What are Keywords, Datatypes, Variables, Literals in java?

Before start Java Programming, we have knowledge about these important words :

Keywords: In java Programming , A set of reserve word that is define for special purpose.  These words can not be identifiers (e.g. Variable name, function name, class name,object name etc.).

In Java , There are 50 keywords.

abstract	continue	for	new	switch
assert	default	goto	package	synchronized
boolean	do	if	private	this
break	double	implements	protected	throw
byte	else	import	public	throws
case	enum	instanceof	return	transient
catch	extends	int	short	try
char	final	interface	static	void
class	finally	long	strictfp	volatile
const	float	native	super	while

For example, if you try and create a new class and name it using a reserved word:
// you can't use try as it's a reserved word!
class try{
public static void main(String[] args) {
//class code..
}
}

In above example I am using finally as class name. It will not compile, instead you will get the following error:
<identifier> expected

Datatypes - We can define data type in a programming language is a set of data with values having predefined characteristics.
Data-types in any language specify the type of data to be stores and the size of memory to be allocated.
In Java, There are two types of data types.
1. primitive datatypes
2. Non-primitive datatypes

 

 Primitive datatypes- Primitive data types are predefined types of data, which are supported by the programming language. For example, integer, character, and string are all primitive data types.

Non-primitive datatypes - Non-primitive data types are not defined by the programming language, but that is  created by the programmer.It is  sometimes called "reference variables," or "object references.

Lets see memory allocated  and  default value of data type.

Data Type	Memory Size	Default value	Declaration
byte	8 bits	0	byte a=9;
short	16 bits	0	short b=99;
int	32 bits	0	int c=999;
long	64 bits	0	long=999999;
float	32 bits	0.0f	float b=37.8f;
double	64 bits	0.0	double c =27.098
char	16 bits	'\u0000'	char a ='e';
boolean	JVM Dependent	false	boolean a =true;

The true & false are keywords in java which form as value for Boolean data variable.
ASCII (American Standard Code for Information Interchange) System- Supports 256 characters.
Unicode (universal international standard character encoding) System- Supports 65535 characters
C/C++ supports ASCII system and Java Supports Unicode system. So we can say java supports multiple language's characters.

Variable - Variables are temporary memory locations used to store a value. The value stored in a variable can change or vary.

Naming conversion of Variable:-

• It should start with an alphabet, underscore ( _ ) OR dollar ($).
• It can contain alphabets, digits, underscore OR dollar.
• No Spaces, NO Special Characters except underscore and dollar.
• we can not use keyword as Variable.
•  It should be unique within its scope

 Syntax for Initialization Variable:

<variableName> = <value>;

 Example:
num = 10;

Initialization with Declaration Syntax:
   <data-type> <variableName> = <value>;
    Example:
     int num = 10;

There are Three types of Variable:

. local variable: A variable that is declared inside the method or block or loop is called local variable.

for example:

public void sum()

{

int a=10;

}

local variables are only visible to the method or block in which they are declared.

Instance Variable:
A variable that is declared inside the class but outside the method is called instance variable . It is not declared as static.Instance variable are also non-static variable because their values are unique to each instance of class.

for example:

 class A{
int a=6;
}

Static variable:
An instance variable is any field declared with static modifier, it tells the compiler that there is exactly only one copy of this variable. It cannot be local.
for example 
static  int num = 5;


Here example of all variable in a class

class MyClass{

int i=50; //instance variable

static int s=100; //static variable

void method(){
int loc=90; //local variable
}

}

Literals :

In Java Literals are used to specify the type of value. Literals in java are a sequence of characters (digits,letters and other characters) that represent constant values to be stored in variables.

 Types of literal

Data-types                            Literals
long                                          l
float                                          f
double                                     d
char                                         ‘ ’
String                                       “ ”

Examples

int i=10                                        //by default int value
long l = 10l                                  //long value with l literal
double d =10.5                           //by default double value
double d =10.5d                        //double value with d literal
float f =10.5f                             //float value with f literal
char c =‘A’                                //char value with ‘ ’ literal
String str = “apple”                   //String value with “ ” literal

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