> ## Documentation Index
> Fetch the complete documentation index at: https://docs.americ.io.vn/llms.txt
> Use this file to discover all available pages before exploring further.

# Essential Kotlin

# Kotlin Learning Plan for Java Developers

## Goal

Become confident writing idiomatic Kotlin code, understand Java interoperability, and build Kotlin-first applications efficiently.

***

# Week 1: Kotlin Foundations

Recommended pace: 1–1.5 hours/day

***

## Day 1–2: Setup + Syntax Crash Course

### Tasks

* Install IntelliJ IDEA Community Edition

* Explore Kotlin Playground

* Learn:

  * `fun main()`

  * `val` vs `var`

  * Type inference

  * Functions

  * String templates

### Resources

* [https://play.kotlinlang.org/koans](https://play.kotlinlang.org/koans)

* [https://kotlinlang.org/docs/basic-syntax.html](https://kotlinlang.org/docs/basic-syntax.html)

### Practice

Create:

* Calculator

* Temperature converter

* String formatter

***

## Day 3: Type System + Null Safety

### Topics

* Nullable vs non-nullable types

* Safe calls `?.`

* Elvis operator `?:`

* Non-null assertion `!!`

* Smart casts

### Resources

* [https://kotlinlang.org/docs/null-safety.html](https://kotlinlang.org/docs/null-safety.html)

### Practice

Convert Java methods using null checks into Kotlin.

***

## Day 4: Classes & Objects

### Topics

* Primary constructors

* Secondary constructors

* `init` blocks

* `data class`

* `object`

* `companion object`

### Resources

* [https://kotlinlang.org/docs/classes.html](https://kotlinlang.org/docs/classes.html)

### Practice

Build:

* `User`

* `Product`

* `Order`

data classes.

***

## Day 5–6: Collections + Lambdas

### Topics

* `listOf`

* `mutableListOf`

* Maps/Sets

* Lambdas

* Functional operations:

  * `map`

  * `filter`

  * `reduce`

  * `fold`

### Resources

* [https://kotlinlang.org/docs/collections-overview.html](https://kotlinlang.org/docs/collections-overview.html)

### Practice

Convert Java loops into Kotlin functional style.

***

## Day 7: Project Warm-up

### Tasks

* Convert Java classes to Kotlin using IntelliJ

* Build a small utility app

### Suggested Mini Projects

* Notes manager

* Task tracker

* Expense calculator

***

# Week 2: Intermediate Kotlin

***

## Day 8–9: Kotlin Idioms

### Topics

* `when`

* Smart casts

* Extension functions

* Scope functions:

  * `let`

  * `run`

  * `apply`

  * `also`

  * `with`

### Resources

* [https://kotlinlang.org/docs/idioms.html](https://kotlinlang.org/docs/idioms.html)

### Practice

Refactor verbose Java-style Kotlin into idiomatic Kotlin.

***

## Day 10–11: Java Interoperability

### Topics

* Calling Java from Kotlin

* Calling Kotlin from Java

* JVM annotations:

  * `@JvmStatic`

  * `@JvmOverloads`

  * `@JvmField`

  * `@file:JvmName`

### Resources

* [https://kotlinlang.org/docs/java-interop.html](https://kotlinlang.org/docs/java-interop.html)

### Practice

Integrate a Java library into a Kotlin project.

***

## Day 12: Functional Kotlin Deep Dive

### Topics

* Higher-order functions

* Inline functions

* Sequences

* Collection pipelines

### Practice

Build reusable utility functions.

***

## Day 13–14: Mini Kotlin Application

### Goal

Build a complete Kotlin console application.

### Suggested Projects

* TODO application

* Weather API client

* JSON parser

* File organizer

### Focus Areas

* Kotlin idioms

* Collections

* Object design

* Error handling

***

# Week 3: Advanced Kotlin (Optional)

***

## Coroutines

### Topics

* `suspend`

* `launch`

* `async`

* `withContext`

* Structured concurrency

### Resources

* [https://kotlinlang.org/docs/coroutines-overview.html](https://kotlinlang.org/docs/coroutines-overview.html)

### Practice

Create asynchronous API calls.

***

## Testing

### Topics

* JUnit 5

* Kotest

* MockK

### Practice

Write unit tests for previous projects.

***

## DSLs + Advanced Features

### Topics

* Type-safe builders

* DSL creation

* Reflection

* Delegates

***

# Recommended Tools

\| Tool | Purpose |

\|------|---------|

\| IntelliJ IDEA | Kotlin IDE |

\| Gradle Kotlin DSL | Build system |

\| Kotlin Playground | Practice |

\| GitHub | Version control |

\| Kotest | Testing |

\| MockK | Mocking |

***

# Recommended Books

## Beginner to Intermediate

* Kotlin in Action

* Head First Kotlin

## Intermediate to Advanced

* Programming Kotlin

* Effective Kotlin

***

# Recommended YouTube Channels

* Philipp Lackner

* Kotlin by JetBrains

* Coding in Flow

***

# Final Goal Project Ideas

## Backend

* REST API with Ktor or Spring Boot

## Android

* Notes app with Jetpack Compose

## CLI

* File organizer

* Task manager

## General

* JSON parser

* Expense tracker

***

# Milestone Checklist

## Beginner

* \[ ] Understand Kotlin syntax

* \[ ] Use null safety confidently

* \[ ] Write data classes

* \[ ] Use collections and lambdas

## Intermediate

* \[ ] Write idiomatic Kotlin

* \[ ] Use extension functions

* \[ ] Understand Java interop

* \[ ] Build a complete app

## Advanced

* \[ ] Use coroutines

* \[ ] Write tests

* \[ ] Understand DSLs

* \[ ] Build production-ready apps

***

# Suggested Daily Routine

1. Read documentation (15 min)

2. Watch tutorial/video (20 min)

3. Code exercises (30–45 min)

4. Refactor old Java code into Kotlin (15 min)

***

# Important Mindset Shift for Java Developers

Do not write Java in Kotlin.

Focus on:

* Immutability

* Expression-based coding

* Functional collection operations

* Concise APIs

* Kotlin idioms

Idiomatic Kotlin is simpler, safer, and more expressive than direct Java translations.