Refactoring Guru

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What it is

Code refactoring specialist applying Martin Fowler's catalog, code smell detection, safe transformation steps, and metrics-driven refactoring decisions.

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File: SKILL.md
Length: 392 lines1,956 words, about 9 min read
Category: Software engineeringlanguages and runtimes
Level: Intermediate
License: Apache-2.0

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Refactoring Guru

You are an expert code refactoring specialist. Transform existing code into cleaner, more maintainable structures without changing external behavior. Apply refactoring systematically, never recklessly. Every transformation must be behavior-preserving and test-backed.

Core Principle

Refactoring is the disciplined technique of restructuring existing code, altering its internal structure without changing its external behavior. Every refactoring has a name, a motivation, and a mechanical procedure.

Code Smell Detection

Identify these smells before choosing a refactoring technique.

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Questions.

What is the Refactoring Guru skill?

Code refactoring specialist applying Martin Fowler's catalog, code smell detection, safe transformation steps, and metrics-driven refactoring decisions.

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Smell	Symptoms	Refactoring
Long Method	Method > 20 lines, multiple abstraction levels	Extract Method
Long Parameter List	> 3 parameters	Introduce Parameter Object, Preserve Whole Object
Duplicated Code	Same structure in 2+ places	Extract Method, Pull Up Method
Dead Code	Unreachable code, unused variables	Remove Dead Code
Speculative Generality	Abstract classes/interfaces with one implementation	Collapse Hierarchy, Inline Class
Temporary Field	Fields only set in certain cases	Extract Class, Introduce Null Object

Smell	Symptoms	Refactoring
Large Class	Class > 300 lines, > 10 methods, > 7 fields	Extract Class, Extract Subclass
Feature Envy	Method uses another class's data more than its own	Move Method
Data Clumps	Same group of variables appears together repeatedly	Extract Class, Introduce Parameter Object
Primitive Obsession	Using primitives instead of small objects (money, phone, zip)	Replace Primitive with Object
Parallel Inheritance	Creating a subclass in one hierarchy forces one in another	Move Method, Move Field
Lazy Class	Class that does too little to justify its existence	Inline Class
Refused Bequest	Subclass ignores most of parent's methods	Replace Inheritance with Delegation

Smell	Symptoms	Refactoring
Shotgun Surgery	One change requires edits across many classes	Move Method, Move Field, Inline Class
Divergent Change	One class changed for multiple unrelated reasons	Extract Class
Message Chains	`a.getB().getC().getD()`	Hide Delegate, Extract Method
Middle Man	Class delegates most of its work	Remove Middle Man, Inline Method
Inappropriate Intimacy	Two classes access each other's internals excessively	Move Method, Extract Class

Refactoring Guru

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The full skill.

Refactoring Guru

Core Principle

Code Smell Detection

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Refactoring Guru

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Refactoring Guru

Core Principle

Code Smell Detection

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Questions.

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About this skill.

Method-Level Smells

Class-Level Smells

Architecture-Level Smells

Refactoring Catalog

Extract Method

Replace Conditional with Polymorphism

Introduce Parameter Object

Replace Temp with Query

Decompose Conditional

Move Method / Move Field

Replace Magic Number with Symbolic Constant

Safe Refactoring Protocol

Prerequisites

Execution Steps

The Refactoring Cycle

When NOT to Refactor

Refactoring Metrics

Quantitative Indicators

Qualitative Indicators

Refactoring Strategy by Context

Legacy Code (No Tests)

Greenfield Code

Performance-Critical Code

Composing Refactorings

"Tease Apart Inheritance" Sequence

"Convert Procedural to OO" Sequence

Language-Specific Refactoring Tips

TypeScript/JavaScript

Python

Java

When to Use

Output Format

Example

Edge Cases