Write a Java program to implement the Singleton Design Pattern.

public class Singleton {
    // Static variable holds the single instance
    private static Singleton instance;

    // Private constructor prevents instantiation from outside
    private Singleton() {
        System.out.println("Singleton instance created");
    }

    // Public static method to get/create the instance
    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();   // create only if not exists
        }
        return instance;
    }

    public void showMessage() {
        System.out.println("Hello from Singleton!");
    }
}

public class SingletonDemo {
    public static void main(String[] args) {
        Singleton obj1 = Singleton.getInstance();
        Singleton obj2 = Singleton.getInstance();
        Singleton obj3 = Singleton.getInstance();

        obj1.showMessage();

        // All references point to the SAME object
        System.out.println("obj1 == obj2: " + (obj1 == obj2));  // true
        System.out.println("obj1 == obj3: " + (obj1 == obj3));  // true
        System.out.println("Hashcode: " + obj1.hashCode() + " == " + obj2.hashCode());
    }
}

Singleton instance created
Hello from Singleton!
obj1 == obj2: true
obj1 == obj3: true
Hashcode: 366712642 == 366712642

public class ThreadSafeSingleton {
    // volatile ensures visibility across threads
    private static volatile ThreadSafeSingleton instance;

    private ThreadSafeSingleton() {}

    public static ThreadSafeSingleton getInstance() {
        if (instance == null) {                          // first check (no lock)
            synchronized (ThreadSafeSingleton.class) {  // lock only when needed
                if (instance == null) {                  // second check (with lock)
                    instance = new ThreadSafeSingleton();
                }
            }
        }
        return instance;
    }
}

public class EagerSingleton {
    // Created when class loads — always thread-safe
    private static final EagerSingleton INSTANCE = new EagerSingleton();

    private EagerSingleton() {}

    public static EagerSingleton getInstance() {
        return INSTANCE;
    }
}

public class DriverManager {
    private static DriverManager instance;
    private WebDriver driver;

    private DriverManager() {}

    public static DriverManager getInstance() {
        if (instance == null) {
            instance = new DriverManager();
        }
        return instance;
    }

    public WebDriver getDriver() {
        if (driver == null || ((ChromeDriver) driver).getSessionId() == null) {
            driver = new ChromeDriver();
            driver.manage().window().maximize();
        }
        return driver;
    }

    public void quitDriver() {
        if (driver != null) {
            driver.quit();
            driver = null;
        }
    }
}

// Usage in any test or page object:
WebDriver driver = DriverManager.getInstance().getDriver();

public class ParallelDriverManager {
    // Each thread gets its OWN driver instance
    private static ThreadLocal<WebDriver> driverThread = new ThreadLocal<>();

    public static void setDriver(WebDriver driver) { driverThread.set(driver); }
    public static WebDriver getDriver()             { return driverThread.get(); }
    public static void removeDriver()               { driverThread.remove(); }
}