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Simplifying Method Calls — Interview Q&A

50+ questions on the 14 techniques.

Conceptual (Q1–Q14)

Q1. What does Rename Method do? A. Gives a method a better name. Most-used IDE refactoring.

Q2. What does Add Parameter do? A. Adds a new parameter when the method needs more information.

Q3. What does Remove Parameter do? A. Removes an unused parameter.

Q4. What's Separate Query from Modifier? A. A method returning a value should not have side effects. Split it.

Q5. What's Parameterize Method? A. Combine several methods that differ by a constant into one with a parameter.

Q6. What's Replace Parameter with Explicit Methods? A. Opposite of Parameterize: a parameter triggering different code paths becomes separate methods.

Q7. What's Preserve Whole Object? A. Pass the whole object instead of a handful of its fields.

Q8. What's Replace Parameter with Method Call? A. The caller passes a value the callee can compute itself.

Q9. What's Introduce Parameter Object? A. Bundle related parameters into a value object.

Q10. What's Remove Setting Method? A. Drop a setter for a field that should be immutable after construction.

Q11. What's Hide Method? A. Make a method less visible (private, package).

Q12. What's Replace Constructor with Factory Method? A. Replace new Type(...) with a static method that constructs and returns the instance.

Q13. What's Encapsulate Downcast? A. Push a downcast inside the method that returns the value, eliminating it at all callers.

Q14. What's Replace Error Code with Exception, and the inverse? A. Replace integer error codes with thrown exceptions for exceptional failures. Replace Exception with Test for expected errors.

When to apply (Q15–Q30)

Q15. When is Add Parameter the wrong move? A. When you'd be the 5th+ parameter — consider Introduce Parameter Object.

Q16. When does Remove Parameter break tests? A. When tests passed the parameter to control behavior; check before removing.

Q17. When is Separate Query from Modifier wrong? A. When the side effect is intrinsic (iterator next(), counter increment).

Q18. When does Parameterize Method overgeneralize? A. When the variants are conceptually different operations, not values of one operation.

Q19. When should Replace Parameter with Explicit Methods apply? A. When a boolean / enum parameter selects entirely different behavior, especially if callers always know statically.

Q20. When does Preserve Whole Object hurt? A. When the callee doesn't need the whole object — overspecifies dependency, complicates testing.

Q21. When should you NOT Replace Parameter with Method Call? A. When the computation is expensive and the caller already has the value cached.

Q22. When is Introduce Parameter Object overkill? A. For 2-3 parameters that don't recur — overkill.

Q23. When is Remove Setting Method right? A. For identifiers, immutable fields, anything set once at construction.

Q24. When is Hide Method the default? A. Always — default to private; expand visibility only when needed.

Q25. When does Replace Constructor with Factory Method help? A. Need a name (Money.fromCents), polymorphic creation, validation, or caching.

Q26. When is Encapsulate Downcast a smell rather than a fix? A. When the type was wide for no reason; better to use generics or a narrower return type.

Q27. When are exceptions the right error mechanism? A. When the failure is exceptional, rare, and hard to detect at the call site.

Q28. When should errors be values (return code, Result type)? A. When failures are expected, frequent, or part of normal flow; performance-sensitive paths.

Q29. What's the boolean parameter trap? A. Multi-boolean parameter lists where call sites become unreadable rows of true, false, true.

Q30. When should a public method be hidden? A. When same-class callers are the only callers.

Code-smell mapping (Q31–Q40)

Q31. Which technique cures Long Parameter List? A. Introduce Parameter Object, Preserve Whole Object, Replace Parameter with Method Call.

Q32. Which technique cures Comments? A. Rename Method (the better name replaces the comment).

Q33. Which technique cures Inappropriate Intimacy? A. Hide Method, Remove Setting Method.

Q34. Which technique cures Speculative Generality? A. Remove Parameter (when the parameter was added "in case we need it").

Q35. Which technique cures Switch Statements? A. Replace Constructor with Factory Method (when the switch is on type/kind).

Q36. Which technique cures Duplicate Code (when methods differ by a value)? A. Parameterize Method.

Q37. Why is the boolean parameter trap a Switch Statement smell? A. Because each boolean combination is effectively a case — an in-place type code.

Q38. Which technique helps with Data Clumps in parameter lists? A. Introduce Parameter Object.

Q39. Why is Replace Constructor with Factory Method a creational pattern? A. Because it shifts construction from a fixed new to a flexible static method that can return cached or polymorphic instances. (See Factory Method pattern.)

Q40. Which technique cures the Long Parameter List smell most generally? A. Introduce Parameter Object — the workhorse cure.

Architecture (Q41–Q50)

Q41. What's the semver impact of renaming a public method? A. MAJOR — breaking change.

Q42. What's the safe way to rename a public API? A. Add the new method; mark the old @Deprecated; migrate consumers; remove in next major version.

Q43. When should you migrate from exceptions to Result types? A. When errors are normal (not exceptional), perf matters, or you want compile-time enforcement.

Q44. What's the functional options pattern in Go? A. A factory takes variadic Option functions that mutate the constructed value, providing named-argument-like ergonomics.

Q45. Why does Java's List.of(...) use a factory method instead of a constructor? A. To return cached / specialized implementations (List.of() → ImmutableCollections.ListN, of(1) → List12) and prevent extension.

Q46. What's wrong with process(o, true, false, false)? A. Boolean parameter trap — call sites are unreadable.

Q47. What's the cost of stack trace capture in a thrown exception? A. ~5-50 microseconds per throw, dominated by stack walking.

Q48. How to throw without stack trace? A. Override fillInStackTrace() to return this, or pre-allocate a singleton exception. (Use sparingly — loses debuggability.)

Q49. What's the effect of Encapsulate Downcast post-JIT? A. Often eliminated entirely; the JIT proves the cast is valid based on inlining.

Q50. Why does varargs cost an array allocation per call? A. Java represents varargs as an array; each call allocates one. Hot loggers provide non-varargs overloads.

Bonus (Q51–Q60)

Q51. What's the difference between a constructor and a factory method? A. Constructor: always returns a fresh instance of exactly its declared type. Factory: can return cached, polymorphic, or null instances.

Q52. Is Money.of(100, "USD") a factory method? A. Yes — by Java convention.

Q53. What's the most common factory method naming convention in modern Java? A. of(...), from(...), parse(...), valueOf(...).

Q54. When does Lombok @Builder make Builder pattern free? A. Most cases — generates the builder class at compile time.

Q55. Can you Replace Constructor with Factory Method and keep the constructor for serialization? A. Yes — keep a private constructor for Jackson / serialization; expose factory methods for normal use.

Q56. What's @VisibleForTesting? A. An annotation indicating the visibility was widened for tests but the method shouldn't be called by production code.

Q57. Why is "wide-to-narrow" parameter typing a refactoring direction? A. Narrower types fail-fast at the boundary; wider types push failures into the body where they cause runtime ClassCastExceptions.

Q58. Why is Encapsulate Downcast preferable to a downcast at every caller? A. One place to maintain; the IDE can replace the cast with generics in a single edit.

Q59. What's "permanent deprecation"? A. An anti-pattern: methods marked @Deprecated since 2018 still in 2026 use. Either remove or remove the annotation.

Q60. What's the relation between Hide Method and the principle of least privilege? A. Smaller API surface means fewer ways for callers to misuse — both a security and a maintainability principle.

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