refactoring-patterns

name	refactoring-patterns
description	Apply named refactoring transformations to improve code structure without changing behavior. Use when the user mentions "refactor this", "code smells", "extract method", "replace conditional", or "technical debt". Covers smell-driven refactoring, safe transformation sequences, and testing guards. For code quality foundations, see clean-code. For managing complexity, see software-design-philosophy.
license	MIT
metadata	{"author":"wondelai","version":"1.0.0"}

Refactoring Patterns Framework

A disciplined approach to improving the internal structure of existing code without changing its observable behavior. Apply these named transformations when reviewing code, reducing technical debt, or preparing code for new features. Every refactoring follows the same loop: verify tests pass, apply one small structural change, verify tests still pass.

Core Principle

Refactoring is not rewriting. It is a sequence of small, behavior-preserving transformations, each backed by tests. You never change what the code does -- you change how the code is organized. The discipline of taking tiny verified steps is what makes refactoring safe. Big-bang rewrites fail because they combine structural change with behavioral change, making it impossible to know which broke things.

The foundation: Bad code is not a character flaw -- it is a natural consequence of delivering features under time pressure. Code smells are objective signals that structure has degraded. Named refactorings are the proven mechanical recipes for fixing each smell. The catalog of smells tells you where to look; the catalog of refactorings tells you what to do.

Scoring

Goal: 10/10. When reviewing or refactoring code, rate the structural quality 0-10 based on adherence to the principles below. A 10/10 means: no obvious smells remain, each function does one thing, names reveal intent, duplication is eliminated, and the test suite covers the refactored paths. Always provide the current score and specific refactorings needed to reach 10/10.

The Refactoring Patterns Framework

Six areas of focus for systematically improving code structure:

1. Code Smells as Triggers

Core concept: Code smells are surface indicators of deeper structural problems. They are not bugs -- the code works -- but they signal that the design is making the code harder to understand, extend, or maintain. Each smell maps to one or more named refactorings that fix it.

Why it works: Without a shared vocabulary of smells, code review devolves into subjective "I don't like this." Named smells give teams objective criteria: "This is Feature Envy -- the method uses six fields from another class and only one of its own." The name points directly to the fix.

Key insights:

• Smells cluster into five families: Bloaters, Object-Orientation Abusers, Change Preventers, Dispensables, and Couplers
• Long Method is the most common smell and the gateway to most other refactorings
• Duplicate Code is the single biggest driver of maintenance cost
• A method that needs a comment to explain what it does is a smell -- extract and name the block instead
• Shotgun Surgery (one change requires edits in many classes) and Divergent Change (one class changes for many reasons) are opposites that both signal misplaced responsibilities
• Primitive Obsession -- using raw strings, ints, or arrays instead of small domain objects -- causes errors and duplication throughout the codebase

Code applications:

Context	Pattern	Example
Method > 10 lines	Extract Method	Pull the loop body into calculateLineTotal()
Class > 200 lines	Extract Class	Move shipping logic into a ShippingCalculator
Switch on type code	Replace Conditional with Polymorphism	Create subclasses for each order type
Multiple methods use same params	Introduce Parameter Object	Group startDate, endDate into DateRange
Method uses another object's data	Move Method	Move calculateDiscount() to the Customer class
Copy-pasted logic	Extract Method + Pull Up Method	Share via a common method or base class

See: references/smell-catalog.md

2. Composing Methods

Core concept: Most refactoring starts here. Long methods are broken into smaller, well-named pieces. Each extracted piece should do one thing and its name should say what that thing is. The goal is methods you can read like prose -- a sequence of high-level steps, each delegating to a clearly named helper.

Why it works: Short methods with intention-revealing names eliminate the need for comments, make bugs obvious (each method is small enough to verify at a glance), and enable reuse. The cognitive cost of a method call is near zero when the name tells you everything.

