Creational Patterns

Creational patterns in LLD context

Creational patterns answer: who constructs this object, and how do we hide the complexity? In interviews, you rarely name the pattern — you reach for it when constructors explode or construction logic scatters across callers.

Pattern	Use when	Skip when
Factory Method	Subclass decides which product to create (DocumentParser.create())	One product type, trivial new
Abstract Factory	Families of related products (UI theme: Button + Checkbox)	Single product, no family constraint
Builder	Many optional fields, stepwise validation (HttpRequest, PizzaOrder)	≤ 4 required fields
Singleton	Truly one instance in the process (config loader)	Testability, DI, or "global state" smell

Factory vs Builder

Factory encapsulates which concrete type to instantiate. Builder encapsulates how to assemble a complex immutable object. They compose: HttpClient client = HttpClientBuilder.create().timeout(30).build(); — Builder constructs, Factory may select the builder implementation.

Singleton controversy

Singleton is a global variable with a dress code. Problems: hidden dependencies, hard to mock, lifecycle tied to class loader. Prefer dependency injection of a single shared instance (Spring @Bean, manual constructor injection). If you must use Singleton in Java, enum singleton (Effective Java Item 3) is the safest variant.

Object pool awareness

Object Pool (less common in LLD) reuses expensive instances (DB connections). Mention only if interviewer brings up connection limits — otherwise Factory + DI covers 95% of creational needs in loops.

Senior-level signal

Say: "I'd inject a VehicleFactory so ParkingLot doesn't know about Car vs Truck — that keeps OCP intact and makes tests pass a MockVehicleFactory." For Builder, walk through why telescoping constructors fail: parameter order ambiguity and invalid partial states.

Where this goes next

Structural Patterns covers wrapping and layering — Adapter, Decorator, Facade, and Proxy — the patterns you use once objects exist and need to interact.