Java Variables: Local, Instance, and Static Variables with Memory Model

A variable is a reserved container in computer memory designed to hold a data value during program execution. In Java, every variable has a statically declared type, a name (identifier), and an assigned value.

The 3 Types of Variables in Java

Java classifies variables into three distinct categories based on where they are declared and their scope in memory:

Comprehensive Code Demonstration

Let us observe all three variable types operating together inside a single program:

public class Employee {
    // 1. Static Variable: Shared across all Employee objects
    public static String company = "Veewom Tech Corp";
    public static int employeeCounter = 0;

    // 2. Instance Variables: Unique copy per employee object
    private int empId;
    private String empName;
    private double salary;

    // Constructor
    public Employee(String name, double salary) {
        employeeCounter++;
        this.empId = employeeCounter;
        this.empName = name;
        this.salary = salary;
    }

    // Method demonstrating Local Variables
    public void calculateBonus(double performanceFactor) {
        // 3. Local Variable: Exists only within this method call
        double bonusAmount = this.salary * performanceFactor;
        System.out.println(empName + " Bonus: $" + bonusAmount);
    }

    public static void main(String[] args) {
        // e1 and e2 are local reference variables stored on the Stack
        Employee e1 = new Employee("Aarav", 75000);
        Employee e2 = new Employee("Sneha", 88000);

        e1.calculateBonus(0.10); // 10% bonus
        e2.calculateBonus(0.15); // 15% bonus

        System.out.println("Total Employees Created: " + Employee.employeeCounter);
    }
}

Default Values of Variables in Java

Instance and static variables are automatically initialized to safe default values by the JVM during memory allocation. Local variables have NO default values and must be initialized before use:

int x;
System.out.println(x);