Key insights:

• Extract Method is the single most important refactoring -- master it first
• If you feel the urge to write a comment, extract the code block and use the comment as the method name
• Inline Method when a method body is as clear as the name -- indirection without value is noise
• Replace Temp with Query when a temporary variable holds a computed value that is used in multiple places
• Split Temporary Variable when one variable is reused for two different purposes
• Replace Method with Method Object when a method is too tangled to extract from (many local variables referencing each other)

Code applications:

Context	Pattern	Example
Block with a comment	Extract Method	// check eligibility becomes isEligible()
Temp used once	Inline Variable	Remove const price = order.getPrice() if used once
Temp used in multiple places	Replace Temp with Query	Replace let discount = getDiscount() with method calls
Temp assigned twice for different reasons	Split Temporary Variable	Introduce perimeterWidth and perimeterHeight
Trivial delegating method	Inline Method	Inline moreThanFiveDeliveries() if it's return deliveries > 5 and only used once
Complex method with many locals	Replace Method with Method Object	Move the method into its own class where locals become fields

See: references/composing-methods.md

3. Moving Features Between Objects

Core concept: The key decision in object-oriented design is where to put responsibilities. When a method or field is in the wrong class -- evidenced by Feature Envy, excessive coupling, or unbalanced class sizes -- move it to where it belongs.

Why it works: Well-placed responsibilities reduce coupling and increase cohesion. When a method lives in the class whose data it uses, changes to that data affect only one class. Misplaced methods create invisible dependencies that cause Shotgun Surgery.

Key insights:

• Move Method when a method uses more features of another class than its own
• Move Field when a field is used more by another class than the class it lives in
• Extract Class when one class does two things -- split along the axis of change
• Inline Class when a class does too little to justify its existence
• Hide Delegate to enforce the Law of Demeter -- a client shouldn't navigate a chain of objects
• Remove Middle Man when a class does nothing but forward calls
• The tension between Hide Delegate and Remove Middle Man is resolved case by case: hide the delegate when the chain is unstable; remove the middle man when forwarding becomes the entire class

Code applications:

Context	Pattern	Example
Method envies another class	Move Method	Move calculateShipping() from Order to ShippingPolicy
Field used by another class constantly	Move Field	Move discountRate from Order to Customer
God class with 500+ lines	Extract Class	Pull Address fields and methods into their own class
Tiny class with one field	Inline Class	Merge PhoneNumber back into Contact if no behavior
Client calls a.getB().getC()	Hide Delegate	Add a.getCThroughB() so client doesn't know about C
Class only forwards calls	Remove Middle Man	Let client call the delegate directly

See: references/moving-features.md

4. Organizing Data

Core concept: Raw data -- magic numbers, exposed fields, type codes represented as integers, parallel arrays -- creates subtle bugs and scatters domain knowledge. Replace primitive representations with objects that encapsulate behavior and enforce invariants.

Why it works: An int representing a currency amount has no concept of rounding rules, currency codes, or formatting. A Money object encapsulates all of that. When domain concepts are represented as first-class objects, business rules live in one place, validation happens automatically, and the type system catches errors at compile time.

Key insights:

• Replace Magic Number with Symbolic Constant as the simplest data refactoring -- it names the intent
• Replace Data Value with Object (Primitive Obsession cure) -- wrap strings and numbers in domain objects (EmailAddress, Money, Temperature)
• Encapsulate Field -- never expose a raw field; a getter/setter allows you to add validation, logging, or computation later
• Encapsulate Collection -- return an unmodifiable view; never let callers mutate your internal list
• Replace Type Code with Subclasses when the type code affects behavior; use Strategy when subclassing is impractical
• Change Value to Reference when you need identity semantics (one shared Customer object, not copies)

Code applications:

Context	Pattern	Example
if (status == 2)	Replace Magic Number with Symbolic Constant	if (status == ORDER_SHIPPED)
String email passed everywhere	Replace Data Value with Object	Create EmailAddress class with validation
Public field	Encapsulate Field	Replace order.total with order.getTotal()
Getter returns mutable list	Encapsulate Collection	Return Collections.unmodifiableList(items)
int typeCode with switch	Replace Type Code with Subclasses	Employee -> Engineer, Manager, Salesperson
Duplicated customer records	Change Value to Reference	Share one Customer instance via a registry

See: references/organizing-data.md

5. Simplifying Conditional Logic

Core concept: Complex conditionals -- deeply nested if/else trees, long switch statements, null checks scattered everywhere -- are the hardest code to read and the most likely to contain bugs. Named refactorings decompose, consolidate, and replace conditionals with clearer structures.

