Java Refactoring Catalog Skill

A comprehensive, technique-by-technique catalog of refactoring best practices for Java, sourced from Martin Fowler's Refactoring: Improving the Design of Existing Code (2nd Edition) and Alexander Shvets' Refactoring in Java (Refactoring Guru).

Core Philosophy

Refactoring is the process of changing the internal structure of code without altering its observable behavior. It is a disciplined technique, not a random cleanup. The golden rule is: Cover → Modify → Refactor (always have tests before you start).

How to Use This Skill

1. Diagnose first: Identify the code smell (see techniques/00-code-smells-diagnostic.md)
2. Select technique: Each smell maps to one or more refactoring techniques
3. Read the technique file: Each technique has its own markdown file in techniques/ with problem description, motivation, step-by-step mechanics, and Java 8 + Java 11 examples
4. Apply incrementally: Small steps, test after each change, commit frequently

Technique Categories

The techniques are organized in 7 groups. Each technique has its own file in the techniques/ directory.

Group 1: Composing Methods (techniques/01-XX)

Techniques for building clean, well-structured methods. The foundation of all refactoring.

• 01-extract-method.md — Extract a code fragment into a named method
• 02-inline-method.md — Replace a method call with the method body
• 03-extract-variable.md — Give a name to a complex expression
• 04-inline-variable.md — Remove a variable that adds no clarity
• 05-replace-temp-with-query.md — Replace temp variables with method calls
• 06-replace-method-with-method-object.md — Turn a complex method into its own class
• 07-substitute-algorithm.md — Replace an algorithm with a clearer version

Group 2: Moving Features (techniques/02-XX)

Techniques for placing code where it truly belongs.

• 08-move-method.md — Move a method to where it has more cohesion
• 09-move-field.md — Move a field to the class that uses it most
• 10-extract-class.md — Split a class with multiple responsibilities
• 11-inline-class.md — Merge a class that does too little
• 12-hide-delegate.md — Encapsulate chain navigation behind a simpler interface
• 13-remove-middle-man.md — Remove unnecessary delegation
• 14-move-statements.md — Move statements into/out of functions, slide statements
• 15-split-loop.md — Separate a loop that does multiple things
• 16-replace-loop-with-pipeline.md — Use Stream API instead of imperative loops
• 17-remove-dead-code.md — Delete unused code

Group 3: Organizing Data (techniques/03-XX)

Techniques for enriching data with behavior and protecting internal state.

• 18-encapsulate-variable.md — Wrap data access with getters/setters
• 19-encapsulate-record.md — Convert data structures into objects
• 20-encapsulate-collection.md — Protect collections from external mutation
• 21-replace-primitive-with-object.md — Create domain types instead of using raw primitives
• 22-split-variable.md — Give each purpose its own variable
• 23-rename-field.md — Improve field names for clarity
• 24-replace-derived-variable-with-query.md — Calculate values on demand
• 25-change-reference-to-value.md — Make objects immutable (Value Objects)
• 26-change-value-to-reference.md — Share a single instance across consumers
• 27-replace-type-code-with-subclasses.md — Convert type codes to polymorphic hierarchy

Group 4: Simplifying Conditionals (techniques/04-XX)

Techniques for taming conditional complexity.

• 28-decompose-conditional.md — Name condition and branches
• 29-consolidate-conditional.md — Merge related conditions
• 30-replace-nested-conditional-with-guard-clauses.md — Early returns for special cases
• 31-replace-conditional-with-polymorphism.md — Use OOP instead of switch/if-type
• 32-introduce-special-case.md — Null Object pattern for default behavior
• 33-introduce-assertion.md — Document invariants with executable assertions
• 34-replace-control-flag.md — Replace boolean flags with break/return

Group 5: Simplifying Method Calls / API Design (techniques/05-XX)

Techniques for building self-documenting interfaces.

• 35-change-function-declaration.md — Rename functions and change parameters
• 36-introduce-parameter-object.md — Group related parameters into an object
• 37-parameterize-function.md — Unify similar functions with a parameter
• 38-remove-flag-argument.md — Replace boolean params with named methods
• 39-preserve-whole-object.md — Pass the object instead of extracted values
• 40-replace-parameter-with-query.md — Let the function calculate what it needs
• 41-replace-query-with-parameter.md — Pass value as param for purity/testability
• 42-remove-setting-method.md — Make properties read-only
• 43-replace-constructor-with-factory.md — Use factory methods for flexible creation
• 44-replace-function-with-command.md — Encapsulate function as object
• 45-separate-query-from-modifier.md — CQS: separate reads from writes

Group 6: Dealing with Inheritance (techniques/06-XX)

Techniques for refactoring class hierarchies.

• 46-pull-up-method.md — Move duplicated methods to superclass
• 47-push-down-method.md — Move specialized methods to subclass
• 48-pull-up-constructor-body.md — Unify constructor initialization
• 49-extract-superclass.md — Create common superclass for shared behavior
• 50-extract-interface.md — Define a contract without implementation
• 51-collapse-hierarchy.md — Merge unnecessary hierarchy levels
• 52-form-template-method.md — Template Method pattern from GoF
• 53-replace-subclass-with-delegate.md — Composition over inheritance
• 54-replace-superclass-with-delegate.md — Replace extends with has-a
• 55-replace-inheritance-with-delegation.md — General inheritance to delegation

Group 7: Additional Techniques (techniques/07-XX)

Cross-cutting techniques from both sources.

• 56-combine-functions-into-class.md — Group functions that share data
• 57-combine-functions-into-transform.md — Enrich read-only data
• 58-split-phase.md — Separate code into processing phases
• 59-introduce-foreign-method.md — Extend third-party classes you can't modify
• 60-introduce-local-extension.md — Subclass or wrapper for library extension
• 61-replace-error-code-with-exception.md — Modernize error handling
• 62-replace-exception-with-test.md — Don't use exceptions for control flow

Diagnostic Guide

Start with techniques/00-code-smells-diagnostic.md to identify which techniques apply to your code. The diagnostic maps 24 code smells to their recommended refactoring techniques.

Applying the Skill

When given Java code to refactor:

1. Read techniques/00-code-smells-diagnostic.md to identify the smells present
2. For each identified smell, read the corresponding technique file(s)
3. Apply techniques in small steps, always testing between changes
4. Provide both Java 8 and Java 11 versions when the language features differ meaningfully
5. Explain WHY each refactoring improves the code, not just HOW to do it

Key Principles

These principles underpin every technique in the catalog:

1. Names matter more than length — a well-named 1-line method is better than an inline expression
2. Small steps — extract small fragments, test, commit. Never batch multiple changes
3. Intention over implementation — code should communicate WHAT, not HOW
4. Data and logic that change together should live together — cohesion is king
5. Prefer composition over inheritance — delegation is more flexible than extends
6. Immutability is a powerful preservative — immutable data is easier to reason about
7. CQS (Command-Query Separation) — a method either returns a value or modifies state, never both