Datatype and Variable

Datatype is a spacial keyword used to allocate sufficient memory space for the data, in other words Data type is used for representing the data in main memory (RAM) of the computer. Based on the data type, the operating system allocates memory and decides what can be stored in the reserved memory. Therefore, by assigning different data types to variables, you can store integers, decimals, or characters in these variables.

There are two data types available in Java:

- Primitive Data Types
- Reference/Object Data Types

Primitive data types are those whose variables allows us to store only one value but they never allows us to store multiple values of same type. This is a data type whose variable can hold maximum one value at a time. Although Java is object oriented, not all types are objects. It is built on top of basic variable types called primitives.

Here is a list of all primitives in Java:

byte (number, 1 byte)
short (number, 2 bytes)
int (number, 4 bytes)
long (number, 8 bytes)
float (float number, 4 bytes)
double (float number, 8 bytes)
char (a character, 2 bytes)
boolean (true or false, 1 byte)

char uses 2 byte in java becuase java uses unicode system rather than ASCII code system. Java support more than 18 international languages so java take 2 byte for characters, because for 18 international language 1 byte of memory is not sufficient for storing all characters and symbols present in 18 languages.

boolean data type takes zero bytes of main memory space because Boolean data type of java implemented by Sun Micro System with a concept of flip - flop. A flip - flop is a general purpose register which stores one bit of information (one true and zero false). .

Reference/Object Data Types : data types are those whose variables allow us to store multiple values of same type. But they never allows to store multiple values of different types. Object are any instantiable class as well as arrays like String , Array etc.

Variables are nothing but reserved memory locations to store values. This means that when you create a variable you reserve some space in memory. Variable is an identifier which can be used to identify input data in a program.

Syntax

Variable_name = value;

Rules to declare a Variable

Every variable name should start with either alphabets or underscore ( _ ) or dollar ( $ ) symbol.
No space are allowed in the variable declarations.
Except underscore ( _ ) no special symbol are allowed in the middle of variable declaration
Variable name always should exist in the left hand side of assignment operators.
Maximum length of variable is 64 characters.
No keywords should access variable name.

Datatype variable_name;

byte b1;

There are three types of variables in java

local variable :A variable that is declared inside the method is called local variable.
instance variable : A variable that is declared inside the class but outside the method is called instance variable . It is not declared as static.
static variable : A variable that is declared as static is called static variable. It cannot be local.

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