==========
Chaper-1
==========

1) What is Java
2) Java Features
3) Java Environment Setup
4) JDK vs JRE vs JVM
5) Java Programs Execution Flow
6) Java Programs Development (Compilation & Execution)
7) Variables
8) Data Types
9) Identifiers
10) Reserved Words (53)
11) Java Coding Standards (Naming Conventions)
12) Java Comments




===========
What is Java 
===========

-> Java is a  programming language

-> Java language developed by Sun Microsystem in 1991  (OAK)

-> James Gosling is the lead for the team who developed Java Language

-> The first version of java came into market in 1995 

Note: Oracle Corporation acquired Sun Microsystem

-> Now java is under license of Oracle corporation

-> Java is a free software & open source


=======================
Java is divided into 3 parts
=======================

1) J2SE 
2) J2EE 
3) J2ME 


J2SE / JSE  ---> JAVA STANDARD EDITION

		-> STAND-ALONE APPS
		-> RUNS ONLY IN ONE MACHINE

		EX: CALC, GAMES, NOTEPAD ETC......


J2EE / JEE ---> JAVA ENTERPRISE EDITION


		-> web applications

		-> Everybody can access web applications using internet

		ex: gmail, youtube, facebook, naukri, irctc etc.........


J2ME / JME ---> JAVA MICRO / MOBILE EDITION

		 -> Mobile apps

		Ex: whatsapp, messgender, phonepay, gpay etc.....

=======================
What we can do using Java
=======================

1) Stand-alone applications

2) Web applications

3) Mobile Applications


==============
Java Features
==============

1) Simple : The complex topics of C & C++ are eliminated in Java

	Ex: Operators overloading, pointers, memory mgmt etc...

2) Platform Independent

	-> Java programs can be executed on any machine
	-> JVM made java as platform independent
	-> JVM stands for Java Virtual Machine
	-> JVM is responsible to run/execute java programs

3) Robust (Strong)

	-> Automatic Memory Management
	-> Exception Handling

4) OOPS (Object Oriented Programming System)

	-> Everything will be represented in objects format
	-> Code Re-Usability

5) Secure

6) Distributed 

7) Portable

8) Dynamic

Java Slogan : WORA (Write Once Run Anywhere)

==================
Environment Setup
==================

1) Download and Install Java Software

		- JDK (Java Development Kit)
		- JRE (Java Runtime Environment)

Q) What is the difference between JDK, JRE & JVM ?

		- JDK contains set of tools to develop java programs
		- JRE providing a platform to run our java programs
		- JVM will take care of program execution

2) Set Path for Java

		Path = C:\Program Files\Java\jdk1.8.0_202\bin
	
		-> Go To Environment Variables
		-> Go To System Environment Variables
		-> Edit Path
		-> Add JDK BIN path


=========================
Java Programs Development
========================

-> We can write java programs in any text editor

			- Note Pad
			- Note Pad++
			- Edit Plus

-> In companies we will use IDE to develop java programs/projects

			- Integrated Development Environment

					- Eclipse
				        - MyEclipse
				        - Netbeans
					- STS (Spring Tool Suite)
					- IntelliJ

========================
Java Program Structure
=========================

package statements

import statements

class declaration

variables

methods



-----------hello.java---------
class hello {

 public static void main(String... args) {
     System.out.println("Welcome To Ashok IT...!!");

    System.out.println("Welcome to Java");
 }

}
----------------------------

javac hello.java

java hello

---------------------------
class demo {
 
   public static void main (String... args){
		System.out.println("Hello World");
	      System.out.println("Welcome to Java");
   }
}

-----------------------------------

-> javac measns java compiler which is used to compile java programs

-> java compiler is called as translator


===============
Translators
===============

-> It is used to convert from one format to antoher format

-> 3 types of translators available

		1) Interpreter
	
		2) Compiler

		3) Assembler


->  Interpreter will convert the program line by line  ( performance is slow )

-> Compiler will convert all the lines of program at a time ( performance is fast )

-> Assembler is used to convert assembler programming languages into machine language


========
JVM
========

-> JVM stands for Java Virtual Machine (We can't see with our eyes)

-> JVM will be part of JRE

-> JVM is responsible for executing java programs

-> JVM will allocate memory required for program execution & de-allocate memory when it is not used

-> JVM will convert byte code into machine understandable format


================
JVM Architecture
===============

1) Classloader subsystem : It will load .class file into JVM

2) Method Area   : Class code will be stored here

3) Heap area : Objects will be stored into heap area

4) Java Stack : Method execution information will be stored here

5) PC Register : It will maintain next line information to execute

6) Native Stack : It will maintain non-java code execution information

7) Execution Engine   ( Interpreter + JIT ) : It is responsible to execute the program and provide output/result

8) Native Interface : It will load native libraries into jvm

9) Native Libraries : Non-java libraries which are required for native code execution


=======
variables
========

-> variables are used to store the data

			name   -  ashok
       
                        age - 30

                        gender - m

                        isStudent - false

			mysalary - 400.56


-> We need to specify type of the variable to store the data

-> To specify type of data we will use 'data types'


=========
data types
=========

-> Data types are used to specify type of the data

-> Data types are divided into 2 categories

1) Primitive / Pre-Defined Data Types 

		1) Integral
				- byte
				- short
				- int
				- long

		2) Decimal
				- float
				- double

		3) Character
				- char

		4) Boolean
			       - boolean

2)  Non-Primitive / Referenced Data Types

			- Arrays
			- Strings
			- Classes


================
Integral data types
=================

--> Integral data types are used to store numbers without decimal points

-->  We can store both positive and negative numbers using integral data types


Ex:

age = 30

phno = 66868686868

studentscnt = 40

balance = - 3000

-> We have 4 data types in this category

-> For These 4 data types memory & range is different

		1)  byte   ----> default value is 0  ----> 1 byte

		2)  short	----> default value is 0  ----> 2 bytes
	
		3)  int    ----> default value is 0  ----> 4 bytes

		4)  long   ----> default value is 0l   ----> 8 bytes


=================
Decimal data types
==================

-> Decimal data types are used to store numbers with decimal values

-> We can store both postive and negative values


Ex: 

petrol price  = 110.567979
stockPrice = 334.3279797979797979
percentage = 9.8
weight = 55.6
height = 5.6
length = 10.2

