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Java Introduction
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Computers and Computer
Languages
• Computers are everywhere
– how many computers do you own?
• Computers are useful because they run various programs
– program is simply a set of instructions to complete some task
– how many different programs do you use in a day?
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Definitions
• Program: A set of instructions that are to be carried
out by a computer.
• Program execution: The act of carrying out the
instructions contained in a program.
– this is done by feeding the instructions to the CPU
• Programming language: A systematic set of rules
used to describe computations, generally in a format
that is editable by humans.
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Java – An Introduction
• Programming language developed by Sun Microsystems in 1991.
• Originally called Oak by James Gosling
• Originally created for consumer electronics ( TV, VCR, Mobile Phone, etc.)
• Internet and Web was just emerging, so Sun turned it into a language of Internet
Programming.
• Pure Object oriented language
• Java is a whole platform, with a huge library, containing lots of reusable code, and
an execution environment that provides services such as

security

portability across operating systems

automatic garbage collection.
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Need for Java
• Many different types of controllers with different set of CPU are used in electronic
devices.
• The problem with C and C++ is that designed to be compiled for a specific target.
• An attempt to solve these problems, Gosling and others began work on a portable,
platform-independent language that on a variety of CPUs under differing
environments.
• Second force was the introduction of World wide web demand a language that
could useful in creating portable application.
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Java “White Paper” Buzzwords
• Simple
• Object Oriented - technique for programming that focuses on the data (= objects)
and on the interfaces to that object.
• Portable & Architecture Neutral - By generating bytecode instructions, make it
executable on many processors, given the presence of the Java runtime system.
• Interpreted - Java interpreter can execute Java bytecodes directly on any machine.
• Network-Savvy - Java has an extensive library of routines for coping with TCP/IP
protocols like HTTP and FTP.
• High Performance – JIT Compiler monitor the code and optimize the code for speed.
• Robust - Emphasis on early checking for possible problems, later dynamic (runtime)
checking, and eliminating situations that are error-prone.
• Multithreaded
• Secure - intended to be used in networked/distributed environments.
• Dynamic
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Architectural Neutral & Portable
• Porting the java system to any new platform involves writing an java
interpreter
• The interpreter will convert the byte code to machine level
instructions
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Object oriented Programming
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Why OOP?
• Greater Reliability
o Break complex Software projects into small, self-contained, and modular
objects.
• Maintainability
o Modular objects make locating bugs easier, with less impact on the overall
project.
• Greater Productivity
o By reusing the components we increase productivity
• Faster Design and Modelling
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OOP Constructs'
• Objects – Real time entity
• Classes – Blue print of an object. It gives structure to the objects
• Inheritance – Deriving base class properties and behavior to child
class
• Polymorphism – One object exists in different forms.
• Encapsulation – Hiding the irrelevant details to irrelevant entity
• Abstraction – Revealing the relevant details
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Java Milestones
Year
Milestones
1990
Sun decided to develop a software that could be used for consumer
electronics. 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
Demonstrate a new language to control a list of home appliances
1994
Team developed a new Web browser called “Hot Java” to locate and run
applets.
1995
Oak was renamed to Java. Companies such as Netscape, Microsoft
announced their support for Java
1996
Java became the language for Internet Programming and General
purpose OO language.
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Java Applications
Java is used to develop two types of application program:
•
Stand-alone applications
•
Web applications (applets)
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Java Environment
JDK
- Java Development Kit ( Program enable users to create java applications)
JRE
IDE
- Java Runtime Environment ( Software to run java programs)
- Eclipse, Jcreator, NetBeans
Setting Path
MyComputer -> Environment Variables ->Advanced -> Path -> C:\Jdk1.7\bin
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Structure of Java Program
Package declaration;
Import statements
Class Declaration
{
//comments
Variable declaration / definition
Methods declaration and definition
Main methods - // entry point of java program
}
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Sample Helloworld application
/* Simple Helloworld Java application */
class test
{
public static void main(String[] args)
{
System.out.println(“Helloworld Java”);
}
}
// save it as test.java
Compile : javac test.java
Execute : java test
Helloworld Java
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Process of Building and Running Java Applications
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A Picture is Worth…
The output of the
compiler is .class
file
The Interpreter's are sometimes referred to as the Java Virtual
Machines
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Sample Java Execution
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More Definitions
• Code or source code: The sequence of
instructions in a particular program.
– The code in this program instructs the computer to print
a message of Hello, world! on the screen.
• Output: The messages printed to the computer
user by a program.
• Console: The text box or window onto which
output is printed.
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Java is Compiled and Interpreted
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How different from compiled languages?
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Compiling and Running
• Compiler: a program that converts a program in one
language to another language
– compile from C++ to machine code
– compile Java to bytecode
• Bytecode: a language for an imaginary cpu
• Interpreter: A converts one instruction or line of
code from one language to another and then executes
that instruction
– When java programs are run the bytecode produced by the
compiler is fed to an interpreter that converts it to machine
code for a particular CPU
– on my machine it converts it to instructions for a Pentium
cpu
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The command line
To run a Java program using your
Command Prompt:
• change to the directory of your
program
cd
• compile the program
javac Hello.java
• execute the program
java Hello
source code
(Hello.java)
compile
byte code
(Hello.class)
execute
output
CS305j Introduction to
Computing
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Platform Independency
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JVM
• Provides platform independent way of executing code, by abstracting
differences between operating systems and CPU.
• JVM is Write Once-Run Anywhere(WORA) software
• JVM forms the larger part of JRE
• JVM role is to interpret the java byte code and translate the actions to
OS calls.
• JVM is OS dependent which makes java source code to machine
independent form.
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JVM
• JVM is the interpreter used to convert the byte code to machine code on the fly and
execute it.
• Byte code is optimized instruction set which independent of machine.
JVM Architecture
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JVM
Advantages
• Enable java program run in protected environment
• Write once, run anywhere (one size fits all)
• Browser can cache the downloaded code and reuse it later
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Input Processing
import java.util
public class InputDemo
{
Scanner in;
public static void main(String[] args)
{
in=new Scanner(System.in);
System.out.println(“Enter a value”);
int i=in.nextInt();
System.out.println(“User has Entered :” + i );
}
}
nextLine, next, nextDouble, hasNext, hasNextInt, hasNextDouble
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Reading Input
Using Console class – Java.io
import java.util
public class InputDemo
{
public static void main(String[] args)
{
Console con= System.Console();
String name=con.readLine();
String pass=con.readPassword();
System.out.println(“User Name : “+name +” and Password :” + pass);
}
}
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Another Java program
public class Hello2 {
public static void main(String[] args) {
System.out.println("Hello, world!");
System.out.println();
System.out.println("This program produces");
System.out.println("four lines of output");
}
}
• The code in this program instructs the computer to print four
messages on the screen.
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Java terminology
• class:
(a) A module that can contain executable code.
(b) A description of a type of objects. (seen later)
• statement: An executable piece of code that represents a complete
command to the computer.
– every basic Java statement ends with a semicolon ;
• method: A named sequence of statements that can be executed
together to perform a particular action or computation.
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Syntax and syntax errors
• syntax: The set of legal structures and commands that
can be used in a particular programming language.
• syntax error or compiler error: A problem in the
structure of a program that causes the compiler to fail.
– If you type your Java program incorrectly, you may violate
Java's syntax and see a syntax error.
public class Hello {
pooblic static void main(String[] args) {
System.owt.println("Hello, world!")_
}
}
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