Kotlin — Refactoring Masterclass

★ A hand-authored, 30-lesson course built around Martin Fowler's *Refactoring* catalogue, retold for idiomatic Kotlin. Every challenge gives you a real-world smelly snippet (long parameter lists, magic strings, nested if-pyramids, scattered guards, leaked mutable collections) and asks you to apply a named refactor — Extract Function, Decompose Conditional, Replace Primitive with Object, Replace Loop with `map`, Replace `if` chain with `when`, and 25 more. The headline UX feature: **progressive hint chains**. Each challenge ships 3-6 mechanical step hints that the magic-hint button reveals one at a time, so a refactor reads as a granular checklist instead of a before-after cliff. Behaviour-preserving tests verify each refactor with stdin/stdout. 30 lessons · 30 challenges · ~120 step hints · culminates in a 5-refactor capstone on a production-flavoured order validator.

51 challenges64 topics

How it works1Pick a challenge2Read the requirements3Code in a full IDE4Run the tests

51 challenges

What is refactoring?

Code smells — the catalog of "something's off"

Smell → Refactor reference table

Extract Function

Inline Function

Extract Variable

Inline Variable

Rename Function / Variable / Class

Replace Magic Number with Named Constant

Encapsulate Collection

Move Statements — slide related code together

Move Function — fix Feature Envy

Move Field — relocate state to where it's used

Split Phase — separate parsing from processing

Decompose Conditional

Consolidate Conditional Expression

Replace Nested Conditional with Guard Clauses

Replace Conditional with Polymorphism

Substitute Algorithm — swap the body, preserve the contract

Replace Nullable Chain with Kotlin null-safety

Replace Primitive with Object

Replace Magic String with Enum / sealed class

Combine Functions into Class

Replace Constructor with Factory Function

Extract Class — split a god class along a seam

Parameterize Function

Remove Flag Argument

Preserve Whole Object

Replace Error Code with Result type

Replace Loop with map / filter / sumOf

Split Loop — one pass, one purpose

Replace if-else chain with `when`

Replace setter boilerplate with `apply { }`

Replace `if (x != null)` with `?.let { }`

Replace utility function with extension function

Capstone — Order validation: 5 refactors in sequence

Capstone II — Legacy ETL pipeline: 6 refactors in sequence

Replace Temp with Query

Introduce Parameter Object

Replace Inheritance with Delegation (`by`)

Replace eager chain with a lazy Sequence

Replace try/catch with `runCatching`

Replace mutable state with immutability + `copy`

Replace callback with a returned stream

Replace Conditional Dispatch with a Lookup Map

Replace Accumulator Loop with fold

Replace Nested Loops with flatMap

Replace Temp Plumbing with Scope-Function Chains

Extract Interface to Create a Testing Seam

Replace Boolean+Message Pairs with a Sealed Result

Wrap-up — the refactorer's mindset