Introduction to Java

Overview

• History of Java

• Java Releases - Evolution

• Java Features

• Java White Paper Buzzwords

• Java & Internet

Java Features• Java is a slang term for coffee.• Java – city in Iowa in Indonesia.• SUN - Sun Microsystems, Inc. is a multinational

vendor of computers, computer components, computer software, and information technology services, founded on 24 February 1982.

• The company is headquartered in Santa Clara, California (part of Silicon Valley).

Java Features

• Java is related to C++.• Java inherited from both C and C++; Syntax

form C and Object-oriented features from C++.• Every p/l fulfilled some needs in programming.• C – need for structured, efficient, and high-

level language – to replace assembly code.

Java Features

• C++ - in structured programming – once a project reaches a certain size, its complexity exceeds what a programmer can manage.– New way of programming – OOP

• Java – WWW and Internet programming – taking features from different programming languages.

Java Features

• Java is an – object-oriented, – multi-threaded programming, – concurrent, – platform independent,– distributed programming language.

• Many of Java’s characteristics comes from other languages.

Java White Paper Buzzwords• http://java.sun.com/docs/white/

langenv/Intro.doc.html• Simple– syntax for Java cleaned up version of C++.

• Object Oriented– modern technique for programming

Java White Paper Buzzwords• Distributed– Networking capabilities of Java– Internet Programming using Java– Remote Method Invocation (RMI) using Java– Java EE – very large scale distributed applications

• Robust– Reliable application– Check for possible errors– No pointers !!!

• Secure– Virus-free

****Java White Paper Buzzwords• Architecture Neutral

– The compiler generates an architecture-neutral object file format – this can be executed on any processor that has a JVM

– Java compiler generates byte code instructions which have nothing to do with particular computer architecture – these are interpreted on any machine and easily translated into native machine code on the fly.

• Portable– No “implementation-dependent” aspects of the specification.

• Interpreted– Java can execute Java Bytecodes directly on any machine

Portability by Java

JAVA processing

Java White Paper Buzzwords• High Performance– The bytecode can be translated on the fly

• Multithreaded– multithreading are better interactive

responsiveness and real-time behavior• Dynamic– Code is added to a run time program

Java & Internet

• Users will download bytecodes form the Internet

and run them on their m/cs. Such Java program

that work on web pages are called applets.

• Client-side programming – input validation.

• Server-side programming – application servers –

scalable server applications

History of Java

• 1991 SUN (Standford University Network) engineers – Jame Gosling & Team wanted to design a small computer language for consumer electronic devices.

• These devices – manufactured by different vendors – different CPUs.

• So needed a language that is not tight to any architecture – independent.

• The project was named – “Green”.

• Main requirement – – small,

– tight, and

– p/f independent code

• They took the idea of virtual machine(VM) from PASCAL.

• A portable language that generated intermediate code for a hypothetical machine (VM) – Hence JVM

• Language syntax from C++ - Object-oriented language - Named – “Oak” – later Java.

History of Java - 2

• Initially, it was unsuccessful – as no companies were interested.

• Later, in mid 1994 – WWW part of Internet was evolving.

• The key to the Web is program (browser) that translates the hypertext page to the screen.

• SUN took this idea and built a browser – HotJava browser – executes code in the web page.

• Success of Java – 1st version was released – 1996.

History of Java - 3

Java Releases

• JDK 1.0 (January 23, 1996)• JDK 1.1 (February 19, 1997)• J2SE 1.2 (December 8, 1998) – Released with editions.

• J2SE 1.3 (May 8, 2000)• J2SE 1.4 (February 6, 2002)• J2SE 5.0 (September 30, 2004) (Java 1.5)• Java SE 6 (December 11, 2006)– 7.1 Java SE 6 Update 10

• Java SE 7

Java Programming Envt

Aum Amriteshwaryai Namah

Java Programs

• Java supports various building of various

types of programs:-

– Console Prgs

– Graphical Apps

– Applets

–Web Applications

Requirements

• To execute Java programs, the following are the requirements:- – JDK – Java Development Kit or – an IDE that sophisticates Java Development.

• To run the JDK tools, use the shell window

Installing JDK

• Download the JDK

– Visit the SUN’s site – http://java.sun.com/

– Choose the platform for which you want the JDK

– Only the installation and compilation instructions

for Java are system dependent.

– Path preferable is:- C:\jdk1.6.0_01 (Windows)

• Setting the Execution Path

– Under windows

• Control Panel>System>Advanced Tab>Environment

Variables

• Edit the Path property by adding “C:\jdk1.6.0_01\bin”.

