Kotlin by Example

Kotlin by Example is a hands-on introduction to Kotlin via annotated example programs. It is inspired by Go by Example and zig-by-example: read the code, read the notes, run it yourself.

Kotlin is a concise, statically-typed language for the JVM (also Native, JS, and Wasm). It emphasizes null safety, conciseness, and seamless Java interop, letting you write expressive code that runs anywhere the JVM does and beyond.

Getting started

The examples are run with the Kotlin toolchain and its kotlin command-line tool.

0. Install the toolchain

macOS / Linux:

curl -fsSL https://kotl.in/install.sh | sh

Windows (PowerShell):

powershell -ExecutionPolicy ByPass -c "irm 'https://kotl.in/install.ps1' | iex"

You can also sdk install kotlintoolchain via SDKMAN. Check it works with kotlin --version.

1. Create a scratch project

The kotlin tool runs projects, not loose .kt files. Set up one small project and reuse it for every example:

my-kotlin/
├─ module.yaml
└─ src/
   └─ main.kt

module.yaml:

product: jvm/app

2. Run an example

Paste an example's code into src/main.kt, then run it from the project root:

$ kotlin run
Hello, World!

kotlin run also prints a couple of INFO compiler lines as it builds; add --log-level=warn (kotlin --log-level=warn run) to show only your program's output, as the examples below do. Pass command-line arguments after --, e.g. kotlin run -- Alice 42.

3. Add dependencies when needed

A few examples (coroutines, serialization, reflection, testing) need extra libraries. Declare them in module.yaml:

product: jvm/app

dependencies:
  - org.jetbrains.kotlinx:kotlinx-coroutines-core:1.10.2

Each example notes which dependency it requires. See the Kotlin toolchain tutorial for more.

Examples

Basics

1. Hello World
2. Values
3. Variables
4. Basic Types & Numbers
5. Type Checks & Casts
6. Strings & Templates
7. Null Safety
8. Equality

Control Flow

9. If & When
10. Ranges & Progressions
11. Loops & Labels
12. Functions
13. Default & Named Arguments
14. Varargs
15. Infix Functions
16. Recursion & tailrec
17. Lambdas & Higher-Order Functions
18. Closures

Collections

19. Arrays
20. Lists
21. Sets
22. Maps
23. Collection Operations
24. Grouping & Aggregation
25. Sequences

Types & OOP

26. Classes & Constructors
27. Properties & Accessors
28. Inheritance & Overriding
29. Abstract Classes
30. Interfaces
31. Visibility Modifiers
32. Data Classes
33. Enum Classes
34. Sealed Classes & Interfaces
35. Objects & Companion Objects
36. Nested & Inner Classes
37. Extension Functions & Properties
38. Generics & Variance
39. Reified Type Parameters
40. Type Aliases
41. Inline Value Classes
42. Operator Overloading
43. Annotations
44. Reflection

Functional & Advanced

45. Scope Functions
46. Destructuring Declarations
47. Class Delegation
48. Property Delegation
49. Inline Functions
50. Exceptions
51. Result & runCatching

Concurrency

52. Coroutines Basics
53. Suspending Functions
54. async/await & Structured Concurrency
55. Dispatchers & Context
56. Channels
57. Flows

Practicals

58. Reading & Writing Files
59. JSON Serialization
60. Regular Expressions
61. String Formatting
62. Sorting & Comparators
63. Date & Time
64. Random
65. Command-Line Arguments
66. Building a Type-Safe DSL
67. Testing

Resources

• Kotlin docs
• Kotlin Playground
• Kotlin Koans
• Standard library reference

License

Released under CC BY 4.0.