Why it works: A conditional with six branches and nested sub-conditions requires the reader to simulate every path mentally. Decomposing the condition into well-named methods makes each branch self-documenting. Replacing conditionals with polymorphism eliminates entire categories of "forgot to handle this case" bugs.

Key insights:

• Decompose Conditional: extract the condition, the then-branch, and the else-branch into named methods
• Consolidate Conditional Expression: merge multiple conditions that lead to the same result into one named check
• Replace Nested Conditional with Guard Clauses: handle edge cases early and return, leaving the main path unindented
• Replace Conditional with Polymorphism: the gold standard for type-based conditionals -- each type knows its own behavior
• Introduce Special Case (Null Object): eliminate if (x == null) checks by providing an object that represents "nothing" with safe default behavior
• Introduce Assertion: make assumptions explicit so they fail fast in development

Code applications:

Context	Pattern	Example
Long if with complex condition	Decompose Conditional	Extract isSummer(date) and summerCharge()
Multiple ifs return same value	Consolidate Conditional	Combine into isDisabled() returning early
Deeply nested if/else	Replace with Guard Clauses	Check edge cases first, return early, flatten the main path
Switch on object type	Replace Conditional with Polymorphism	Each type implements its own calculatePay()
if (customer == null) everywhere	Introduce Special Case	Create NullCustomer with default behavior
Hidden assumption in code	Introduce Assertion	assert quantity > 0 at method entry

See: references/simplifying-conditionals.md

6. Safe Refactoring Workflow

Core concept: Refactoring is only safe when wrapped in tests. The workflow is mechanical: run tests (green), apply one small transformation, run tests (green), commit. If tests go red, revert the last change -- don't debug a broken refactoring.

Why it works: Small steps make it trivial to find what went wrong (it was the last thing you did). Reverting a failed step costs seconds. Debugging a failed big-bang rewrite costs days. Frequent commits create save points you can return to.

Key insights:

• The refactoring cycle: test -> refactor -> test -> commit (repeat)
• Rule of Three: tolerate duplication once, note it twice, refactor on the third occurrence
• Preparatory refactoring: restructure code to make the feature easy before adding the feature
• Comprehension refactoring: refactor to understand code as you read it -- leave it clearer than you found it
• Litter-pickup refactoring: small improvements whenever you touch a file (Boy Scout Rule)
• When NOT to refactor: when it's easier to rewrite from scratch, when there are no tests and adding them first isn't feasible, or when the code will be deleted soon
• Refactoring and performance: refactor for clarity first, then profile and optimize the measured bottleneck -- refactored code is easier to tune because the hot path is isolated
• Branch by Abstraction and Parallel Change enable large refactorings in production systems without feature branches

Code applications:

Context	Pattern	Example
About to add a feature	Preparatory Refactoring	Extract method to make new feature's insertion point clean
Reading unfamiliar code	Comprehension Refactoring	Rename variables and extract methods to understand intent
Saw a small issue while working	Litter-Pickup Refactoring	Fix the smell before moving on (Boy Scout Rule)
Third copy of same logic	Rule of Three	Extract the shared logic into a common method
Large API change in production	Branch by Abstraction	Introduce abstraction layer, migrate callers, remove old path
Renaming a widely-used method	Parallel Change	Add new name, deprecate old, migrate callers, remove old