Installing JDK

Installing JDK• Choosing a Development Environment– Basic JDK contains tools that need to be typed in

the shell window.– There are 3 choices for a Java development

environment:-• Use JDK and any text editor; Compile and launch

programs in a shell window• Use any integrated evnt such as Eclipse, NetBeans.• Use JDK and a text editor that is integrated with the

JDK eg Emacs, TextPad or JEdit. Compile and launch the programs inside the editor.

Installing JDK

• Using the Command Line Tools:-

– >javac Welcome.java

– >java Welcome

– The javac program is the Java compiler. It compiles the

file Welcome.java into the file Welcome.class. The Java

program launches the Java virtual machine. It executes

the bytecodes that the compiler placed in the class file.

FirstProgram.java/* My first Java program Author : Bri. Jayashree File Name : FirstProgram.java */public class FirstProgram{

public static void main(String args[]){

System.out.println("Aum Namah Shivaya!!!");}

}

A Closer look into the program

Java is case sensitive Keyword public is called the access

modifier; these modifiers control the level of access.

• Keyword class is used to define a container that holds code in it

• Everything in a Java program must lie inside a class.

• The name of the class is an identifier – keywords cannot be used as class names - start with uppercase

• The file name should be the name of the public class with the extension .java

• Launch the javac compiler to compile the program:-– > javac FirstProgram.java– Notice the FirstProgram.class file created by the

compiler• Launch the java interpreter to execute the

program– > java FirstProgram

A Closer look into the program

Java Execution

• When java ClassName is used to run the

program, the JVM always starts execution with

the code in the main method.

• { } – define the parts in a program.

• static

• void – return type of main()

General Program Structure

public class ClassName

{

public static void main(String args[])

{

Program statements

}

}

Program Statements• Java statements can be considered as sentences of the

language.

• Every statement must end with a semicolon.

System.out.println("Aum Namah Shivaya!!!");

• We are using the System.out object and calling its println

method.

• Object.method(parameters) -- Syntax for calling a method.

Comments

Comments – do not show up in the executable program

Java has three ways of marking comments:-1) //2) /* */3) /** */ to generate documentation

automatically.

Running the program

• Launch the shell program

• Go to the path where the file is.

• javac FirstProgram.java

• java FirstProgram

Data Types

Java is a strongly typed language – every variable must have a declared type.

There are 8 primitive types in Java:-1)4 of them integer types2)2 floating-point types3)1 as character type4)1 as Boolean type

Integer types• These are for numbers without fractional parts. • Negative values are allowed

Floating-point types

• Numbers with fractional parts.

– Float – 4 bytes

– Double – 8 bytes

char type• Java uses the 16-bit Unicode characters

boolean type

The Boolean type has two values – true and false.

No type conversion b/w integers and Boolean values.

Variables• In Java, every variable has a type.

• Syntax:- type variablename;

• A variable name must begin with a letter and must be sequence of letters or digits – reserved words cannot be used.

• All characters in the name of a variable are significant and case is also significant.

Variables• The length of a variable name is unlimited.

• Uninitialized variables cannot be used.

• Variable declaration can be kept anywhere in

the code.

Constants

final keyword is used to denote a constant.

The constant variables are assigned once

and its value cannot be changed.

Its customary to name constants in all

uppercase.

E.g. final int YEAR = 2009;

Operators• Arithmetic operators:- +,- ,*,/,%,++,--.• Relational operators - &&,||,==,!==• Conditional ternary operator:- ?:• Bitwise operators:- &,|,^,~.

Mathematical Functions and constants

Like math.h in C, in Java Math class an assortment of mathematical functions. e.g. sqrt().

pow(),sin()…Usage:- Math.function();

Conversion b/w numeric types

• Explicit type conversion is done by means of cast.

• E.g double x=9.99; int y=(int) x;

Operator Precedence

• Self Study

Control Flow

• Like other programming langs, in Java, conditional

statements and the looping statements determine

control flow.

• A block or compound statement is any Java

statements that are surrounded by a pair of braces.

• Blocks define the scope of the variables.

Conditional Statements

Syntax of if statement:-if (condition) statement else statemente.g

if (marks>50) status = ‘P’;

else status = ‘F’;

Looping Statements

• while loop executes a statement while a condition is true.

