Can you provide examples of different ways to implement the Singleton pattern?

In Java, the Singleton pattern can be implemented in several ways:

1. Lazy initialization with synchronization

public class Singleton {
    private static Singleton instance;

    private Singleton() {}

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

2. Lazy initialization with double-checked locking

public class Singleton {
    private static volatile Singleton instance;

    private Singleton() {}

    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

3. Eager initialization at class loading

public class Singleton {
    private static final Singleton instance = new Singleton();

    private Singleton() {}

    public static Singleton getInstance() {
        return instance;
    }
}

4. Using enum (recommended approach since Java 5+)

public enum Singleton {
    INSTANCE;

    public void someMethod() {
        // ...
    }
}

This method guarantees thread safety and protection against serialization.

Each approach has its pros and cons regarding thread safety, performance, and implementation simplicity.