Loading...
Factory, builder, singleton, and prototype, including when each is over-engineering.
3 questions in Low-Level Design & OOD.
Replace the switch with a registry the application builds once at startup: a map from discriminator to a factory, validated for duplicate and missing keys as it is constructed, injected rather than global, and with a decided answer for a type the consumer has never seen.
A constructor is the default and needs no justification. A static factory earns its place when construction can return a cached or alternative implementation, or needs a name; a builder when there are many optional fields. Singleton is usually a testability liability.
The mechanics are easy - an enum in Java, a module-level value elsewhere - and are not the interesting part. The objection is that a singleton bundles two unrelated decisions, one instance and globally reachable, and the second is what makes dependencies invisible and tests order-dependent. It also connects creational patterns to the point an interviewer is testing.