Singleton Pattern: Definition and Core Purpose

Definition
Singleton Pattern is a creational design pattern that ensures a class has only one instance throughout the application lifetime while providing a global point of access to that instance.
What Problem Does It Solve?
When multiple parts of your application need to share a single resource or coordinator, creating multiple instances leads to inconsistent state, resource wastage, or conflicting operations. The Singleton Pattern guarantees exactly one instance exists, preventing duplication.
Common Use Cases:
First, Configuration Management: A single ConfigurationManager loads settings once and shares them across all components.

Second, Logging Service: A Logger writes to one file handle, preventing file corruption from concurrent writes.

Third, Connection Pooling: A DatabaseConnectionPool manages limited database connections centrally.

Fourth, Cache Management: A CacheManager maintains one in-memory cache, avoiding data duplication.
Interview Tip: Always explain that Singleton controls instance creation (structural concern), not just providing global access. The pattern is about lifecycle management.
Key Constraint:
The class itself is responsible for tracking and returning its single instance. External code cannot create new instances directly because the constructor is made private or protected.

💡 Key Takeaways

✓Guarantees exactly one instance of a class exists in the application

✓Provides global access point through a static method like getInstance()

✓Prevents instantiation from outside by making constructor private

✓Solves resource sharing and state consistency problems

✓Different from global variables because it controls instantiation lifecycle

📌 Examples

1ConfigurationManager that loads application settings once

2Logger that writes to a single file handle

3DatabaseConnectionPool managing connection limits

4CacheManager maintaining one in-memory data store

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