What is Kotlin? — Complete Beginner's Guide

In this tutorial, you will learn about What is Kotlin?. We cover key concepts, practical examples, and best practices to help you master this topic.

Kotlin is a modern statically typed programming language that runs on the Java Virtual Machine, compiles to JavaScript, and targets native platforms through LLVM, making it versatile for Android, backend, and cross-platform development.

What You'll Learn

• What Kotlin is and why JetBrains created it
• Key features: null safety, extension functions, data classes, coroutines
• How Kotlin interoperates with Java
• Kotlin Multiplatform and its target platforms
• Real-world adoption and use cases
• How to get started with Kotlin development

Why It Matters

Kotlin is the official language for Android development, endorsed by Google. It solves long-standing Java pain points: null pointer exceptions, verbose syntax, and lack of functional programming support. Companies like Google, Netflix, Trello, and Square use Kotlin in production. Kotlin also powers server-side applications through frameworks like Ktor and Spring Boot, and cross-platform apps through Kotlin Multiplatform. Learning Kotlin opens doors to Android development, backend engineering, and multiplatform projects with a single language.

Real-World Use

DodaTech uses Kotlin for its Android configuration utility and internal API services. The same business logic compiles to both Android and backend JARs, reducing duplication. Trello rewrote their Android app in Kotlin and reported a 30% reduction in code size. Netflix uses Kotlin extensively for their Android app, citing improved safety and developer productivity over Java.

Learning Path

flowchart LR
  A[Start Here] --> B[What is Kotlin?\nYou are here]
  B --> C[Installation & Setup]
  style B fill:#f90,color:#fff

What Makes Kotlin Special

Kotlin was created by JetBrains, the company behind IntelliJ IDEA. It first appeared in 2011 and reached version 1.0 in 2016. Google announced first-class support for Kotlin on Android in 2017, and it became the recommended language in 2019.

Null Safety

The most frequent cause of crashes in Java is NullPointerException. Kotlin eliminates this by making null part of the type system.

var name: String = "Alice"   // Cannot be null
var nullableName: String? = null  // Can be null

fun getLength(text: String?): Int {
    return text?.length ?: 0  // Safe call with elvis operator
}

fun main() {
    println(getLength(nullableName))  // Output: 0
    println(getLength("Hello"))       // Output: 5
}

Output: The function returns 0 when text is null and 5 when it contains "Hello", without any risk of NullPointerException.

Concise Syntax

Kotlin removes boilerplate common in Java. Data classes replace getters, setters, equals, hashCode, and toString in one line.

data class User(val id: Int, val name: String, val email: String)

fun main() {
    val user = User(1, "Alice", "alice@example.com")
    println(user)  // Output: User(id=1, name=Alice, email=alice@example.com)
    println(user.component1())  // Output: 1
}

Output: The data class automatically generates toString, component functions, copy, equals, and hashCode.

Functional Programming

Kotlin supports higher-order functions, lambdas, and immutable data.

fun main() {
    val numbers = listOf(1, 2, 3, 4, 5, 6)
    val evenSquares = numbers
        .filter { it % 2 == 0 }
        .map { it * it }
    
    println(evenSquares)  // Output: [4, 16, 36]
    
    val sum = numbers.reduce { acc, n -> acc + n }
    println(sum)  // Output: 21
}

Output: The filter and map operations return a new list without modifying the original. reduce computes the sum of all elements.

Kotlin Platforms

Kotlin targets multiple platforms through different compiler backends.

Kotlin Multiplatform (KMP) lets you write shared code once and use it on Android, iOS, and web. Platform-specific code goes in expect/actual declarations.

Interoperability with Java

Kotlin compiles to JVM bytecode. You can call Java code from Kotlin and Kotlin code from Java seamlessly.

// Calling Java from Kotlin
import java.util.Date

fun main() {
    val date = Date()
    println("Current time: $date")
}

Java can call Kotlin code with some annotations to handle Kotlin-specific features like default parameters and nullability. The @JvmOverloads annotation generates Java overloads for functions with default parameters.

Common Mistakes

1. Overusing the !! operator: The double-bang operator throws a NullPointerException if the value is null. It defeats Kotlin's null safety. Use safe calls (?.) and the elvis operator (?:) instead.

2. Treating Kotlin as "Java with different syntax": Kotlin has its own idioms. Using Java patterns like getters/setters, utility classes with static methods, or raw loops instead of collection operations leads to unidiomatic code.

3. Ignoring visibility modifiers: Kotlin's visibility defaults to public. For library code, mark internal or private anything that should not be exposed to consumers.

4. Not using data classes for model objects: Writing manual equals, hashCode, and toString in Kotlin is wasteful. Data classes generate them automatically.

5. Forgetting semicolons are optional: Mixing semicolons inconsistently is harmless but looks messy. Pick one style and stick with it.

6. Misunderstanding val versus var: val is read-only (immutable reference), not necessarily immutable content. A val list can still have elements added. Use val by default and var only when reassignment is needed.

7. Calling Kotlin from Java without nullability annotations: Java sees all Kotlin types as platform types. Add @Nullable and @NotNull annotations for clean interop.

Practice Questions

1. What is the difference between val and var in Kotlin?

Answer: val declares an immutable reference (cannot be reassigned, like final in Java). var declares a mutable reference that can be reassigned. Both can point to mutable objects.

2. How does Kotlin prevent null pointer exceptions?

Answer: Kotlin makes nullability part of the type system. Types ending with ? are nullable and require safe access with ?. or !! operators. Non-nullable types cannot hold null, preventing NPEs at compile time.

3. What platforms can Kotlin target?

Answer: Kotlin targets JVM, JavaScript, Native (iOS, macOS, Windows, Linux), and WebAssembly through Kotlin Multiplatform.

4. What is a data class and what does it generate automatically?

Answer: A data class is a class marked with the data keyword. It automatically generates equals, hashCode, toString, copy, and componentN functions based on constructor properties.

5. Challenge: Write a Kotlin program that processes a list of strings, filters out nulls and empty strings, converts the rest to uppercase, and prints them sorted by length.

Answer:

fun main() {
    val items = listOf("apple", null, "", "BANANA", "cherry", null, "date")
    val result = items
        .filterNotNull()
        .filter { it.isNotEmpty() }
        .map { it.uppercase() }
        .sortedBy { it.length }
    
    println(result)  // Output: [DATE, APPLE, CHERRY, BANANA]
}

Mini Project

Create a command-line user directory that stores contacts and searches them. Requirements:

• Define a data class Contact with name, phone, and email properties
• Store contacts in a mutable list
• Implement search by name (case-insensitive substring match)
• Handle nullable phone and email fields with safe access
• Use functional operations (filter, map, forEach) for data processing
• Write unit tests with kotlin.test

This project practices data classes, null safety, collections, and functional programming all at once.

FAQ

What's Next

Now that you understand what Kotlin is, proceed to installation and setup to get your development environment ready. You can also explore Kotlin basics for a hands-on introduction to syntax and language features.