Explain the Singleton Design Pattern in Java
💡 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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