can be invoked from
command line . A program that has a “main”
method
Applet-
A program embedded in a web page , to be run
when the page is browsed . A program that
contains no “main” method
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Applets v/s Applications
 Different ways to run a Java executable are
Application –Executed by the Java interpreter.
Applet- Java enabled web browser.
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Significance of downloading
Applets
 Interactive WWW
 Flashy animation instead of static web pages
 Applets react to users input and dynamically change
 Display of dynamic data
 WWW with Java - more than a document publishing
medium
 http://www.javasoft.com/applets/alpha/applets/Stock
Demo/standalone.html
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Java Development Kit
Tools in JDK Purpose
javac It is java complier. It complies java source code into the
bytecode which can be understood by JVM.
java Java Interpreter – It interprets the bytecode into machine
code & then executes it
jdb Java debugger – detect the errors in java program
appletviewer Executes the special type of java program known as
applet
javadoc Used to create documentation in HTML
javah Produces a c header file for use with special type of java
method , known as native methods
Javap Java disassembler – Helps to convert bytecode file into
java program description
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Process of Building and Running
Java Programs
Text Editor
Java Source
Code
javac
Java Class File
java
Outout
javadoc
javah
jdb
HTML Files
Header Files
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Let us Try Out
Building your first Java Program
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Creating, Compiling, and
Running Programs
Source Code
Create/Modify Source Code
Compile Source Code
i.e., javac Welcome.java
Bytecode
Run Byteode
i.e., java Welcome
Result
If compilation errors
If runtime errors or incorrect result
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
}
}
…
Method Welcome()
0 aload_0
…
Method void main(java.lang.String[])
0 getstatic #2 …
3 ldc #3 <String "Welcome to
Java!">
5 invokevirtual #4 …
8 return
Saved on the disk
stored on the disk
Source code (developed by the programmer)
Byte code (generated by the compiler for JVM
to read and interpret, not for you to understand)
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Demo program
// HelloWorld.java
class HelloWorld
{
public static void main(String args[])
{
System.out.println(“Hello World”);
}
}
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Program Processing
 Compilation
# javac hello.java
results is HelloInternet.class
 Execution
# java HelloInternet
Hello Internet
#
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public static void main
Access to all / can be accessed from
everywhere
Does not return
anything
main Method
Means the main
method is shared by all
objects in the class.
Access without object
name
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System.out.println(“…”);
Out is the output stream
used to connect the
console
To print the
specific string onto
output device in
next line.
System is predefined class
to provide access to the
system
System: System refers to current java program.
Out : out refers to output device. by default it is monitor.
println : to print the specific string onto output device in
next line
OR
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Program Processing
 Compilation
C:> javac HelloWorld.java
results in HelloWorld.class
 Execution
c:> java HelloWorld
Hello World
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Fundamental structure Java prog
/* a comment */
// a comment
[package declarations]
[import statements]
class declaration
{
}
An example is given below:
import java.lang.*;
class Demo
{
public static void main(String[] args)
{
Sytem.out.println("Hello! World");
}
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Simple Java Applet
//HelloWorld.java
import java.applet.Applet;
import java.awt.*;
public class HelloWorld extends Applet {
public void paint(Graphics g) {
g.drawString (“Hello World !”,25,
25);
}
}
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Calling an Applet
<HTML>
<TITLE>
HELLO WORLD APPLET
</TITLE>
<HEAD>
THE HELLO WORLD APPLET
</HEAD>
<APPLET CODE=“HelloWorld.class” width=500
height=500>
</APPLET>
</HTML> 41RAJESHREE KHANDE

Applet Execution
Using AppletViewer
Using Browser
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Class & Object
 What Are Classes?
A class is a blueprint or prototype that
defines the variables and the methods
common to all objects of a certain kind.
 What Is an Object?
An object is a software bundle of variables
and related methods. Software objects are
often used to model real-world objects you
find in everyday life
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Class vs. Object
 Specifies the
structure (the
number and types)
of its objects’
attributes — the
same for all of its
objects
 Specifies the
possible behaviors
of its objects
 Holds specific
values of attributes;
these values can
change while the
program is running
 Behaves
appropriately when
called upon
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Class vs. Object
 A piece of the
program’s source
code
 Written by a
programmer
 An entity in a
running program
 Created when the
program is running
(by the main method
or a constructor or
another method)
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Classes
public class Car
{
...
}
Car.java By convention, the
name of a class (and
its source file) always
starts with a capital
letter.
(In Java, all names are case-sensitive.)
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public class SomeClass
 Fields
 Constructors
 Methods
}
Attributes / variables that define the
object’s state; can hold numbers,
characters, strings, other objects
Procedures for constructing
a new object of this class
and initializing its fields
Actions that an object
of this class can take
(behaviors)
{
Class header
SomeClass.java
import ... import statements
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Classes
 A class is a collection of fields (data) and methods
(procedure or function) that operate on that data.
 The basic syntax for a class definition:
 Bare bone class – no fields, no methods
public class Circle {
// my circle class
}
ClassName]
{
[fields declaration]
[methods declaration]
}
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Classes
 Classes describe the data held by each of its
objects
 A class may describe any number of objects
 A class may describe a single object, or even
no objects at all
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Dr. Rajeshree Khande :Intoduction to java

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