🎯 Kotlin Handbook

Overview

Kotlin is a modern, statically typed JVM language developed by JetBrains. It is fully interoperable with Java and is the preferred language for Android development. Also used for server-side development.

Platform JVM, Android, JS, Native  Paradigm OOP, Functional  Typing Static

Core Concepts

Variables

• val — immutable (like Java final)
• var — mutable
• Type inference: val name = "HCL"
• Null safety: String? vs String
• Safe call: name?.length, Elvis: name ?: "default"

Functions

• fun greet(name: String): String
• Default & named parameters
• Extension functions: fun String.shout() = uppercase()
• Lambda: val square = { x: Int -> x * x }
• Inline functions for performance

Classes

• Data class: data class User(val id: Int, val name: String)
• Sealed class for restricted hierarchies
• Object (singleton): object Config { ... }
• Companion object (static-like members)

Coroutines (Async)

• suspend fun — suspending functions
• launch — fire and forget coroutine
• async / await — concurrent with result
• Flow — cold asynchronous streams
• Structured concurrency with CoroutineScope

Key Frameworks

Category	Framework / Library
Android	Android SDK, Jetpack Compose
Server-side	Ktor, Spring Boot
Testing	JUnit5, Kotest, MockK
DI	Koin, Hilt (Android)
Build	Gradle (Kotlin DSL)

Common Interview Topics

• Null safety and !! vs ?.
• Data class vs regular class
• Coroutines vs threads
• Sealed class use cases
• Kotlin vs Java interoperability
• Higher-order functions and lambdas
• Scope functions: let, run, apply, also, with