Builder

Builder assembles complex objects step by step with readable fluency, defaults, and validation before `build()`. It beats telescoping constructors when optional fields combine explosively. In application code, builders often appear for HTTP clients, test fixtures, query objects, and configuration records.

On interviews: compare Builder with constructors, factories, and object literals. Discuss immutability and validation timing.

Common pitfalls: builders that allow invalid partial states; mutable builders shared across threads; duplicating defaults in multiple builder copies.

The trade-off is flexibility versus complexity—know when the simpler path is enough.

Checklist:

• Validate invariants at build time.
• Provide sensible defaults for optional fields.
• Keep builder API discoverable and ordered logically.
• Prefer immutable products after build completes.