Kotlin to Dart

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How to convert from Kotlin to Dart

As a proficient Kotlin developer stepping into the world of Dart, you may encounter certain challenges. Although both are modern, statically-typed languages that are widely used in mobile development (Kotlin for Android and Dart for Flutter), there are several syntactical and semantic differences that require careful consideration. This guide aims to facilitate your transition by detailing the nuances of converting commonly used Kotlin constructs into Dart.

Understanding the Basics: Variable Declaration

In Kotlin, variables are declared using val for read-only values and var for mutable values. Dart uses final and var for similar purposes, with the const keyword additionally available for compile-time constants.


val readOnly: String = "I am immutable"
var mutable: Int = 10


final readOnly = "I am immutable";
var mutable = 10;
const compileTimeConstant = 3.14;

Functions and Methods

Kotlin's approach to functions is quite flexible, supporting both traditional and expression body syntax. Dart's syntax is somewhat similar but requires a different approach to arrow functions.


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

fun subtract(a: Int, b: Int) = a - b


int add(int a, int b) {
    return a + b;

int subtract(int a, int b) => a - b;

Classes and Constructors

Both Kotlin and Dart support classes and primary constructors, but the initialization syntax is different. In Kotlin, the primary constructor is part of the class header, while Dart requires a more explicit definition.


class Person(val name: String, var age: Int)


class Person {
  final String name;
  int age;

  Person(this.name, this.age);

Working with Null Safety

Kotlin’s null safety features such as the ? operator and Elvis operator (?:) provide robust ways to handle nullable types. Dart has adopted similar null safety features, making the transition easier but still requiring attention to detail during conversion.


val name: String? = null
val length = name?.length ?: -1


String? name = null;
int length = name?.length ?? -1;

Type Inference and Casting

Both languages provide type inference, but casting is managed differently in Dart. Where Kotlin uses the as operator and smart casts, Dart employs as and type checks via is.


val obj: Any = "A String"
if (obj is String) {


var obj = "A String";
if (obj is String) {

Collections and Iteration

Collections such as lists and maps are fundamental in both languages. The syntax for initializing and iterating through these collections varies slightly.


val numbers = listOf(1, 2, 3)
for (number in numbers) {


var numbers = [1, 2, 3];
for (var number in numbers) {


Converting from Kotlin to Dart involves understanding and adapting to the unique syntax and features of each language. While the overall structure and concepts remain consistent, the detailed nuances in syntax need careful attention.

By mastering these conversions, you can leverage your Kotlin expertise to effectively develop in Dart, particularly for Flutter applications. Keep experimenting with code and refer back to this guide as a quick reference to ensure a smooth transition. Happy coding!

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