What is Singleton Design Pattern?

✅ What is Singleton Design Pattern?

The Singleton Design Pattern is a creational design pattern that ensures only one object of a class is created throughout the application and provides a global access point to that object.

👉 In simple words:
A class using Singleton allows only one instance and prevents multiple object creation.

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✅ Why Use Singleton?

Singleton is useful when exactly one object is required, such as:

• Database connection
• Logger class
• Configuration settings
• Cache manager

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✅ Key Rules of Singleton Class

1. Constructor must be private (to stop object creation using new).
2. Create a static instance of the class.
3. Provide a public static method to access the instance.

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✅ Basic Singleton Example (Lazy Initialization)

class Singleton {

    private static Singleton instance;

    private Singleton() {
        System.out.println("Object Created");
    }

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

public class Test {
    public static void main(String[] args) {
        Singleton s1 = Singleton.getInstance();
        Singleton s2 = Singleton.getInstance();

        System.out.println(s1 == s2); // true
    }
}

✔ Both references point to the same object.

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✅ Types of Singleton Implementation

1️⃣ Eager Initialization

Object created at class loading time.

private static Singleton instance = new Singleton();

2️⃣ Lazy Initialization

Object created only when needed.

3️⃣ Thread-Safe Singleton

Used in multithreading environments.

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

4️⃣ Bill Pugh Singleton (Best Practice)

Uses static inner class — efficient and thread-safe.

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✅ Advantages

• Saves memory
• Controlled access to resources
• Global access point
• Prevents multiple object creation

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