Java Variables | Types, Scope & Best Practices

In Java, variables are used to store data values that can be referenced and manipulated in a program. Every variable in Java has a data type, a name, and a value.

Primitive Data Types
Reference Data Types

Primitive Data Types:

Directly store values (e.g., numbers, characters).
Default Values: Automatically initialized for instance/static variables.

Type	Size	Default Value	Description	Example
byte	8 bits	0	Small integer range (-128 to 127).	byte b = 10;
short	16 bits	0	Medium integer range (-32,768 to 32,767).	short s = 3000;
int	32 bits	0	Standard integer range (-2^31 to 2^31-1).	int age = 25;
long	64 bits	0L	Large integer range (-2^63 to 2^63-1).	long distance = 100000L;
float	32 bits	0.0f	Decimal with 6-7 digits of precision.	float pi = 3.14f;
double	64 bits	0.0d	Decimal with 15-16 digits of precision.	double e = 2.71828;
char	16 bits	\u0000	A single character (Unicode).	char grade = 'A';
boolean	8 bits	false	Logical true/false.	boolean isActive = true;

Example

byte b = 10; // Small Integer variable

short s = 3000; // Medium Integer variable

int age = 25; // Standard Integer variable

long distance = 100000L; // Long Integer variable

float pi = 3.14f; // Decimal with 6-7 digits of precision

double price = 19.99; // Decimal with 15-16 digits of precision

char grade = 'A'; // Character variable

boolean isActive = true; // Boolean variable

Try it yourself

Reference Data Types

Definition: Reference variables store the memory address (reference) of an object instead of the actual value.
Characteristics:
Used to reference objects (like arrays, strings, or user-defined objects).
Default value is null.
You can call methods on reference types.

Type	Description	Default Value	Example
String	Stores a sequence of characters. Immutable in nature.	null	String name = "John";
Array	Stores a fixed-size collection of elements of the same data type.	null	int[] numbers = {1, 2, 3};
Class/Object	Represents an instance of a user-defined class or an object.	null	MyClass obj = new MyClass();

Example

String name = "John";

int[] numbers = {1, 2, 3};

Try it yourself

Variable Naming Rules:

Must start with a letter, _, or $.
Cannot start with a number.
Cannot use reserved keywords.
Should be descriptive and use camelCase.

Example

int validVariable = 10; // Valid

int $amount = 100; // Valid

int _counter = 5; // Vali

int 1number = 20; // Invalid (starts with a number)

int class = 30; // Invalid (uses reserved keyword)

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Primitive Data Types:

Type

Size

Default Value

Description

Example

Reference Data Types

Type

Description

Default Value

Example

Variable Naming Rules: