Calling Java from Kotlin seamlessly

Introduction

Sometimes you want to use Java code inside your Kotlin program without extra work. Kotlin lets you call Java code easily, so you can mix both languages smoothly.

You have existing Java libraries and want to use them in your new Kotlin app.

You want to reuse Java code without rewriting it in Kotlin.

You are working on a project with both Java and Kotlin files.

You want to call Java methods or classes directly from Kotlin.

You want to gradually convert a Java project to Kotlin.

Syntax

Kotlin

val javaObject = JavaClass()
javaObject.javaMethod()

You can create Java objects and call their methods just like Kotlin objects.

Kotlin automatically understands Java classes, methods, and fields.

Examples

Creating a Java class object and calling its method from Kotlin.

Kotlin

val person = Person("Alice")
println(person.getName())

Using Java's ArrayList class in Kotlin code.

Kotlin

val list = ArrayList<String>()
list.add("Hello")
println(list.size)

Sample Program

This Kotlin program creates an object of a Java class Car and calls its methods to print details.

Kotlin

fun main() {
    val car = Car("Toyota", 2020)
    println("Car brand: ${car.brand}")
    println("Car year: ${car.year}")
}

// Java class in the same project
// public class Car {
//     private String brand;
//     private int year;
//     public Car(String brand, int year) {
//         this.brand = brand;
//         this.year = year;
//     }
//     public String getBrand() { return brand; }
//     public int getYear() { return year; }
// }

Important Notes

Kotlin treats Java getters and setters as properties, so you can also write car.brand instead of car.getBrand().

Static Java methods can be called using the class name directly in Kotlin.

Null safety is important: Java types may be nullable in Kotlin, so check for nulls.

Summary

Kotlin can use Java classes and methods directly without extra code.

You can create Java objects and call their methods just like Kotlin objects.

This makes mixing Java and Kotlin easy and helps reuse existing code.