• while loop tests at the top – entry control loop• Syntax

while(condition) statementint i=10;while(i>1){

System.out.println(i +") Amma ");i--;

}

Looping Statements• do..while loop executes the statement and

then only checks for the condition• do..while is an exit control loop.• Syntax:-

do statement while(condition);

• determinate loops:– the for loop is a general construct to support

iteration that is controlled by a counter that is updated after every iteration.for(counter initialization; condition; counter updating)

Multiple Selections

• Java has same switch stmt as C,C++.switch(choice)

{case 1:break;

case 2:break;default:break;

…}

• Case labels must be integers or enumerated constants, strings cannot be used.

Control Statements

Statements that break control flow:-o break o continue :- breaks the regular flow of

control – it transfers control to the header of the innermost enclosing loop.

o goto (no recommended to use)o labeled break – when multiple loops are

used

Arrays

• An array is a data structure that stores a

collection of values of the same type.

• Individual value is accessed through an

integer index

• E.g. if a is an array of integers, then a[i] is the

ith integer in the array.

Arrays

• Array declaration syntax:-

– datatype [] name or datatype name[]

– e.g. int [] marks;

– or int marks[];

• The above statement has only declared a variable.

• It has not been initialized/ created.

Arrays

• For this we have to use the new operator to

create the array.

• int [] a= new int[100];

• Index starts with 0; here a indexes from 0-99.

Arrays

• To find the number of elements of an array user

array.length.

• Once an array is created, its size cannot be changed.

• “for each” loop

– To traverse an entire collection the following can be used

– for(variable : collection) statement

Arrays

• for (int i : marks){

sum+=i;}

• Array initializationint[] a = {1,2,3,4,5};

• Array Sorting– Arrays.sort(arrayname);

Multidimensional array

• Datatype[][] name; //declaration

• Creation two ways:-– name= new type[5][5];

– or name=new type[5][]; //then each arrays can be created individually.

– E.g

int [][] t2D=new int[3][3];

or

int [][]t2D;

t2D=new int[3][];

for(int i=0;i<3;i++)

t2D[i]=new int[3];

• Multidimensional arrays are actually arrays of arrays.

Strings in Java

• Java strings are sequences of Unicode characters.• No built-in string type.• The std Java library contains a predefined class

called – String• Each quoted string is an instance of String class Java strings are implemented as

sequences of char values. String greeting = “Hello”;

– int n= greeting.length();

Strings

• Individual characters cannot be changed – instances of string are immutable.

• To concatenate two strings, use + operator.– String greeting =”Hello ”;– String greeting2 = “All!”;– String greeting 3 = greeting + greeting2 + “Welcome to

Java Learning” + 123;– When value to be concatenated is not a string, the latter

will be converted to a string automatically.

• More on Strings? Online Documentation

String• To compare two strings, use equals() method.– boolean status = greeting.equals(greeting2);– returns true if the strings are equal and false otherwise.– == operator cannot be used for comparison, as it checks

or not the strings are stored in the same location.– Other methods:-

• equalsIgnoreCase() – same as equals() but ignores the case of the letters

• compareTo() – works exactly same as strcmp() of C.• toLowerCase() – converts the string to lowercase.• toUpperCase() – converts the string to uppercase .

Command Line Arguments• Every Java program has a main method with

String[] args as parameter.• This parameter indicates that the main method

receives an array of strings, the arguments specified on the command line.

public class CommandLineTest{

public static void main(String [] args){

int s=args.length;System.out.println("Number of arguments:"

+ s);for(String a:args){

System.out.println(a);}

}}

Input and Output• First construct a Scanner (added in 5.0

version) that is attached to the std input stream System.in.Scanner in = new Scanner(System.in);

• You need to import the class using the import statement.

• Scanner is in java.util package• To import a class from package useimport package.Classname

Input and Outputimport java.util.Scanner;

public class InputTest

{

public static void main(String arg[])

{

Scanner in = new Scanner(System.in);

  //String input

System.out.print("Name? ");

String name=in.nextLine();

System.out.println("Hello " + name + "!!");

//output to console

  //Interger Input

System.out.print("Age? ");

int age=in.nextInt();

System.out.println("Hello " + name + "!!" + "\n You are "+ age + " years old!!");

}

}

Input and Output

• in.nextLine() will read line of input

• in.nextInt() will read and convert the next character sequence into integer.

• in.next() will read next word

• Displaying to the console System.out.println()

Input and Output

• Formatting output– System.out.printf() – is used formatting output – similar

to C’s printf().

– E.g. System.out.printf(“%8.2f”, x); -- 8 characters in float with 2 precision.

– There are many format specifiers that start with % and is replaced with the corresponding argument.

– Conversion character that ends a format specifier indicates the type of the value to be formatted.