Kotlin vs Swift Comparison : Which One Should I learn?

Mobile app development continues to evolve rapidly, with Kotlin and Swift emerging as the preferred languages for Android and iOS development, respectively. Both languages were created to address shortcomings in their predecessors (Java and Objective-C) and offer modern features that enhance developer productivity and code safety.

In this comprehensive comparison, we’ll examine how Kotlin and Swift stack up against each other across multiple dimensions including syntax, performance, ecosystem, and learning curve. Whether you’re deciding which language to learn next or planning a cross-platform project, this guide will help you understand the similarities and differences between these powerful programming languages.

Language Origins and Adoption

Kotlin gained significant momentum when Google announced first-class support for it in 2017, while Swift has been Apple’s recommended language for iOS development since its introduction in 2014.

Syntax Comparison

Basic Variable Declaration

Kotlin :

// Kotlin
val immutableVariable = "Cannot be changed"  // Type inferred
var mutableVariable = 42                     // Type inferred
var explicitType: String = "Type specified"

Swift:

// Swift
let immutableVariable = "Cannot be changed"  // Type inferred
var mutableVariable = 42                     // Type inferred
var explicitType: String = "Type specified"

Both languages use type inference and have similar syntax for declaring variables. Kotlin uses val for immutable variables, while Swift uses let.

Functions

Kotlin:

// Kotlin
fun add(a: Int, b: Int): Int {
    return a + b
}

// Kotlin one-liner
fun multiply(a: Int, b: Int) = a * b

Swift:

// Swift
func add(a: Int, b: Int) -> Int {
    return a + b
}

// Swift one-liner
func multiply(a: Int, b: Int) -> Int { a * b }

Null Safety

Both languages prioritize null safety but implement it differently:

More about Kotlin Null Safety

Kotlin

// Kotlin
var nullableString: String? = "Hello"
nullableString = null  // Valid

// Non-nullable variable
var nonNullString: String = "Hello"
// nonNullString = null  // Compilation error

// Safe call operator
val length = nullableString?.length  // Returns null if nullableString is null

// Elvis operator (null coalescing)
val nonNullLength = nullableString?.length ?: 0  // Returns 0 if nullableString is null


// Force unwrap (not recommended)
val forcedLength = nullableString!!.length  // Throws NullPointerException if null

Swift

// Swift
var nullableString: String? = "Hello"
nullableString = nil  // Valid

// Non-nullable variable
var nonNullString: String = "Hello"
// nonNullString = nil  // Compilation error

// Optional binding
if let safeString = nullableString {
    print(safeString.count)  // Safely accessed
}

// Nil coalescing operator
let nonNullLength = nullableString?.count ?? 0  // Returns 0 if nullableString is nil

// Force unwrap (not recommended)
let forcedLength = nullableString!.count  // Crashes if nil

In both Kotlin and Swift, there’s a distinction between variables that can hold null values (nullable) and those that cannot (non-nullable).

For the line var nonNullString: String = “Hello”:

• var indicates this is a mutable variable that can be changed later.
• nonNullString is the variable name.
• String (without any question mark or optional indicator) specifies that this is a non-nullable String type.
• “Hello” is the initial value assigned to the variable.

The key aspect here is that when you declare a variable as a non-nullable type (just String rather than String? in Kotlin or String? in Swift), the compiler enforces that this variable can never hold a null/nil value.
The commented line // nonNullString = null // Compilation error is telling you what would happen if you tried to assign null to this variable:

In Kotlin: If you wrote nonNullString = null, the compiler would produce an error like “Null can not be a value of a non-null type String”
In Swift: If you wrote nonNullString = nil, the compiler would give an error like “Nil cannot be assigned to type ‘String'”

This is one of the core safety features in both languages. By distinguishing between nullable and non-nullable types at compile time, these languages help prevent the infamous NullPointerException (in Java) or “unexpectedly found nil” runtime crashes (in Objective-C).

This is different from languages like Java (pre-Java 8) or Objective-C where any reference type could potentially be null/nil, leading to runtime crashes when developers forgot to check for null values before accessing properties or methods.

Both Kotlin and Swift force developers to explicitly handle the possibility of null values in their code, making applications more robust and less prone to runtime crashes.

