Day 4: Understanding Java Variables and Their Types, Static and Non-Static Variables

What is a Variable in Java?

A variable is used to store data in a program.

Each variable has:

• A type (int, String, etc.)
• A name
• A value

Example:

int age = 10;

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Types of Variables in Java

Java variables can be mainly classified into

1. Global variables
2. Local variables
3. Static variables
4. Non-static (Instance) variables

Let’s understand these using the below example.

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Example Code

public class School {

 // Global variable / Instance variable / Non-static variable
 String name;
 int std;

 // Global and non-static variable
 int age = 10;

 // Global variable and static variable
 static String school_name = "ABC.scl";

 public static void main(String[] args) {

  School tamil_teacher1 = new School(); // Object or Instance
  tamil_teacher1.name = "Mukesh";
  System.out.println(tamil_teacher1.name);

  School tamil_teacher2 = new School();
  tamil_teacher2.name = "Karthick";
  System.out.println(tamil_teacher2.name);

  School English_teacher = new School(); // Object or Instance
  English_teacher.name = "Guru";
  System.out.println(English_teacher.name);

  System.out.println(School.school_name);
  test();
 }

 public static void test() {
  School math_teacher = new School();
  math_teacher.name = "Yogesh";
  System.out.println(math_teacher.name);

  // Local variable
  int age = 10;
  System.out.println(age);
 }
}

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1. Global Variables

Variables declared inside a class but outside methods are called global variables.

Example:

String name;
int age;

These variables:

• Belong to the class
• Can be accessed using an object

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2. Non-Static (Instance) Variables

Non-static variables are also called instance variables because:

• Each object gets its own copy
• Values can be different for each object

Example:

School tamil_teacher1 = new School();
tamil_teacher1.name = "Mukesh";

School tamil_teacher2 = new School();
tamil_teacher2.name = "Karthick";

Here, both objects have different name values.

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3. Static Variables

A static variable belongs to the class, not to objects.

Example:

static String school_name = "ABC.scl";

Features:

• Only one copy exists
• Shared by all objects
• Accessed using the class name

Example:

System.out.println(School.school_name);

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4. Local Variables

A local variable is declared inside a method.

Example:

int age = 10;

Local variables:

• Exist only inside the method
• Cannot be accessed outside the method
• Must be initialized before use

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Important note:

• Static methods cannot directly access non-static variables
• Non-static variables need an object to be accessed

That’s why we create objects inside methods to access name.