See: references/refactoring-workflow.md

Common Mistakes

Mistake	Why It Fails	Fix
Refactoring without tests	No safety net -- you can't tell if behavior changed	Write characterization tests first, then refactor
Big-bang rewrite instead of incremental steps	Combines structural and behavioral changes; impossible to debug	Take the smallest step possible, run tests after each
Refactoring and adding features at the same time	Two hats at once -- you can't verify either change in isolation	Separate the hats: refactor first (commit), then add feature (commit)
Renaming without updating all callers	Breaks the build or introduces dead code	Use IDE rename refactoring; search for all references
Extracting too many tiny methods	Creates indirection without clarity when names are poor	Each extracted method must have a name that removes the need to read the body
Ignoring the smell catalog	Reinventing fixes instead of applying proven recipes	Learn the named smells; each one maps to specific refactorings
Refactoring code that will be deleted	Wasted effort -- polish on condemned code	Ask first: is this code's lifespan long enough to justify the investment?
Optimizing prematurely during refactoring	Conflates clarity with performance; optimized code is often harder to read	Refactor for clarity first, then profile, then optimize the measured hot path only

Quick Diagnostic

Question	If No	Action
Do tests pass before you start?	You have no safety net	Write or fix tests first -- do not refactor without green tests
Can you name the smell you're fixing?	You're refactoring by instinct, not by catalog	Identify the smell from the catalog, then apply its prescribed refactoring
Is each method under ~10 lines?	Long Methods are likely present	Apply Extract Method to break long methods into named steps
Does each class have a single reason to change?	Divergent Change or Large Class smell	Apply Extract Class to separate responsibilities
Are there duplicated code blocks?	Duplicate Code is the most expensive smell	Extract shared logic into a common method or base class
Do conditionals use polymorphism where appropriate?	Switch Statements or complex if/else trees remain	Apply Replace Conditional with Polymorphism
Are you committing after each refactoring step?	You risk losing work and mixing changes	Commit after every green-to-green transformation
Is the code easier to read after your change?	The refactoring may have added complexity	Revert and try a different approach

Reference Files

• smell-catalog.md: Comprehensive catalog of code smells organized by family -- Bloaters, Object-Orientation Abusers, Change Preventers, Dispensables, and Couplers -- with detection heuristics and fix mappings
• composing-methods.md: Extract Method, Inline Method, Extract Variable, Inline Variable, Replace Temp with Query, Split Temporary Variable, Remove Assignments to Parameters, Replace Method with Method Object -- motivation, mechanics, and examples
• moving-features.md: Move Method, Move Field, Extract Class, Inline Class, Hide Delegate, Remove Middle Man -- when and how to redistribute responsibilities between objects
• organizing-data.md: Replace Data Value with Object, Change Value to Reference, Replace Array with Object, Replace Magic Number, Encapsulate Field, Encapsulate Collection, Replace Type Code with Class/Subclasses/Strategy
• simplifying-conditionals.md: Decompose Conditional, Consolidate Conditional, Replace Nested Conditional with Guard Clauses, Replace Conditional with Polymorphism, Introduce Special Case, Introduce Assertion -- with before/after examples
• refactoring-workflow.md: The refactoring cycle, when to refactor, when NOT to refactor, refactoring and performance, Branch by Abstraction, Parallel Change

Further Reading

This skill is based on the definitive guide to improving the design of existing code:

• "Refactoring: Improving the Design of Existing Code (2nd Edition)" by Martin Fowler
• "Working Effectively with Legacy Code" by Michael Feathers (companion for code without tests)
• "Clean Code: A Handbook of Agile Software Craftsmanship" by Robert C. Martin (complementary naming and style principles)

About the Author

Martin Fowler is the Chief Scientist at Thoughtworks and one of the most influential voices in software engineering. He is the author of Refactoring: Improving the Design of Existing Code (1999, 2nd edition 2018), which introduced the concept of named, catalog-based refactoring transformations to mainstream software development. Fowler is also the author of Patterns of Enterprise Application Architecture, UML Distilled, and numerous influential articles on software design, agile methodology, and continuous delivery. He was a signatory of the Agile Manifesto and has spent decades advocating for evolutionary design -- the practice of continuously improving code structure through disciplined, incremental refactoring rather than upfront big design. His refactoring catalog, originally written in Java, has been adapted to virtually every programming language and is built into the automated refactoring tools of every major IDE.