Classes and Properties

Kotlin:

// Kotlin
class Person(val name: String, var age: Int) {
    // Constructor parameters automatically become properties
    
    // Computed property
    val isAdult: Boolean
        get() = age >= 18
        
    // Custom setter/getter
    var email: String = ""
        set(value) {
            if (!value.contains("@")) {
                throw IllegalArgumentException("Invalid email")
            }
            field = value
        }
}

In Kotlin, the primary constructor is part of the class header. When you add val or var before parameters, these automatically become properties:

• val name: String creates an immutable property (can’t be changed after initialization)
• var age: Int creates a mutable property (can be changed later)

This concise syntax saves you from writing boilerplate code. You don’t need to manually assign constructor parameters to class properties.

Computed Property in Kotlin

// Computed property
val isAdult: Boolean
    get() = age >= 18

This creates a read-only property called isAdult that calculates its value dynamically. Every time you access person.isAdult, it evaluates age >= 18 and returns the result. There’s no backing field storing this value – it’s computed on demand.

Swift:

// Swift
class Person {
    let name: String
    var age: Int
    
    // Init required for non-optional properties
    init(name: String, age: Int) {
        self.name = name
        self.age = age
    }
    
    // Computed property
    var isAdult: Bool {
        return age >= 18
    }
    
    // Property with custom setter/getter
    var email: String = "" {
        willSet {
            print("Will change email to \(newValue)")
        }
        didSet {
            if !email.contains("@") {
                email = oldValue
                print("Invalid email format")
            }
        }
    }
}

In Swift, you declare properties inside the class body, and then initialize them in the init method:

• let name: String creates an immutable property (similar to Kotlin’s val)
• var age: Int creates a mutable property (similar to Kotlin’s var)
• The init method is required to initialize all non-optional properties
• self.name = name means “assign the parameter name to the property name” (using self to disambiguate)

Computed Property in Swift

// Computed property
var isAdult: Bool {
    return age >= 18
}

Similar to Kotlin, this creates a computed property that calculates its value on demand. Every time you access person.isAdult, it evaluates age >= 18.

Control Flow

If Expressions

Kotlin:

// Kotlin - if is an expression
val max = if (a > b) a else b

Swift:

// Swift - if as expression
let max = a > b ? a : b  // Ternary operator

When/Switch Expressions

When in Kotlin:

// Kotlin - when expression
val description = when (x) {
    1 -> "One"
    2, 3 -> "Two or Three"
    in 4..10 -> "Between 4 and 10"
    is String -> "Is a string of length ${x.length}"
    else -> "Something else"
}

Switch in Swift:

// Swift - switch statement
let description: String
switch x {
case 1:
    description = "One"
case 2, 3:
    description = "Two or Three"
case 4...10:
    description = "Between 4 and 10"
case let str as String:
    description = "Is a string of length \(str.count)"
default:
    description = "Something else"
}

Functional Programming Features

Both languages have strong functional programming support:

Kotlin:

// Kotlin
val numbers = listOf(1, 2, 3, 4, 5)
val doubled = numbers.map { it * 2 }
val sum = numbers.reduce { acc, i -> acc + i }
val evenNumbers = numbers.filter { it % 2 == 0 }

// Higher-order function
fun operation(x: Int, y: Int, op: (Int, Int) -> Int): Int {
    return op(x, y)
}
val result = operation(4, 5) { a, b -> a * b }  // 20

Swift:

// Swift
let numbers = [1, 2, 3, 4, 5]
let doubled = numbers.map { $0 * 2 }
let sum = numbers.reduce(0, +)  // More concise with operator references
let evenNumbers = numbers.filter { $0 % 2 == 0 }

// Higher-order function
func operation(x: Int, y: Int, op: (Int, Int) -> Int) -> Int {
    return op(x, y)
}
let result = operation(4, 5) { $0 * $1 }  // 20

Concurrency Model

Kotlin’s coroutines and Swift’s modern async/await system (introduced in Swift 5.5) both offer structured concurrency models, making asynchronous programming more straightforward and less error-prone compared to callback-based approaches.