-> In this category we have 2 data types

			1) float   ----> 4 bytes ---> upto 6 decimal points

			2) double  -----> 8 bytes  --> upto 15 decimal points


================
character data type
=================

-> Character data type is used to store single character

-> Any single character ( alphbet / digit / special character ) we can store using 'char' data type

-> char datatype will occupy 2 bytes of memory

-> When we are storing data into 'char' data type  single quote is mandatory

-> default value is 'u0002'

gender = 'm'

rank = '1'

Note: In C language 'char' will take only 1 byte where as in java 'char' will take 2 bytes

==============
boolean data type
==============

-> It is used to store true or false values only

-> It will occupy 1 bit memory

Note:  8 bits = 1 byte

-> default value for boolean is false

Ex:

isPass;
isFail
isMarried
isOdd
isEven


===========
Variables
===========
->  Variables are used to store the data / value

-> To store the data into variable we need to specify data type

-> To store data into variables we need to perform 2 steps

			1)  Variable Declaration  (defining variable with data type)

				Ex:     byte age ;

			2) Variable Intialization (storing value into variable)

				Ex:   age = abc;


-> We can complete declaration and intialization in single line

					byte age = 20;


=============================== Variables Program ==================================

class var {

      public static void main (String... args) {

			int age = 20;
			System.out.println(age);

			float a = 25.01f;
			System.out.println(a);

			double price = 120.87;
			System.out.println(price);
	
			char gender = 'm';
			System.out.println(gender);
		
			boolean pass = true;
			System.out.println(pass);
					
      }
}



1) Identifiers

2) Keywords

3) Java Naming Conventions


===========
Identifiers
==========

-> All java components requires a name

-> For variables, for classes and for methods we need a name

			int age ;

			class Hello {
				// code 
			}

			main ( ) {
				//logic
			}


-> The name which we are using for packages, variables, classes  & methods is  called as identifier

-> We can use any name for identifiers but we need to follow below rules to work with identifiers

Rule-1 : Java will allow only below charaters for identifiers

				1) a - z
				2) A - Z
				3) 0 to 9
				4) $ (dollar)
				5) _ (underscore)

		Ex:

				name   -----> valid
				name@ -----> invalid
				age#   ------> invalid

Rule-2  : Identifier should not start with digit (first character shouldn't be digit)

				1age --------> invalid
				age2  ------> valid				
				name3  -----> valid
				_name -----> valid
				$name ------> valid
				@name ------> invalid
				$_amt   --------> valid
				_1bill -----------> valid

Rule-3 :  Java reserved words shouldn't be used as identifier   (53 reserved words)

					int byte = 20;    -------> invalid bcz byte is a reserved word

					byte for = 25;   -------> invalid bcz for is a reserved word

					int try = 30; 	------> invalid bcz try is a reserved word

					long phno = 797979799 -----> valid
			

Rule-4 : Spaces are not allowed in identifiers

				int mobile bill = 400;   // invalid

				int mobile_bill = 400 ;   // valid


Rule-5 : Java is case sensitive language 'name' & 'NAME' both are not same


==============================================
Java Naming Conventions  ( Java Coding Standards )
===============================================

-> Java language followed some standards/conventions for pre-defined packages, classes and methods....

-> Java language suggested java programmers also to follow same standards / conventions

-> Following these standards/conventions is not mandatory but highly recommended.

===============================
Naming Convention For Class Name
===============================

-> A class name can contain any no.of words without spaces
-> Recommended to write every word first character as uppercase in class name

Examples:

class Hello  {
}

class HelloWorld {
}

class UserManagementService{
}

class WelcomeRestController {
}

Note: Class Names & Interface Names conventions are same.

=========================
Variables Naming Convention
=========================

-> Variable name can have any no.of words without spaces

-> Recommended to start variable name with lowercase letter

-> If variable name contains multiple words then recommended to write firstword all characters in lowercase and from second word onwards every word first character in Uppercase

Examples:

int age ;

int  userAge;

long creditCardNumber ;

=========================
Method Naming Convention
=========================

> Method name can have any no.of words without spaces

-> Recommended to start method name with lowercase letter

-> If method name contains multiple words then recommended to write firstword all characters in lowercase and from second word onwards every word first character in Uppercase

main ( )  {
}

save ( ) {

}

saveUser( ) {

}

getWelcomeMsg ( ) {

}


Note: Variables & Methods naming conventions are same. But methods will have parenthesis ( () ) variables will not have parenthesis.


==============================
Naming Conventions for Constants
=============================

-> Constant means fixed value (value will not change, it is fixed)

-> Recommended to write constant variable all characters in uppercase

-> If constant variable contains multiple words recommended to use _ (underscore) with all uppercase characters


		final int MIN_AGE = 21;

		final int MAX_AGE = 60 ;

		int PI = 3.14;


===============================
Naming Conventions for Packages
==============================

-> Package name can have any no.of characters & any of words

-> Recommended to use only lowercase letters in package names

-> If package name conatins multiple words then we will use . (dot) to seperate words

Eamples:

java.lang
java.io
java.util
in.ashokit
com.oracle
com.ibm