Creational Patterns — Notes

Why

• Decouple "what to use" from "how to make it".
• Centralise construction logic so it can be swapped, tested, mocked.

Singleton pitfalls

• Acts as global state.
• Hard to test (must reset between tests).
• Multi-threaded init needs care (double-checked locking, volatile, or eager init).
• Frequently better: inject a single instance via the DI container.

Builder vs telescoping constructors

• 4 parameters? Reach for Builder.

• Required vs optional: enforce required in Builder constructor; expose setters for optional.
• Fluent (return this) reads better.

Modern alternatives

• Kotlin: data class + named/default args replaces Builder for many cases.
• Java records (since 14) cover simple POJOs.
• Python: keyword arguments + dataclasses replace most of Builder.
• Go: functional options (WithTimeout(5), WithRetries(3)).

Refs

• Design Patterns: Elements of Reusable Object-Oriented Software — Gamma, Helm, Johnson, Vlissides (GoF, 1994).
• Effective Java (Bloch) — Item 2 (Builder), Item 3 (Singleton).
• Refactoring.guru patterns gallery.