Refactoring: Improving the Design of Existing Code

The foundational text on restructuring existing code. martin-fowler defines refactoring as changing code structure without altering external behavior, then provides a comprehensive catalog of techniques to do exactly that.

Core Philosophy

Bad code accumulates. Features get added, requirements shift, and clean designs decay into tangled messes. Refactoring offers a systematic way out: small, behavior-preserving transformations that gradually improve structure.

The key insight: you don't need a grand rewrite. Incremental improvements compound. Each refactoring makes the next easier.

The Catalog

Fowler organizes refactorings by what they improve:

• Composing Methods - Extract Method, Inline Method, Replace Temp with Query
• Moving Features - Move Method, Move Field, Extract Class
• Organizing Data - Replace Data Value with Object, Change Value to Reference
• Simplifying Conditionals - Decompose Conditional, Replace Conditional with Polymorphism
• Making Method Calls Simpler - Rename Method, Add Parameter, Replace Parameter with Method

Each technique follows the same format: motivation, mechanics, and example. The mechanics section gives step-by-step instructions safe enough to follow mechanically.

Code Smells

Before refactoring, you need to spot problems. Fowler catalogs "code smells" - symptoms that suggest deeper issues:

• Long Method - methods that try to do too much
• Large Class - classes with too many responsibilities
• Feature Envy - methods that use another class's data more than their own
• Data Clumps - groups of data that travel together
• Shotgun Surgery - one change requires edits across many classes

Smells guide you toward which refactorings to apply.

Connection to Tidy First

Kent Beck's tidy-first builds on this foundation. Where Fowler provides the comprehensive catalog, Beck asks: when should you refactor versus ship? Both books share the same core belief - clean code pays dividends.