🔒Singleton Pattern – Overview

🧠 Concept

The Singleton Pattern ensures that only one instance of a class exists and provides a global point of access to it.

👉 Useful when you need a single, shared resource (e.g., configuration, logger, connection pool).

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🧱 Key Points

Concept	Description
Single Instance	Only one object of the class is created.
Global Access Point	Instance is accessible globally through a static method.
Private Constructor	Prevents direct instantiation from outside.

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💻 Java Example – Logger Singleton

public class Logger {
    // Step 1: Create a private static instance
    private static Logger instance;

    // Step 2: Make constructor private
    private Logger() {
        System.out.println("Logger initialized");
    }

    // Step 3: Provide a global access method
    public static Logger getInstance() {
        if (instance == null) {
            instance = new Logger(); // lazy initialization
        }
        return instance;
    }

    public void log(String message) {
        System.out.println("[LOG] " + message);
    }
}

✅ Usage Example:

Logger log1 = Logger.getInstance();
Logger log2 = Logger.getInstance();

log1.log("Starting system...");
log2.log("System running...");

System.out.println(log1 == log2); // true → same instance

Output:

Logger initialized
[LOG] Starting system...
[LOG] System running...
true

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🧠 Flow Summary (Step-by-Step)

Step	Action	Instance State	Output
1️⃣	First getInstance()	Creates new Logger	“Logger initialized”
2️⃣	Second getInstance()	Returns existing instance	Same object used
3️⃣	log() calls	Work on the same shared instance	“Starting system…” etc.

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🪜 Summary

• Ensures only one object exists for the whole app.

• Provides controlled access to that object.

• Saves memory by sharing one instance.

• Commonly used for:

  • Logging

  • Configuration managers

  • Database connection pools

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⚙️ Thread-safe Singleton (Improved)

public class Logger {
    private static volatile Logger instance;

    private Logger() {}

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

✅ Double-checked locking ensures thread safety with minimal synchronization overhead.

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📦 Real-world Analogy

🏢 Company CEO There can be only one CEO in a company. Everyone accesses decisions through that one person — not by creating new CEOs.

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🧭 TL;DR

Singleton = One Object to Rule Them All Ensures only one instance exists and provides a global access point.