Java to Kotlin

Free Java to Kotlin Code Converter

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Transform your code from Java to Kotlin with our free AI-based code convertion tool. If you like what you see, we also create documentation for your code! We don't ever store your code or any representation of it in our databases, but it will be shared with the LLM of our choice for processing.

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How to convert from Java to Kotlin

Converting from Java to Kotlin can be an efficient way to streamline your code by leveraging modern language features. Kotlin, developed by JetBrains, offers enhanced readability, concise syntax, and full interoperability with Java. Let's walk through the steps to convert Java to Kotlin effectively.

Kotlin Advantages Over Java

Before diving into the conversion process, let's first outline why you might want to convert from Java to Kotlin:

  • Concise Syntax: Kotlin significantly reduces the amount of boilerplate code.
  • Null Safety: Built-in null safety reduces NullPointerExceptions.
  • Coroutines: Simplifies asynchronous programming with coroutines.
  • Interoperability: Kotlin is fully interoperable with Java, so your existing Java code can coexist with Kotlin code.

Using IntelliJ's Built-in Converter

The easiest way to convert Java code to Kotlin is by using IntelliJ IDEA's built-in converter. Follow these steps:

  1. Open Your Java File: Open the Java class you want to convert.
  2. Invoke the Converter: Go to the Tools menu, then select Kotlin, and click on Convert Java File to Kotlin.
  3. Review the Converted Code: IntelliJ IDEA will generate Kotlin code which you can review.

Manual Conversion

While an automatic converter speeds up the process, sometimes you might need to manually refine the converted code. Here are fundamental steps for manual conversion:

Basic Syntax Changes

Variable Declarations:


int count = 10;
String message = "Hello, World!";


var count: Int = 10
var message: String = "Hello, World!"

Alternatively, Kotlin can infer types:

var count = 10
var message = "Hello, World!"

Immutable Variables:

In Kotlin, use val for read-only variables:

val count = 10

Null Safety


String message = null;


var message: String? = null

The ? indicates that the variable can hold a null value.

Elvis Operator (?:):

Handling nulls in Kotlin is simplified using the Elvis operator:

val length = message?.length ?: 0

Functions and Methods


public int sum(int a, int b) {
    return a + b;


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

Data Classes


public class User {
   private String name;
   private int age;
   public User(String name, int age) { = name;
       this.age = age;

   // getters and setters...


data class User(val name: String, val age: Int)

Data classes in Kotlin automatically generate equals(), hashCode(), toString(), and copy() methods.

Utilizing Kotlin Features

Once you have converted your code, take advantage of the advanced Kotlin features:

Extension Functions

Extension functions allow adding methods to existing classes without modifying them:

fun String.lastChar(): Char = this[this.length - 1]


val letter = "Kotlin".lastChar()  // 'n'

Coroutines for Asynchronous Programming

Instead of Java’s Thread and Runnable, use Kotlin's coroutines:

import kotlinx.coroutines.*

fun main() = runBlocking {
    launch {



Smart Casts

Kotlin smart casts automatically cast a variable to its type after a check:

fun printLength(obj: Any) {
    if (obj is String) {
        println(obj.length)  // Smart cast to String


A Free Java to Kotlin code converter can facilitate the migration process, but understanding the syntax changes and leveraging Kotlin’s advanced features will ensure the transformation is smooth and beneficial. Enhance your coding experience by adopting Kotlin's modern language features and reduce boilerplate code, introduce null safety, and simplify asynchronous programming.

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