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Organizing Data — Interview Q&A

50+ questions covering the 15 techniques.

Conceptual (Q1–Q15)

Q1. What is the "Organizing Data" category about? A. Reshape how state is represented — encapsulation, type promotion, identity, and shape changes.

Q2. What's Self Encapsulate Field? A. Use accessors (getters/setters) even from inside the same class, to prepare for subclass overrides or future shape changes.

Q3. What's Replace Data Value with Object? A. Promote a primitive (String, int) to a proper class so it can carry behavior and invariants.

Q4. What's the difference between Change Value to Reference and Change Reference to Value? A. Value to Reference makes one shared instance per identity (entity). Reference to Value makes equality content-based, eliminating shared identity.

Q5. What's Replace Array with Object? A. Replace an index-positional array (where each slot has a different meaning) with a class with named fields.

Q6. What's Encapsulate Field? A. Make a field private, expose accessors. The basic OO encapsulation step.

Q7. What's Encapsulate Collection? A. Don't expose a mutable collection externally — return immutable views or copies, and provide controlled add/remove methods.

Q8. What's a Type Code? A. An integer constant standing in for a kind ("status = 1 for active, 2 for inactive"). Almost always a smell.

Q9. What are the three Replace Type Code variants? A. Replace Type Code with Class (just want type safety), with Subclasses (behavior differs by code), with State/Strategy (type changes at runtime).

Q10. What's Replace Subclass with Fields? A. Collapse subclasses that differ only by data (no behavior overrides) into a single class with fields.

Q11. What's Replace Magic Number with Symbolic Constant? A. Give a literal a name. 1.785 → GST_RATE.

Q12. What's Change Unidirectional to Bidirectional Association? A. Add a back-pointer. Order.customer → also Customer.orders.

Q13. What's Duplicate Observed Data? A. Move (or copy) domain data out of UI widgets into the domain model.

Q14. What's the unidirectional inverse of bidirectional? A. Change Bidirectional to Unidirectional Association — drop the unneeded back-pointer.

Q15. What is Self Encapsulate Field a precondition for? A. Many other refactorings — Replace Data Value with Object, Replace Type Code, Move Field, etc.

When to apply (Q16–Q30)

Q16. When does Replace Data Value with Object pay off? A. When the primitive carries semantic meaning (Email, Money) — you want type safety and behavior on it.

Q17. When is Inline Class (the inverse) the right move on a value object? A. When the wrapper class adds nothing — no validation, no behavior beyond getter/setter.

Q18. When do you choose enum vs. subclass? A. Enum for closed sets with simple data; subclass for richer behavior. Enum with abstract methods covers many "subclass-like" cases.

Q19. When do you choose State/Strategy over Subclasses? A. When the type can change during the object's lifetime (Order: DRAFT → SHIPPED).

Q20. When is Replace Subclass with Fields a trap? A. When the subclasses encode planned behavioral differences that haven't materialized yet — collapsing now means rebuilding later.

Q21. When should a magic number become configuration instead of a constant? A. When ops needs to tune it per environment without redeploying.

Q22. When does bidirectional association pay off? A. When traversal both ways is frequent and the alternative (a query) is expensive.

Q23. When should you drop bidirectional? A. When one direction is rarely used or causes serialization / ORM headaches.

Q24. When does Encapsulate Collection matter most? A. When the class enforces invariants on the collection (no duplicates, max size, sorted order).

Q25. When is exposing a mutable collection acceptable? A. Almost never in a public API. Acceptable only as a deliberate, documented, package-private hand-off.

Q26. When does Change Value to Reference apply? A. When two instances representing the "same logical thing" are diverging in state (loyalty points, etc.).

Q27. When is Change Reference to Value the right move? A. For small immutable concepts (Money, Color, ZIP code) where identity carries no meaning.

Q28. When can a String stay a String (not be promoted to Object)? A. When it's purely a label with no invariants — log messages, display strings, debug output.

Q29. Why is Replace Type Code with Subclasses also a Switch Statements cure? A. Polymorphism replaces the switch — each subclass implements its own variant.

Q30. When should you keep getXxx()/setXxx() instead of using records / properties? A. When you have additional logic in accessors (validation, lazy init, derived values), or when stuck on older language versions.

Code-smell mapping (Q31–Q40)

Q31. Which technique cures Primitive Obsession? A. Replace Data Value with Object, Replace Type Code with Class, Replace Array with Object.

Q32. Which technique cures Switch Statements (driven by type code)? A. Replace Type Code with Subclasses, Replace Type Code with State/Strategy.

