Kotlin to Java

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How to convert from Kotlin to Java

Converting code from Kotlin to Java might seem daunting at first, but with the right guidelines, you can make the process seamless. This guide provides detailed instructions on how to convert your Kotlin code to Java. By following these steps, you'll be able to effectively translate your Kotlin codebase into Java.

Understanding Basic Differences

Variable Declarations

In Kotlin, variable declarations are more concise compared to Java. Here's a basic comparison:


val immutableVariable: Int = 1
var mutableVariable: Int = 2


final int immutableVariable = 1;
int mutableVariable = 2;

In Kotlin, val and var are used for immutable and mutable variables, respectively. In Java, final is used to declare a constant, and types are explicitly stated.

Handling Null Safety

Kotlin's null safety is one of its standout features. Converting this to Java requires careful attention:


var nullableString: String? = null


String nullableString = null;

In Kotlin, the nullable type is explicitly declared using ?, whereas in Java, null types are managed manually, necessitating careful null checks.

Function Syntax Differences

Kotlin's succinct function syntax needs to be converted to Java's more verbose style:


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


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

In Java, return types and parameters must always be explicitly stated at the start.

Handling Properties

Kotlin lets you declare properties directly in the primary constructor, which is not possible in Java. Here’s how to deal with this:


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


public class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;

    public String getName() {
        return name;

    public void setName(String name) {
        this.name = name;

    public int getAge() {
        return age;

    public void setAge(int age) {
        this.age = age;

Java requires a more boilerplate code to handle properties, including getter and setter methods.

Dealing with Extensions

Kotlin extensions are a powerful feature that Java does not natively support. You need to convert these into static utility methods.


fun String.removeFirstAndLastChar(): String = this.substring(1, this.length - 1)


public class StringUtils {
    public static String removeFirstAndLastChar(String str) {
        return str.substring(1, str.length() - 1);

Static helper classes in Java can replicate Kotlin's extension functions.

Coroutines vs Thread Management

Kotlin's coroutines simplify asynchronous programming compared to Java's thread management.


GlobalScope.launch {
    println("Hello, World!")


new Thread(new Runnable() {
    public void run() {
        try {
            System.out.println("Hello, World!");
        } catch (InterruptedException e) {

In Java, you need to handle threads and exceptions manually, making the code more verbose.

Higher-Order Functions

Kotlin supports higher-order functions and lambdas natively. Java requires more setup to achieve similar functionality.


fun doOperation(x: Int, y: Int, operation: (Int, Int) -> Int): Int {
    return operation(x, y)

val sum = doOperation(1, 2) { a, b -> a + b }


interface Operation {
    int apply(int a, int b);

public class Main {
    public static int doOperation(int x, int y, Operation operation) {
        return operation.apply(x, y);

    public static void main(String[] args) {
        int sum = doOperation(1, 2, new Operation() {
            public int apply(int a, int b) {
                return a + b;

Java requires defining interfaces and anonymous inner classes to achieve similar results.

Final Notes

While converting Kotlin to Java is not always straightforward, understanding these key differences will make the process easier. The verbosity of Java compared to Kotlin can be seen in various aspects such as variable declarations, null safety, function syntax, property handling, and more. By paying close attention to these differences, you can ensure a smooth transition of your code.

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