Koltin:

// Coroutines
import kotlinx.coroutines.*

fun main() = runBlocking {
    launch {
        delay(1000L)
        println("World!")
    }
    println("Hello,")
}

// Structured concurrency with async/await
suspend fun fetchUserData(): UserData {
    val deferred1 = async { fetchUserProfile() }
    val deferred2 = async { fetchUserSettings() }
    
    return UserData(deferred1.await(), deferred2.await())
}

Swift:

// Swift 5.5+ async/await
func fetchData() async throws -> Data {
    let (data, _) = try await URLSession.shared.data(from: url)
    return data
}

// Task and structured concurrency
func loadUserData() async throws -> UserData {
    async let profile = fetchUserProfile()
    async let settings = fetchUserSettings()
    
    return try await UserData(profile: profile, settings: settings)
}

// Using Task for creation
func updateUI() {
    Task {
        do {
            let data = try await fetchData()
            // Update UI with data
        } catch {
            // Handle errors
        }
    }
}

Performance Considerations

Swift generally offers better raw performance due to its native compilation, while Kotlin’s performance on Android is excellent but includes some JVM overhead.

Key Similarities

• Modern syntax with type inference
• Strong focus on null safety
• First-class functions and functional programming features
• Extension functions/methods
• Smart casting/type inference
• Interoperability with legacy code (Java/Objective-C)
• Strong tooling support
• Structured concurrency models

Key Differences : Kotlin vs Swift

• Interoperability: Kotlin has seamless Java interoperability; Swift’s Objective-C bridge is more complex
• Memory Management: Kotlin uses garbage collection; Swift uses ARC
• Protocol Extensions: Swift’s protocol extensions are more powerful than Kotlin’s interfaces
• Value Types: Swift has stronger emphasis on value types (structs)
• Multi-platform Strategy: Kotlin Multiplatform vs. Swift’s primarily Apple ecosystem focus
• Property Observers: Swift has willSet and didSet; Kotlin uses delegation pattern
• Pattern Matching: Swift’s pattern matching is more extensive

Which One Should I Learn? Kotlin or Swift

The decision between learning Kotlin or Swift depends largely on your career goals, existing skills, and the platforms you want to develop for. Here’s guidance to help you decide:

Choose Kotlin if:

• You want to develop Android apps: Kotlin is the officially recommended language for Android development.
• You already know Java: Kotlin’s interoperability with Java makes the transition easier, and you can gradually migrate existing code.
• You’re interested in cross-platform development: Kotlin Multiplatform allows you to share business logic across platforms while maintaining native UIs.
• You want to work with existing JVM ecosystems: If your work involves Spring, enterprise Java systems, or Android, Kotlin provides significant advantages.
• Your job market has more Android opportunities: In many regions, Android development positions outnumber iOS positions.

Choose Swift if:

• You want to develop for Apple platforms: Swift is essential for iOS, macOS, watchOS, and tvOS development.
• You prefer working with more direct native performance: Swift’s compilation to native code avoids any VM overhead.
• You’re interested in Apple’s ecosystem: If you own Apple devices and want to create apps for them, Swift is the natural choice.
• You already have experience with Objective-C: The transition will be smoother, and you can integrate with existing Objective-C codebases.
• Your local job market has strong iOS demand: Some regions (particularly those with higher-income demographics) have stronger iOS app markets.

Consider Learning Both if:

• You want to be a versatile mobile developer: Being proficient in both ecosystems makes you more marketable.
• You work at a company that develops for both platforms: Understanding both languages helps with code reviews and cross-team collaboration.
• You’re interested in architectural patterns across platforms: Learning how similar problems are solved in different ecosystems deepens your understanding.

The good news is that once you learn one of these languages, picking up the other becomes easier. They share many modern programming concepts, and the skills are transferable. Many developers start with the platform that interests them most, or matches their immediate job prospects, then add the other language later as needed.

Kotlin vs Swift : Conclusion

Kotlin delegated properties offer a powerful way to extract and reuse common property behaviors. Whether you’re implementing lazy initialization, validation, or dynamic binding, delegates can help make your code more maintainable and concise.

By mastering this feature, you’ll have another tool in your Kotlin toolbox to write cleaner, more expressive code. Start by using the standard library delegates, then progress to creating your own as you become more comfortable with the pattern.

Remember that like any powerful feature, delegates should be used thoughtfully. They’re most valuable when they help encapsulate complex property logic that would otherwise be duplicated across multiple classes.

Happy Code!