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Creational/Singleton/Singleton.md

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+# Singleton Pattern
+
+## Definition
+The Singleton Pattern is a creational design pattern that ensures a class has only one instance and provides a global point of access to it. This pattern restricts the instantiation of a class to a single object, often used for managing shared resources or configurations.
+
+## Intent
+- To ensure that a class has only one instance throughout the application.
+- To provide a global point of access to the single instance.
+
+## Problem
+- You need to control the instantiation of a class and ensure that only one instance exists, even in multi-threaded environments.
+- You want to provide a consistent and centralized point of access to that single instance.
+
+## Solution
+- Make the class’s constructor private or protected to prevent direct instantiation from outside the class.
+- Provide a static method that allows the client to access the single instance, ensuring that only one instance is ever created.
+
+## Key Components
+- **Singleton Class:** The class that ensures only one instance exists and provides global access to it.
+- **Instance:** The single instance of the class, created lazily when required.
+- **Static Access Method:** A method, typically static, that returns the single instance of the class.
+
+## When to Use
+- When you need to ensure that only one instance of a class exists, such as for managing shared resources like database connections, logging, or configuration settings.
+- When the overhead of creating and managing multiple instances is unnecessary and undesirable.
+
+## Benefits
+- **Global Access:** The single instance can be accessed globally, making it easy to manage shared resources.
+- **Controlled Instance Creation:** The pattern ensures that the object is created only when needed and maintains its unique instance.
+- **Reduced Memory Usage:** As only one instance exists, it can help reduce memory usage and the overhead of multiple object creations.
+
+## When to Avoid
+- When the system requires multiple instances of the class, as this pattern limits instantiation to one.
+- If the class is used in a multi-threaded environment without proper synchronization, it may lead to issues in creating the instance safely.
+- When unit testing is challenging due to the global state maintained by the Singleton instance.
+
+## Summary
+The Singleton Pattern ensures that a class has only one instance and provides a global point of access to it. This pattern is useful for managing shared resources and configurations, but should be used carefully in multi-threaded applications and situations where multiple instances might be required.
+
+## What to Prepare for Interview
+- **Definition and Use Case:** Be able to explain the singleton pattern and provide practical examples of when to use it, such as managing global settings or logging.
+- **Code Example:** Prepare a simple code example demonstrating the creation of a singleton class and accessing its instance.
+- **Real-World Use Cases:** Understand how the Singleton pattern is applied in scenarios like logging, database connection pooling, or caching.
+- **Comparison with Other Patterns:** Be ready to explain how the singleton pattern differs from other patterns like Factory Method (which creates multiple instances of products) and the Prototype Pattern (which creates new objects from a prototype).
+- **Benefits and Trade-offs:** Discuss the advantages and limitations of using the singleton pattern, especially regarding thread safety, global state, and challenges in testing.