Unit I Java Pro

8/3/2019 Unit I Java Pro

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INTRODUCTION TO JAVA

Ms. Asma A. Mokashi


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Ms. Asma A. Mokashi, Lecturer, MCA

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History

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James Gosling, Mike Sheridan, and Patrick Naughton initiated

the Java language project in June 1991.

Java was originally designed for interactive television, but it

was too advanced for the digital cable television industry at the

time.

The language was initially called Oak after an oak tree that

stood outside Gosling's office.

It was later renamed Java, from Java coffee, said to be

consumed in large quantities by the language's creators.


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Java Milestones

Year Development

1990 Sun decided to developed special software that could be usedfor electronic devices. A project called Green Project created

and head by James Gosling.

1991 Team announced a new language named Oak

1992 The team demonstrated the application of their new languageto control a list of home appliances using a hand held device.

1993 The World Wide Web appeared on the Internet andtransformed the text-based interface to a graphical richenvironment. The team developed Web applets (timeprograms) that could run on all types of computers connectedto the Internet.


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Java Milestones

Year Development

1994 The team developed a new Web browsed called Hot Java tolocate and run Applets. HotJava gained instance success.

1995 Oak was renamed to Java, as it did not survive legalregistration. Many companies such as Netscape and Microsoftannounced their support for Java

1996 Java established itself it self 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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Sun white paper defines Java as:

Simple and Powerful

Safe

Object Oriented Robust

Architecture Neutral and Portable

Interpreted and High Performance Threaded

Dynamic


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Java Attributes

Familiar, Simple, Small

Compiled and Interpreted

Platform-Independent and Portable

Object-Oriented

Robust and Secure

Distributed

Multithreaded and Interactive

High Performance

Dynamic and Extensible


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Java Attributes7

JAVA AttributesJAVA Attributes


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Simple , Small and Familiar

Many features of C and C++ that are either

redundant or sources of reliable code are not part of

Java.

To make the language look familiar to the existing

programmers, it was modelled on C and C++

Languages.


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Java is Compiled and Interpreted

Text Editor Compiler Interpreter

Programmer

Source Code

.java file

Byte Code

.class file

Hardware andOperating System

Notepad,emacs,vi

javac javaappletviewernetscape


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Compiled Languages

Text Editor Compiler linker

Programmer

Source Code

.c file

ObjectCode

.o file

Notepad,emacs,vi

gcc

ExecutableCode

a.out file


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Platform Independent and Portable

JAVA COMPILERJAVA COMPILER

JAVA BYTE CODEJAVA BYTE CODE

JAVA INTERPRETERJAVA INTERPRETER

Windows 95 Macintosh Solaris Windows NT

(translator)

(same forall platforms)

(one foreach differentsystem)


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Platform independence


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Architecture of Java Applications

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Java applications are written astext files

The java compiler createsplatform independent code whichis called bytecode.

Bytecode can be executed by the

java runtime environment. The Java virtual machine is a

program which knows how to runthe bytecode on the operatingsystem the JRE is installed upon.

The JRE translates the bytecodeinto native code, e.g. the native

code for Linux is different then thenative code for Windows.

Java code

is compiled

to produce

byte code

run by Java

Virtual Machine

(JVM) to produce

results

BA Mosbach / Referent Lars Vogel WS 08/09


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Architecture Neutral & Portable

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

Bytecode - an intermediate form, closer tomachine representation

A interpreter (virtual machine) on any targetplatform interprets thebytecode.


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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.

Size of primitive data types are machine

independent


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Rich Class Environment

Core Classes

language

Utilities

Input/Output

Low-Level Networking

Abstract Graphical User Interface

Internet Classes

TCP/IP Networking

WWW and HTML

Distributed Programs


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Object Oriented

Java is a true object oriented language . Almost

everything in Java is an Object

Features of Object Oriented Programming :

1. Object

2. Class

3. Data Abstraction

4. Encapsulation5. Inheritance

6. Polymorphism

7. Binding


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Object Oriented Languages -A Comparison

Feature C++ ObjectiveC

Ada Java

Encapsulation Yes Yes Yes Yes

Inheritance

Yes Yes No YesMultiple Inherit. Yes Yes No No

Polymorphism Yes Yes Yes Yes

Binding(Earlyor Late) Both Both Early Late

Concurrency Poor Poor Difficult Yes

Garbage Collection No Yes No Yes

Genericity Yes No Yes Limited

Class Libraries Yes Yes Limited Yes


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Robust & Secure

Robust :

1. It has strict compile time and runtime checking for

data types .

2. Garbage Collection- Relieving programmers virtually

all memory management problems

3. Exception Handling Which captures series of

errors and eliminates any risk of crashing the system.


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Secure :

1. Java system verify all memory access

2. Also ensues that no viruses are communicated with an

applet.

3. The absence of pointers in Java ensures that programs

cannot gain access to memory locations without

proper authorization.


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Distributed

Java is designed as a language for creating

applications on networks.

