Variable is name of reserved area allocated in memory. In other words, it is a name of memory location. Variables are used to store information to be referenced and manipulated in a computer program. value of the variable can change, depending on conditions or on information passed to the program. variable="vary + able" that means its value can be changed.

It is helpful to think of variables as containers that hold information.

Each variable in Java has a specific type, which determines the size and layout of the variable's memory; the range of values that can be stored within that memory; and the set of operations that can be applied to the variable.

You must declare all variables before they can be used. Following is the basic form of a variable declaration

Variable Declaration and Initialization in two step;

data_type variable_name ;
variable_name = Constant_value ;

int age ;
age = 21 ;

Here,

data_type is int

variable_name is age and

Constant_value 21

Variable Declaration and Initialization in one step

data_type variable_name = Constant_value ;

int age = 21 ;

Here,

data_type is int

variable_name is age and

Constant_value 21

A variable name can be chosen by the programmer in a meaningful way so as to reflect what it represents in the program. some Examples of variable name;

average
height
classStrength
roll
semester
averageLength
etc.

Variable may consist of alphabets, digits, underscore(_), and dollar sign;

Rules for naming variables:

All variable names must begin with a letter of the alphabet, an underscore, or ( _ ), or a dollar sign ($). The convention is to always use a letter of the alphabet. The dollar sign and the underscore are discouraged.
Digit at start is not allowed.
A variable’s name can be any legal identifier.
After the first initial letter, variable names may also contain letters and the digits 0 to 9. No spaces or special characters are allowed.
The name can be of any length, but don't get carried away. Remember that you will have to type this name.
Uppercase characters are distinct from lowercase characters. Using ALL uppercase letters are primarily used to identify constant variables. Remember that variable names are case-sensitive.
You cannot use a java keyword (reserved word) for a variable name.
White space is not permitted.

Some Standard Conventions Used

1. Never Use Dollar Or Underscore as First Letter

Dollar Sign and Underscore as First Character in Variable name is allowed but still its not good programming Style to use it as First Character.
Generally we use Underscore in “Constant Value” variable.

static final int TOTAL_SUM = 10;

static final int MAX_HEIGHT = 20;

*Final is nothing but constant value in Java (In C programming we use const)

2. Capitalization of First Character of Second Word

If variable name contain two words then write first letter of second word in Capital Case.
If variable name contain single word then write that word in small case.

Example : Multiple Words in Variable Name

sumOfNumber

firstRollno

skinColor

Example : Single Word in Variable Name

sum

first

last

Variables in Java: Definition and Usage Explained