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Dealing with Generalization — Interview Q&A

50+ questions on the 12 techniques.

Conceptual (Q1–Q14)

Q1. What does Pull Up Field do? A. Move a field from multiple subclasses up to a common superclass.

Q2. What does Pull Up Method do? A. Move a duplicated method from subclasses up to the superclass.

Q3. What does Pull Up Constructor Body do? A. Move duplicated initialization code from subclass constructors into the superclass constructor.

Q4. What does Push Down Method do? A. Move a method from the superclass into the only subclass that uses it.

Q5. What does Push Down Field do? A. Move a field from the superclass into the only subclass(es) that need it.

Q6. What does Extract Subclass do? A. Split off a subclass to capture features used only sometimes.

Q7. What does Extract Superclass do? A. Create a common superclass for two classes sharing features.

Q8. What does Extract Interface do? A. Define an interface capturing operations shared by multiple classes (without sharing state).

Q9. What does Collapse Hierarchy do? A. Merge a superclass and subclass that aren't different enough.

Q10. What's Form Template Method? A. Two methods doing similar work in different ways → extract the common skeleton with hook methods for variants.

Q11. What's Replace Inheritance with Delegation? A. Convert an "is-a" relationship to "has-a" + delegation when the inheritance was wrong.

Q12. What's Replace Delegation with Inheritance? A. Convert pure pass-through delegation to actual inheritance.

Q13. What's the Liskov Substitution Principle? A. Subclasses must be substitutable for their parent without breaking correctness.

Q14. What's the "is-a" test? A. Before applying inheritance, ask: "Is the subclass really an instance of the parent?" — if not, use delegation.

When to apply (Q15–Q30)

Q15. When is Pull Up Method wrong? A. When the methods look identical but mean different things in each subclass; or when the methods rely on subclass-specific helpers.

Q16. When is Push Down Method right? A. When a parent method is overridden as a no-op (or throws) in some subclasses — Refused Bequest.

Q17. When is Extract Superclass right? A. When two classes share state and behavior, with a meaningful "is-a" relationship.

Q18. When is Extract Interface preferable to Extract Superclass? A. When the classes share behavior but not state, or are otherwise unrelated.

Q19. When is Collapse Hierarchy right? A. When the subclass adds nothing meaningful (no override, no fields), and tests pass with merged version.

Q20. When is Form Template Method right? A. When two methods share a skeleton but vary in steps; the variation aligns naturally with subclass identity.

Q21. When is Replace Inheritance with Delegation right? A. When the subclass uses only part of the parent (Refused Bequest), violates LSP, or exposes wrong API.

Q22. When is Replace Delegation with Inheritance right? A. When every method on the wrapper just forwards to the same delegate, AND the relationship is genuinely "is-a."

Q23. When is "favor composition over inheritance" wrong? A. When polymorphic dispatch is exactly what you need; when Template Method genuinely fits.

Q24. What's the fragile base class problem? A. Changes to a base class can break subclasses invisibly.

Q25. When should you avoid mixins? A. When mixins' expectations conflict (diamond problem); when method resolution becomes hard to trace.

Q26. When are sealed types preferable to open hierarchies? A. When the variants are closed and you want exhaustive pattern matching.

Q27. Why does Java prevent records from extending other classes? A. To preserve the simplicity of records as data carriers; "data inheritance" is intentionally avoided.

Q28. When does Pull Up Method break LSP? A. When the method's contract (preconditions, postconditions) differs subtly across subclasses.

Q29. When should you Extract Subclass? A. When some features are conditionally used (if isInternal), and the conditions persist across multiple methods.

Q30. When is the diamond problem actually a problem? A. In multi-inheritance languages (Python, C++) when the diamond ancestors have conflicting state or methods.

Code-smell mapping (Q31–Q40)

Q31. Which technique cures Duplicate Code in subclasses? A. Pull Up Method, Pull Up Field, Form Template Method, Pull Up Constructor Body.

Q32. Which technique cures Refused Bequest? A. Push Down Method, Push Down Field, Replace Inheritance with Delegation.

Q33. Which technique cures Lazy Class? A. Collapse Hierarchy, Replace Subclass with Fields.

