A variable is a container that holds data. It allows you to store and modify values in memory.

Key Points:

A variable is a named memory location for storing data.
Declaration: Specify the data type and variable name.
Initialization: Assign a value to the variable.

Declaration & Initialization

To declare a variable first write datatype ( type of data i.e. int, float, char, etc.) then space , and then write the name of the variable (Identifier).

Syntax:

 datatype variable_name;

Example:

int a; // Declaring a variable

To initialize a variable we have to assign a value to the variable.

Syntax:

variable_name = value;

Example:

a = 12; // Assigning value

Declaration and Initialization of a variable at once:

int a=20;
String s="Hello";

Types of Variables in Java

Variables are of three types: Instance, Local, Static.

1. Instance Variable

Defined inside a class but outside methods.
Each object gets its own copy of the instance variable.

Example:

package quipohouse;
public class InstanceVariable {
	int var = 10;    // Declaration and initialization of instance variable

	public static void main(String[] args) {
		InstanceVariable c = new InstanceVariable();
		System.out.println(c.var);
	}
}

Output:

2. Local Variable

Declared inside a method or block.
Scope limited to that method/block.

Example:

package quipohouse;
public class LocalVariable {

	public static void main(String[] args) {
		int var2 = 12; // Declaration and initialization of Local Variable
		System.out.println(var2);
	}
}

Output:

3. Static Variable

Declared using static inside a class.
Shared across all objects of the class (single copy).
Can be accessed directly using the class name.

Example:

package quipohouse;
public class StaticVariable {
	static int var; // Declaration of static variable

	public static void main(String[] args) {
		System.out.println(StaticVariable.var); // 0
		System.out.println(var); // 0
	}
}

Output:

0
0