# Modularization

> Use when writing or changing any class, object, function, property, or constant in a multi-module Android/Kotlin project (settings.gradle(.kts) includes more than one module — :app, :core:*, :feature:*, :data, etc.). Symptoms — a top-level declaration left at Kotlin's default public visibility; an implementation class, mapper, helper, constant, or UI-state holder reachable from another module though only used within its own; reaching for public "because the caller needs it"; "should this be public or internal?".

- Skill: `rcosteira79/modularization` (Agent Skill)
- Install (CLI): `npx skillmds@latest add rcosteira79/modularization`
- Raw SKILL.md: https://api.skillmd.com/api/skills/rcosteira79/modularization/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: rcosteira79 (https://skillmd.com/u/rcosteira79)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/rcosteira79/modularization

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# Modularization

**Declare everything at the lowest visibility that still compiles.** Start `private`; widen to `internal` only when another file in the *same module* needs it; widen to `public` only when a *different module* actually consumes it. Kotlin defaults to `public`, and in a multi-module project that default is usually wrong — a `public` symbol joins the module's API surface: it leaks implementation across the boundary and you are then obliged to keep it stable.

- Public interface as the module's API → the interface is `public`; its **implementation class stays `internal`** (DI-bound, never named from another module).
- Mapper / helper / extension / constant / UI-state holder used only inside the module → `internal` (or `private` if a single file uses it). "Rendered by a composable in this module" does **not** require `public` — `internal` is visible to the whole module.
- Member used only within its class → `private`.

Widening a symbol to `public` is a decision, not a default: confirm a real cross-module consumer first.