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Kotlin Learning Plan for Java Developers

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Goal

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

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Week 1: Kotlin Foundations

Recommended pace: 1–1.5 hours/day

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Day 1–2: Setup + Syntax Crash Course

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Tasks

  • Install IntelliJ IDEA Community Edition
  • Explore Kotlin Playground
  • Learn:
    • fun main()
    • val vs var
    • Type inference
    • Functions
    • String templates

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Resources

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Practice

Create:
  • Calculator
  • Temperature converter
  • String formatter

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Day 3: Type System + Null Safety

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Topics

  • Nullable vs non-nullable types
  • Safe calls ?.
  • Elvis operator ?:
  • Non-null assertion !!
  • Smart casts

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Resources

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Practice

Convert Java methods using null checks into Kotlin.

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Day 4: Classes & Objects

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Topics

  • Primary constructors
  • Secondary constructors
  • init blocks
  • data class
  • object
  • companion object

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Resources

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Practice

Build:
  • User
  • Product
  • Order
data classes.

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Day 5–6: Collections + Lambdas

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Topics

  • listOf
  • mutableListOf
  • Maps/Sets
  • Lambdas
  • Functional operations:
    • map
    • filter
    • reduce
    • fold

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Resources

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Practice

Convert Java loops into Kotlin functional style.

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Day 7: Project Warm-up

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Tasks

  • Convert Java classes to Kotlin using IntelliJ
  • Build a small utility app

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Suggested Mini Projects

  • Notes manager
  • Task tracker
  • Expense calculator

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Week 2: Intermediate Kotlin

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Day 8–9: Kotlin Idioms

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Topics

  • when
  • Smart casts
  • Extension functions
  • Scope functions:
    • let
    • run
    • apply
    • also
    • with

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Resources

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Practice

Refactor verbose Java-style Kotlin into idiomatic Kotlin.

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Day 10–11: Java Interoperability

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Topics

  • Calling Java from Kotlin
  • Calling Kotlin from Java
  • JVM annotations:
    • @JvmStatic
    • @JvmOverloads
    • @JvmField
    • @file:JvmName

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Resources

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Practice

Integrate a Java library into a Kotlin project.

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Day 12: Functional Kotlin Deep Dive

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Topics

  • Higher-order functions
  • Inline functions
  • Sequences
  • Collection pipelines

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Practice

Build reusable utility functions.

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Day 13–14: Mini Kotlin Application

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Goal

Build a complete Kotlin console application.

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Suggested Projects

  • TODO application
  • Weather API client
  • JSON parser
  • File organizer

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Focus Areas

  • Kotlin idioms
  • Collections
  • Object design
  • Error handling

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Week 3: Advanced Kotlin (Optional)

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Coroutines

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Topics

  • suspend
  • launch
  • async
  • withContext
  • Structured concurrency

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Resources

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Practice

Create asynchronous API calls.

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Testing

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Topics

  • JUnit 5
  • Kotest
  • MockK

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Practice

Write unit tests for previous projects.

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DSLs + Advanced Features

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Topics

  • Type-safe builders
  • DSL creation
  • Reflection
  • Delegates

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Recommended Tools

| Tool | Purpose | |------|---------| | IntelliJ IDEA | Kotlin IDE | | Gradle Kotlin DSL | Build system | | Kotlin Playground | Practice | | GitHub | Version control | | Kotest | Testing | | MockK | Mocking |

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Recommended Books

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Beginner to Intermediate

  • Kotlin in Action
  • Head First Kotlin

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Intermediate to Advanced

  • Programming Kotlin
  • Effective Kotlin

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Recommended YouTube Channels

  • Philipp Lackner
  • Kotlin by JetBrains
  • Coding in Flow

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Final Goal Project Ideas

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Backend

  • REST API with Ktor or Spring Boot

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Android

  • Notes app with Jetpack Compose

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CLI

  • File organizer
  • Task manager

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General

  • JSON parser
  • Expense tracker

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Milestone Checklist

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Beginner

  • [ ] Understand Kotlin syntax
  • [ ] Use null safety confidently
  • [ ] Write data classes
  • [ ] Use collections and lambdas

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Intermediate

  • [ ] Write idiomatic Kotlin
  • [ ] Use extension functions
  • [ ] Understand Java interop
  • [ ] Build a complete app

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Advanced

  • [ ] Use coroutines
  • [ ] Write tests
  • [ ] Understand DSLs
  • [ ] Build production-ready apps

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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)

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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.