Java static Keyword
Complete Guide

• ✅Static variable class load hote waqt initialize hota hai (object banane se pehle)
• ✅Sabhi objects ke beech shared hai — ek mein change → sab ko effect
• ✅ClassName.varName se access karna best practice hai
• ❌Object-specific data ko static variable mein mat rakhein — wrong design hai
• ❌Multi-threading mein static variables thread-safety issues cause karte hain

public class ObjectCounter {

    // static variable — sirf ek copy, sabhi objects share karte hain
    static int count = 0;

    // instance variable — har object ka apna
    String name;
    int    id;

    ObjectCounter(String name) {
        count++;               // shared counter badho
        this.id   = count;
        this.name = name;
        System.out.println("Object Created: " + name + " | ID: " + id + " | Total: " + count);
    }

    static void showTotal() {
        System.out.println("Total objects created: " + count);
    }

    public static void main(String[] args) {

        System.out.println("--- Before creating objects ---");
        ObjectCounter.showTotal();  // class name se access — BEST way

        ObjectCounter s1 = new ObjectCounter("Rahul");
        ObjectCounter s2 = new ObjectCounter("Amit");
        ObjectCounter s3 = new ObjectCounter("Priya");

        System.out.println("\n--- After creating 3 objects ---");
        System.out.println("Class access   : " + ObjectCounter.count);  // 3
        System.out.println("Via s1         : " + s1.count);               // 3
        System.out.println("Via s2         : " + s2.count);               // 3 — sab same!

        System.out.println("\ns1.name = " + s1.name + " | s2.name = " + s2.name);
        // name alag-alag (instance variable)
    }
}

--- Before creating objects ---
Total objects created: 0
Object Created: Rahul | ID: 1 | Total: 1
Object Created: Amit  | ID: 2 | Total: 2
Object Created: Priya | ID: 3 | Total: 3

--- After creating 3 objects ---
Class access   : 3
Via s1         : 3
Via s2         : 3   ← sab same memory location!

s1.name = Rahul | s2.name = Amit

class BankAccount {

    // ─── Static (shared by all accounts) ───────────────────────
    static String  bankName     = "State Bank of India";
    static double  interestRate = 7.5;
    static int     totalAccounts = 0;

    // ─── Instance (individual per account) ─────────────────────
    String holder;
    long   accNo;
    double balance;

    BankAccount(String holder, long accNo, double balance) {
        this.holder  = holder;
        this.accNo   = accNo;
        this.balance = balance;
        totalAccounts++;
    }

    void showDetails() {
        double interest = balance * interestRate / 100;
        System.out.printf("[%-15s] Acc: %d | Bal: ₹%8.0f | Rate: %.1f%% | Interest: ₹%.0f%n",
                           holder, accNo, balance, interestRate, interest);
    }

    public static void main(String[] args) {
        BankAccount a1 = new BankAccount("Rahul Sharma", 10012345L, 50000);
        BankAccount a2 = new BankAccount("Priya Singh",  10067890L, 80000);
        BankAccount a3 = new BankAccount("Amit Kumar",   10099001L, 120000);

        System.out.println("Bank: " + BankAccount.bankName);
        System.out.println("Total Accounts: " + BankAccount.totalAccounts);
        System.out.println("─".repeat(75));
        a1.showDetails();
        a2.showDetails();
        a3.showDetails();

        System.out.println("\n★ RBI ne interest rate change kiya to 8.0%...");
        BankAccount.interestRate = 8.0;   // ek jagah badla → sabko effect!
        System.out.println("─".repeat(75));
        a1.showDetails();
        a2.showDetails();
        a3.showDetails();
    }
}

Bank: State Bank of India
Total Accounts: 3
───────────────────────────────────────────────────────────────────────────
[Rahul Sharma   ] Acc: 10012345 | Bal: ₹  50000 | Rate: 7.5% | Interest: ₹3750
[Priya Singh    ] Acc: 10067890 | Bal: ₹  80000 | Rate: 7.5% | Interest: ₹6000
[Amit Kumar     ] Acc: 10099001 | Bal: ₹ 120000 | Rate: 7.5% | Interest: ₹9000

★ RBI ne interest rate change kiya to 8.0%...
───────────────────────────────────────────────────────────────────────────
[Rahul Sharma   ] Acc: 10012345 | Bal: ₹  50000 | Rate: 8.0% | Interest: ₹4000
[Priya Singh    ] Acc: 10067890 | Bal: ₹  80000 | Rate: 8.0% | Interest: ₹6400
[Amit Kumar     ] Acc: 10099001 | Bal: ₹ 120000 | Rate: 8.0% | Interest: ₹9600

class AppConstants {

    // static final = constant — change karne pe COMPILE ERROR!
    static final double PI          = 3.14159265358979;
    static final int    MAX_USERS   = 1000;
    static final String APP_NAME    = "BackendApp";
    static final String VERSION     = "v2.5.0";
    static final double TAX_RATE    = 0.18;