It has the ability to share both data and programs .

Java applications can open and access remote objects

on internet .

This enables multiple programmers at multiple

remote locations to collborates and work togetheron a single project.


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Multithreaded and Interactive

Multithreaded means handling multiple task

simultaneously. This means that we need not wait

for the application to finish one task before

beginning another.

This features greatly improves the interactive

performance of graphical applications.


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High Performance

Java performance is impressive for an interpreted

langauge , mainly due to the use of intermediate

bytecode.

Incorporation of multithreading enhances the overall

execution speed of Java Programs.


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Dyanamic & Extensible

Java is capable of dynamically linking in new class

libraries, methods, and objects.

Java programs support functions written in other

languages such as C and C++. these function are

known as native methods

Native mthods are linked dynamically at runtime.


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Hello Java!26

A simple Java Program

The virtual machine will start the main method of this class if called via javaHelloWorld

The filename must be equal to the class name. The extension must be.java.

class HelloWorld {public static void main (String[] args) {

System.out.println(Hello Java!);

}

}

BA Mosbach / Referent Lars Vogel WS 08/09


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Example java program// My first Java Program

import java.util.*;

public class HelloWorld {

public static void main(String[] args) {System.out.println("Hello, Java ");

}}

source file:HelloWorld.java

All code must be inside a named class & thefilename must match the classname exactly(case-sensitive)


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Deconstructing a Java program

// My First Java Program

There are 3 kinds ofcomments in Java

1. //

2. /* */

1. /** */

Implementation comments: tells aprogrammer reading your codeabout your implementation

Documentation comments: used byjavadoc to generate info for users ofyour classes


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Deconstructing a java program// My First Java Program

import java.lang.*;


public static void main(String[] args) {

System.out.println("Hello, Java ");

}

}

Similar to #include inC/C++;

but only loads a classdynamically, if needed atruntime

Never import the java.lang API

it is always automatically imported


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Deconstructing a java program


Defines a new class. The source file must be theexact same name followed by extension .java

A file can contain more than one class definition, butonly one can be public.


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Deconstructing a Java program

public static void main(String[] args) {

public the main method is publicly visible

The main program must have this exact header:

static allows the main program to be called withoutcreating an object

void no value is returned

String[] array of Strings allows info to be given to themain program ( String args[] is also OK )


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Deconstructing a java program

System.out.println("Hello, it's: ");

Prints a string to the standard output stream

System is a java class

out is an instance of an OutputStream;

defaults to the console


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Compiling & Running a Java Program

Step 1: compile into bytecodes

javacHelloWorld.java

Step 2: run

javaHelloWorld


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Step 1: compile into bytecodes

javacHelloWorld.java

java compiler createsHelloWorld.class file

(machine independent

bytecodes)

must include the.java extension


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Step 2: run

javaHelloWorld

loads the classes from HelloDate.class, fromjava.lang, and java.util.Date

verifies the bytecodes

runs the main program


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The Java Virtual Machine

Java programs are notcompiled to machine code in thesame way as conventional programming language.

To support safe execution of compiled code on multipleplatforms (portability, security), they are compiled toinstructions for an abstract machine called theJavaVirtualMachine (JVM).

The JVM is a specification originally published by SunMicrosystems.

JVM instructions are called Java byte codes. They are storedin a class file.

The JVM is a program that runs on a real computer. So


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JIT

In one form of Just-In-Time compilation, methods

may be compiled to machine code immediately

before they are executed for the first time. Then

subsequent calls to the method just involve jumping

into the machine code.

The JIT compiler reads the bytecodes in many

sections (or in full rarely) and compiles theminteractively into machine language so the program

can run faster. Java performs runtime checks on

various sections of the code and this is the reason

the entire code is not compiled at once.[


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Java Tokens

Keywords

Identifiers

Literals

Operator

Separators


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Keywords

abstract boolean break byte case catch

char class const * continue default do

double else extends final finally floatfor goto * if implements import instanceof

int interface long native new null

package private protected public return short

static super switch synchronized this throw

throws transient try void volatile while


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Data types

byte: The byte data type is an 8-bit signed two's

complement integer. It has a minimum value of -

128 and a maximum value of 127 (inclusive). The

byte data type can be useful for saving memory inlarge arrays, where the memory savings actually

matters. They can also be used in place of int where

their limits help to clarify your code; the fact that a

variable's range is limited can serve as a form of

documentation.

short: The short data type is a 16-bit signed two's


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float: The float data type is a single-precision 32-bit

IEEE 754 floating point. Its range of values is

beyond the scope of this discussion, but is specified

in section 4.2.3 of the Java Language Specification.As with the recommendations for byte and short,

use a float (instead of double) if you need to save

memory in large arrays of floating point numbers.

This data type should never be used for precise

values, such as currency. For that, you will need to

use the java.math.BigDecimal class instead.

Numbers and Strings covers BigDecimal and other

useful classes rovided b the Java latform

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Unit I Java Pro