Refactoring Patterns

The Refactoring Pattern Catalog

Pattern	What It Does	When to Use	Risk
Extract Method	Break long function into named steps	Function exceeds 5-7 lines or mixes abstraction levels	Low
Inline Method	Remove unnecessary indirection	Method adds no clarity; just delegates	Low
Replace Type Code with Classes	Turn enums/constants into class hierarchies	if/else or switch on type codes	Medium
Push Code into Classes	Move logic from callers into the class it belongs to	Logic about a class lives outside it	Medium
Specialize Method	Create focused versions of overly general methods	Method has parameters used only in some call sites	Low
Try Delete Then Compile	Delete code and let the compiler verify safety	Suspected dead code, unused parameters	Low
Unify Similar Classes	Merge classes that differ only in constants	Near-duplicate classes with identical structure	Medium
Combine Ifs	Merge consecutive ifs with identical bodies	Repeated conditional blocks	Low
Introduce Strategy Pattern	Extract varying behavior into strategy objects	Multiple classes share structure but differ in behavior	High
Extract Interface	Create interface from existing implementation	Need to decouple consumers from concrete class	Medium
Eliminate Getter/Setter	Move behavior to where the data lives	External code pulls data, operates, pushes back	Medium
Encapsulate Data	Wrap related variables in a class	Loose variables travel together across functions	Medium
Enforce Sequence	Use constructors to guarantee operation ordering	Methods must be called in a specific order	Medium

When to Refactor

Is there a concrete problem?
├── No → Don't refactor. "If it ain't broke, don't fix it."
└── Yes → What kind of problem?
    ├── Hard to understand → Extract Method, rename, Encapsulate Data
    ├── Hard to change → Push Code into Classes, Extract Interface
    ├── Duplicated logic → Unify Similar Classes, Introduce Strategy
    ├── Scattered conditionals → Replace Type Code with Classes
    ├── Wrong abstraction level → Inline Method, Specialize Method
    └── Dead or unused code → Try Delete Then Compile

Smell-to-Pattern Mapping

Structural Smells

Smell	Signal	Refactoring Response
Long method	Function exceeds 5-7 lines of logic	Extract Method
Mixed abstraction levels	Function both calls methods and does low-level work	Extract Method
Deep nesting	Conditionals nested 3+ levels	Extract Method, guard clauses
if/else chains	Type-checking with if/else on enums	Replace Type Code with Classes
Getters and setters	External code pulls data, transforms, pushes back	Eliminate Getter/Setter
Common prefixes/suffixes	Multiple functions share a prefix like `player_*`	Encapsulate Data
Primitive obsession	Related primitives passed together	Encapsulate Data

Dependency Smells

Smell	Signal	Refactoring Response
Hard-coded dependencies	Constructor creates its own collaborators	Inject dependencies through constructor
Concrete class coupling	Code depends on implementation, not interface	Extract Interface
Feature envy	Method uses another class's data more than its own	Push Code into Classes
Leaking implementation details	Return types expose internal layers	Wrap in domain-appropriate types
Global state	Shared mutable state makes code unsafe	Inject shared state as explicit dependency

The Invariant Hierarchy

Push invariants as high up this hierarchy as possible. Levels 1-2 are dramatically more reliable than 3-6.

Level	Mechanism	Reliability
1. Eliminate the invariant	Redesign so the problem can't occur	Highest
2. Compiler-enforced	Type system prevents violations	High
3. Runtime-enforced	Assertions, guards, validation	Medium
4. Documented	Comments, README, wiki	Low
5. Verbal	Told during onboarding	Very low
6. Hoped for	Undocumented assumption	None

"Which refactoring do I need?"

You're struggling with...	Primary Pattern	Supporting Pattern
Function too long to understand	Extract Method	Specialize Method
if/else chains on type codes	Replace Type Code with Classes	Push Code into Classes
Changing one thing breaks another	Extract Interface	Dependency injection
Duplicated code across classes	Unify Similar Classes	Introduce Strategy Pattern
Data exposed through getters	Eliminate Getter/Setter	Push Code into Classes
Related variables scattered	Encapsulate Data	Enforce Sequence
Suspected dead code	Try Delete Then Compile	—
Methods must be called in order	Enforce Sequence	Encapsulate Data
Deep inheritance tree	Extract Interface + composition	Introduce Strategy Pattern

Refactoring Checklist

Before:

Tests exist for the code being refactored (or write them first)
Problem is concrete — not refactoring for aesthetics
Scope is bounded — refactoring one thing at a time

During:

One pattern at a time — apply, verify, commit, then next
Tests pass after each step
Behavior unchanged — refactoring changes structure, not behavior

After:

All tests still pass
Code is simpler — if complexity increased, reconsider
Names updated — new structure deserves new names
Dead code deleted — refactoring often reveals unnecessary code

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