What is Singleton design pattern?

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

What is the thread-safe way to implement Singleton?

You can use double-checked locking, Bill Pugh pattern, or Enum to ensure thread-safety.

Explain the Singleton Design Pattern in Java | FullStackPrep.Dev

💡 Concept: Singleton Pattern

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

📘 Quick Intro

Singleton pattern restricts object creation of a class to one "single" instance for system-wide use.

🧠 Analogy

Think of a Singleton like the government — there’s only one central government (object) everyone refers to. Multiple governments (instances) would create chaos.

🔧 Technical Explanation

Uses a private static variable to hold the single instance.
Constructor is private to restrict instantiation.
A public static method provides the global access point.
Thread-safe implementations use synchronization or Bill Pugh pattern.
Singleton can be eager, lazy, or double-checked.

🎯 Use Cases

✅ Configuration managers
✅ Logger instances
✅ Database connections
✅ Cache objects

💻 Example: Thread-safe Singleton


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;
    }
}

❓ Interview Q&A

Q1: What is Singleton pattern?
A: A pattern that allows only one instance of a class.

Q2: How is Singleton implemented?
A: Using a private constructor and static instance.

Q3: What is double-checked locking?
A: Checks instance twice with synchronization for efficiency.

Q4: What are the types of Singleton?
A: Eager, Lazy, Bill Pugh, Enum.

Q5: Can Singleton be broken?
A: Yes, via reflection, cloning, or serialization.

Q6: How to make Singleton serialization-safe?
A: Override readResolve() method.

Q7: Why is Singleton considered an anti-pattern sometimes?
A: Because it introduces global state and tight coupling.

Q8: Is Enum Singleton thread-safe?
A: Yes, by default.

Q9: Can Spring beans be Singleton?
A: Yes, default scope is Singleton.

Q10: What's the Bill Pugh Singleton implementation?
A: Uses inner static helper class to hold instance.

📝 MCQs

Q1. What is the purpose of Singleton pattern?

Provide multiple objects
Hide constructor
Ensure only one instance of a class exists
Enable inheritance

Q2. How is Singleton object accessed?

With new keyword
Via constructor
Using a static method
Via factory method

Q3. Which is thread-safe Singleton implementation?

Lazy loading
Eager loading
Double-checked locking
Cloning

Q4. What is the role of readResolve()?

Reset object
Log serialization
Prevent breaking Singleton during deserialization
None

Q5. Which is easiest way to create Singleton in Java?

Static block
Bill Pugh
Using Enum
Reflection

Q6. Why make constructor private?

Improve speed
Prevent external instantiation
Allow inheritance
Enable serialization

Q7. What design problem does Singleton address?

Multiple inheritance
Observer management
Global access to a single instance
Object pooling

Q8. Can Singleton be used in multithreading?

No
Yes, always safe
Yes, with proper synchronization
Only in Java 11+

Q9. What is common use case of Singleton?

Servlet
Logger
View
Controller

Q10. Which keyword is used in Bill Pugh Singleton?

abstract
final
static inner class
super

💡 Bonus Insight

The Enum-based Singleton is the simplest and safest Singleton implementation recommended since Java 1.5.

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Explain the Singleton Design Pattern in Java