    // static mutable variable (shared state)
    static int    activeUsers  = 0;
    static String currentTheme = "light";

    public static void main(String[] args) {

        System.out.println("=== App Configuration ===");
        System.out.println("App     : " + AppConstants.APP_NAME);
        System.out.println("Version : " + AppConstants.VERSION);
        System.out.println("MaxUsers: " + AppConstants.MAX_USERS);
        System.out.println("TaxRate : " + (AppConstants.TAX_RATE * 100) + "%");
        System.out.println("PI      : " + AppConstants.PI);

        // Tax calculation using constant
        double price = 1500.0;
        double total = price + (price * AppConstants.TAX_RATE);
        System.out.printf("\nPrice: ₹%.0f | Tax: ₹%.0f | Total: ₹%.0f%n",
                           price, price * AppConstants.TAX_RATE, total);

        // Mutable static
        AppConstants.activeUsers = 45;
        System.out.println("\nActive users: " + AppConstants.activeUsers);
        AppConstants.activeUsers++;
        System.out.println("After login : " + AppConstants.activeUsers);

        // PI.PI = 3.0; ← COMPILE ERROR: cannot assign a value to final variable
        System.out.println("\nCircle area (r=7): " + AppConstants.PI * 7 * 7);
    }
}

=== App Configuration ===
App     : BackendApp
Version : v2.5.0
MaxUsers: 1000
TaxRate : 18.0%
PI      : 3.14159265358979

Price: ₹1500 | Tax: ₹270 | Total: ₹1770

Active users: 45
After login : 46

Circle area (r=7): 153.9380...

class DatabaseConnection {

    // static instance — sirf ek object kabhi bhi exist karega
    private static DatabaseConnection instance = null;
    private static int connectionAttempts = 0;

    private String url;
    private String status;

    // private constructor — bahar se new nahi bana sakte!
    private DatabaseConnection() {
        this.url    = "jdbc:mysql://localhost:3306/mydb";
        this.status = "Connected";
        System.out.println("[DB] Connection established: " + url);
    }

    // static factory method — ek hi instance deta hai
    public static DatabaseConnection getInstance() {
        connectionAttempts++;
        if (instance == null) {
            System.out.println("[DB] Creating new connection (first time)...");
            instance = new DatabaseConnection();
        } else {
            System.out.println("[DB] Reusing existing connection.");
        }
        return instance;
    }

    void query(String sql) {
        System.out.println("[DB] Executing: " + sql);
    }

    static int getAttempts() { return connectionAttempts; }

    public static void main(String[] args) {
        DatabaseConnection db1 = DatabaseConnection.getInstance();
        DatabaseConnection db2 = DatabaseConnection.getInstance();
        DatabaseConnection db3 = DatabaseConnection.getInstance();

        System.out.println("\ndb1 == db2: " + (db1 == db2));  // true!
        System.out.println("db2 == db3: " + (db2 == db3));  // true!
        System.out.println("Connection attempts: " + DatabaseConnection.getAttempts());
        System.out.println("Actual connections created: 1 (Singleton!)");

        db1.query("SELECT * FROM users WHERE id = 5");
        db3.query("INSERT INTO orders VALUES (101, 'Rahul', 500)");
    }
}

[DB] Creating new connection (first time)...
[DB] Connection established: jdbc:mysql://localhost:3306/mydb
[DB] Reusing existing connection.
[DB] Reusing existing connection.

db1 == db2: true
db2 == db3: true
Connection attempts: 3
Actual connections created: 1 (Singleton!)
[DB] Executing: SELECT * FROM users WHERE id = 5
[DB] Executing: INSERT INTO orders VALUES (101, 'Rahul', 500)

class Vehicle {
    // ── Static (class-level shared) ───────────────────────────────
    static String manufacturer  = "Tata Motors";
    static int    totalVehicles = 0;

    // ── Instance (per-object individual) ──────────────────────────
    String model;
    int    year;
    double price;

    Vehicle(String model, int year, double price) {
        this.model = model;
        this.year  = year;
        this.price = price;
        totalVehicles++;
    }

    void info() {
        System.out.printf("%-10s %-6d ₹%-10.0f  %s%n", model, year, price, manufacturer);
    }

    public static void main(String[] args) {
        Vehicle v1 = new Vehicle("Nexon",   2023, 900000);
        Vehicle v2 = new Vehicle("Punch",   2022, 700000);
        Vehicle v3 = new Vehicle("Safari",  2024, 1800000);
        Vehicle v4 = new Vehicle("Harrier", 2023, 1500000);

        System.out.println("Model      Year   Price         Manufacturer");
        System.out.println("─".repeat(52));
        v1.info(); v2.info(); v3.info(); v4.info();
        System.out.println("Total: " + Vehicle.totalVehicles + " vehicles");

        System.out.println("\n★ Manufacturer name update:");
        Vehicle.manufacturer = "Tata Motors Ltd.";   // ek jagah badla → sab pe effect
        v1.info(); v2.info();
    }
}

Model      Year   Price         Manufacturer
────────────────────────────────────────────────────
Nexon      2023   ₹900000      Tata Motors
Punch      2022   ₹700000      Tata Motors
Safari     2024   ₹1800000     Tata Motors
Harrier    2023   ₹1500000     Tata Motors
Total: 4 vehicles

★ Manufacturer name update:
Nexon      2023   ₹900000      Tata Motors Ltd.
Punch      2022   ₹700000      Tata Motors Ltd.