Book Refactoring Guru Mini

OBEY Refactoring.Guru

When to use

Use when changing existing code where code smells, refactoring technique choice, behavior preservation, and cleanup scope control matter.

Primary bias to correct

Refactoring is not general cleanup or pattern application. It is a small, smell-driven, behavior-preserving treatment with verification and a stop condition.

Decision rules

• Separate refactoring from feature work and bug fixes. If behavior changes, name it as behavior change and isolate it from structural edits.
• Diagnose the smell before choosing a technique: symptom, maintenance cost, scope, expected cleaner end state, verification path, and stop condition.
• Prefer the smallest treatment that directly reduces the diagnosed smell; escalate only when the smaller technique is blocked.
• Keep the code runnable and understandable through small named transformations rather than broad redesign.
• Run relevant checks after risky moves, public interface changes, state-flow changes, or algorithm substitution.
• Stop when the named smell is gone or materially reduced; record new smells separately unless they block the current change.
• Use the Rule of Three: tolerate uncertain duplication early, but refactor the third similar occurrence unless the similarity is coincidental.
• Treat technical debt as compounding cost; pay down the debt that slows current change speed, correctness, or team understanding.
• Scan smells by category: bloaters, object-orientation abusers, change preventers, dispensables, couplers, and incomplete library gaps.
• For bloaters, prefer extraction, parameter/data modeling, and responsibility splits before creating method objects, subclasses, or interfaces.
• For switch/type-code smells, isolate the decision first; use polymorphism, subclasses, or state/strategy only when variation is stable and repeated.
• For change preventers, move behavior and data toward the owner of the changing concept so one conceptual change has one main edit site.
• For dispensables, delete or inline unused structure, but check public, generated, reflected, serialized, plugin-facing, and framework extension uses first.
• For couplers, reduce navigation and private knowledge; keep delegating layers only when they hide volatile structure, policy, or a real boundary.
• Use comments for rationale, constraints, contracts, or hard algorithms; use names, variables, methods, or assertions when comments explain unclear code.
• Keep behavior with the data it changes unless separation deliberately supports interchangeable behavior.
• Encapsulation is not finished by adding getters and setters; move behavior inward when callers are still manipulating exposed data.
• Avoid speculative abstractions: do not create wrappers, parameter objects, interfaces, superclasses, or hierarchy variants without a real concept or client.
• Preserve public compatibility or provide a transition path when changing signatures, constructors, visibility, type hierarchy, or externally reachable APIs.
• Before extraction or movement, identify inputs, outputs, mutated variables, callers, visibility, construction paths, and invariants.
• Before condition consolidation or algorithm substitution, verify side effects, ordering, truth tables, edge cases, and performance-sensitive behavior.
• Before data reorganization, decide identity, value/reference semantics, mutability, equality, lifecycle ownership, association direction, and synchronization.
• Before generalization changes, prove shared behavior is real; preserve substitutability and avoid inheriting unused behavior.
• Choose exceptions deliberately: a simple conditional, useful comment, intentional strategy separation, small extension point, or clear duplication may be better than a mechanical treatment.

Trigger rules

• When a method needs comments, scrolling, or local-state reconstruction, try Extract Method; use Replace Temp with Query, Introduce Parameter Object, or Preserve Whole Object when locals block extraction.
• When a class has multiple reasons to change, use Extract Class; use subclass/interface extraction only for stable variants or real client-facing subsets.
• When primitives, arrays, magic numbers, or type codes carry meaning, model the concept only if the model adds naming, validation, behavior, or safer variation handling.
• When a parameter list grows beyond local reasoning, replace derived parameters, preserve a whole object, or introduce a parameter object only for a real recurring concept.
• When the same change requires edits across many files, move methods/fields or extract ownership so the knowledge is centralized.
• When client code navigates object chains, hide the delegate or move behavior closer to the data; do not add pure forwarding.
• When a class mostly forwards, remove the middle man unless it protects boundary policy or volatile structure.
• When a method both queries and mutates, separate query from modifier unless atomic read-modify behavior is the public contract.
• When branches repeat behavior, decompose, consolidate, or move duplicate fragments only after checking side effects and execution order.
• When null checks dominate, introduce a null object only if absence can obey the same interface; keep absence explicit when it is an error.
• When inheritance creates refused bequest or intimacy, push members down or replace inheritance with delegation.
• When deleting dead or speculative code, verify external reachability and test-only access before removal.
• When a library class is incomplete, use a foreign method for one narrow gap and a local extension only for substantial repeated gaps.
• When cleanup keeps expanding, stop at the diagnosed smell and report the next smell separately.

Final checklist

• Is this change clearly refactoring, feature work, or bug fixing?
• Which smell was diagnosed, and what cost did it create?
• Was the smallest suitable treatment used before riskier structure?
• Did behavior stay preserved under relevant checks?
• Did the named smell become materially better?
• Did the change avoid speculative abstraction and mechanical pattern use?
• Were public compatibility, state flow, and ownership checked?
• Is any intentionally untreated smell documented rather than hidden?