Q33. Which technique cures Inappropriate Intimacy? A. Encapsulate Field, Encapsulate Collection, Change Bidirectional to Unidirectional Association.

Q34. Which technique cures Lazy Class? A. Replace Subclass with Fields (when the lazy class is a near-identical subclass).

Q35. Which technique addresses Magic Numbers? A. Replace Magic Number with Symbolic Constant (or move to configuration).

Q36. Why is Encapsulate Field NOT optional in modern code? A. It's required for any meaningful evolution: validation, lazy init, subclass override, observability.

Q37. Which Organizing Data technique often follows Extract Class? A. Encapsulate Field on the new class's fields (then Move Field to populate).

Q38. What's a typical sequence to fix a Type Code? A. Self Encapsulate Field → Replace Type Code with Class → if behavior differs, evolve to Subclasses → if state changes, evolve to State/Strategy.

Q39. Which technique helps with the "Data Class" smell? A. Move Method (from Moving Features) — give the data class behavior. Replace Data Value with Object also helps when the data class is just a primitive holder.

Q40. How does Replace Magic Number relate to the Comments smell? A. Naming a constant often replaces a comment ("// 0.65 is the conversion factor" → CONVERSION_FACTOR = 0.65).

Architecture & data shape (Q41–Q50)

Q41. Why does data shape determine architecture? A. The shape determines what operations are easy and hard. Bad shape locks in bad code patterns.

Q42. Compare UUID v4 and UUID v7. A. v4 is fully random — terrible for B-tree insert locality. v7 is timestamp-prefixed — sortable, retains distribution.

Q43. What's the expand-contract migration pattern? A. Add new shape alongside old; dual-write; backfill; switch reads; deprecate old.

Q44. When does a closed set deserve an enum vs. a database table? A. Enum if values rarely change (status types, blood groups). Table if ops needs to add new values without redeploy.

Q45. Why should DTOs differ from domain objects? A. They serve different audiences: domain objects encapsulate logic, DTOs serialize for transport. Coupling them leaks internal state.

Q46. What's the "type-driven design" approach to validation? A. Encode invariants in types (Email, NonEmpty<String>) so invalid states are unrepresentable.

Q47. What's the cost of Encapsulate Collection on a hot path? A. Each call allocates a new collection (or wrapper). View-based encapsulation (unmodifiableList) avoids the copy.

Q48. How does Project Valhalla change Replace Data Value with Object? A. Future Java value classes will let you wrap primitives without paying allocation overhead — making the refactor essentially free.

Q49. Why is __slots__ important when applying Replace Data Value with Object in Python? A. Without __slots__, every instance has its own dict — significant memory at scale.

Q50. What's the "type code zombie"? A. Code uses an enum but the database column is still INT — two systems of record. Migrate the schema or write a strict converter.

Bonus (Q51–Q60)

Q51. What's a Brand type in TypeScript? A. A nominal-style wrapper using a unique symbol so the compiler distinguishes Email from plain string without runtime cost.

Q52. What's a "newtype" in Rust? A. A tuple struct with one field, often used to add type safety without runtime overhead.

Q53. What's a Java record? A. An immutable data carrier with auto-generated equals/hashCode/toString. Modern replacement for many "Replace Data Value with Object" cases.

Q54. Why does ORM make bidirectional associations tricky? A. The owning side must be declared explicitly; both sides must be kept consistent in setters; lazy loading and serialization can cycle.

Q55. What's the difference between an entity and a value object in DDD? A. Entity: identity (id), mutable state. Value object: equality by content, immutable.

Q56. Why is Encapsulate Field "free" at runtime in modern JVMs? A. JIT inlines the trivial accessor; resulting machine code is identical to direct field access.

Q57. When is Defensive Copy in Encapsulate Collection too expensive? A. Hot paths with thousands of calls per request. Use unmodifiable views or streams instead.

Q58. What's Collections.unmodifiableList(list) vs. List.copyOf(list)? A. unmodifiableList is a view — sharing storage, no copy, but mutation through the view is blocked. List.copyOf makes a true immutable copy.

Q59. When does a "registry singleton" implementation of Change Value to Reference become a problem? A. In tests (state pollution between tests), in concurrency (race on init), in long-lived processes (memory leak as registry grows).

Q60. What's the difference between an enum and a constant class in Java? A. Enum has static type-checked constants, exhaustive switch, ordinal/values methods. Constant class is just public static final fields — no type safety.

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