Q34. Which technique cures Alternative Classes with Different Interfaces? A. Extract Superclass, Extract Interface (when class names differ but they do similar things).

Q35. Which technique cures Middle Man (when extreme)? A. Replace Delegation with Inheritance.

Q36. Which technique cures Speculative Generality? A. Collapse Hierarchy (when a parent class was created speculatively but never warranted).

Q37. What smell does "5-deep inheritance chain" indicate? A. Often Speculative Generality + Refused Bequest at multiple levels.

Q38. Why is Stack extends Vector (in Java) a textbook bad inheritance? A. Stack inherits Vector's full API (add(int, E)) which lets callers shove items into the middle, breaking stack semantics.

Q39. Which Generalization technique helps with the Visitor pattern? A. Extract Interface (the visitor interface) + Form Template Method (the accept skeleton).

Q40. What's the relationship between Replace Conditional with Polymorphism and this category? A. Replace Conditional gives you subclasses; Generalization techniques manage the resulting hierarchy (pull up, extract, etc.).

Architecture & runtime (Q41–Q50)

Q41. What's the cost of virtual dispatch? A. ~3-5 cycles uncached; ~1-2 cycles with branch prediction; effectively free post-JIT for monomorphic sites.

Q42. What's CHA? A. Class Hierarchy Analysis — JIT analyzes the loaded class hierarchy to prove no overrides exist, enabling devirtualization.

Q43. What's an inline cache (IC)? A. A small per-call-site cache mapping receiver type to method pointer; allows monomorphic dispatch to be ~as fast as direct call.

Q44. When does Form Template Method create a perf cost? A. When the abstract step methods are megamorphic (4+ subclass types observed) — fall back to vtable lookup.

Q45. How do sealed types help JIT? A. Closed hierarchies let JIT fully devirtualize because no new subtypes can appear.

Q46. What's the perf cost of Replace Inheritance with Delegation? A. One extra pointer dereference; JIT inlines, eliminating it. Steady-state cost: zero.

Q47. Why does Pull Up Field sometimes hurt memory? A. Subclasses that don't use the field still carry it — wasted bytes per instance.

Q48. What's the diamond problem and how does Java handle it? A. Multiple inheritance ambiguity. Java disallows multi-class inheritance; default methods on interfaces require explicit resolution.

Q49. What's MRO in Python? A. Method Resolution Order — the C3 linearization algorithm that determines which method wins in multi-inheritance.

Q50. How does Go's lack of inheritance affect this category? A. Most techniques translate to embedding/composition idioms. Pull Up = move method to embedded; Replace Inheritance with Delegation = move from embedding to explicit field.

Bonus (Q51–Q60)

Q51. What's Open-Closed Principle? A. Modules should be open for extension, closed for modification. Polymorphism enables this.

Q52. How does pattern matching change OCP? A. Pattern matching makes adding operations easy (open for new functions) but adding types hard (closed for new variants). Polymorphism is the reverse.

Q53. What's the expression problem? A. The trade-off above: types-axis vs. operations-axis growth.

Q54. What does ArchUnit help with? A. Encoding architectural rules (e.g., "no concrete extends in legacy package") as automated tests.

Q55. When does inheritance inherit the wrong API? A. When the parent's full API is wider than what the subclass should expose (Stack-Vector example).

Q56. Why are tests with deep base-class inheritance fragile? A. Changes to the base test cascade to all subclasses; setup is hard to trace.

Q57. What's a mixin? A. A class designed to be inherited with another for shared capabilities; common in Python.

Q58. What's capability-based design? A. Express each ability as an interface; classes implement multiple capabilities. Inheritance for polymorphism, interfaces for protocols.

Q59. How do Java records change the Generalization landscape? A. Records are final, can't extend other classes. Many former Pull Up / Extract Superclass cases become "use sealed types + records."

Q60. What's the trajectory from inheritance to clean architecture? A. Pull Up to dedupe → Extract Interface for capability → Replace Inheritance with Delegation when "is-a" doesn't fit → Sealed types for closed sets → Pattern matching for operations.

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