#34 | Singleton Pattern explained completely for all Questions | Java Interview

In this video, we deep dive into the Singleton Design Pattern in Java — one of the most commonly asked creational design patterns in interviews. You’ll understand what Singleton really means, why it’s needed, how a basic implementation works, and most importantly, how Singleton breaks in multithreading and how to fix it properly.

Singleton ensures that only one instance of a class is created and provides a global point of access to that instance. This is extremely useful for objects like database connections, thread pools, caches, loggers, configuration managers, and other shared resources.

🕒 Timestamps:
00:00 - About Singleton Design Pattern
01:29 - Normal class with multiple instances
03:34 - Restricting object creation using basic Singleton
07:30 - Why basic Singleton fails in multithreading
08:56 - Solution 1 – Using synchronized getInstance()
10:30 - Solution 2 – Early initialization (eager loading)
12:06 - Solution 3 – Double-checked locking

👨‍💻 What you’ll learn:
• What Singleton Design Pattern is and why it’s a creational pattern
• Real-world use cases like DB connections, caches, and loggers
• Why the constructor must be private
• Why the instance must be static
• How a basic Singleton works
• Why basic Singleton breaks in a multi-threaded environment
• Why synchronizing getInstance() works but hurts performance
• How early initialization solves thread-safety using JVM guarantees
• How double-checked locking works and why it uses two checks
• Performance vs thread-safety tradeoffs interviewers care about

🎯 Interview mindset:
Interviewers are checking whether you understand:
• Object creation control
• JVM class loading guarantees
• Thread safety vs performance
• Why static is required
• Why naïve Singleton implementations fail

This video is perfect for Core Java interviews, design pattern preparation, backend developers, and system design basics.

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