How does Kotlin handle Java interoperability?

Overview

Kotlin is 100% interoperable with Java. You can call Java from Kotlin and vice versa.

Calling Java from Kotlin

kotlinCopy
// Call Java classes directly
val list = ArrayList<String>()
list.add("Kotlin")

// Use Java libraries
val file = File("/path/to/file")
val reader = BufferedReader(FileReader(file))

Calling Kotlin from Java

Kotlin Code

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class User(val name: String) {
    fun greet() = "Hello, $name"
}

object Constants {
    const val API_KEY = "123"
    
    @JvmStatic
    fun getBaseUrl() = "https://api.example.com"
}

Java Code

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User user = new User("Alice");
String greeting = user.greet();

String apiKey = Constants.API_KEY;
String url = Constants.getBaseUrl();

@JvmStatic

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class Utils {
    companion object {
        @JvmStatic
        fun square(x: Int) = x * x
    }
}

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// Java can call as static method
int result = Utils.square(5);

@JvmField

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class Config {
    @JvmField
    val timeout = 30
}

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int t = new Config().timeout; // Direct field access

@JvmOverloads

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fun greet(name: String, greeting: String = "Hello") {
    println("$greeting, $name")
}

Without

@JvmOverloads

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@JvmOverloads
fun greet(name: String, greeting: String = "Hello") { }

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greet("Alice"); // Works now
greet("Bob", "Hi");

Null Safety Bridge

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// Kotlin sees Java types as platform types
val str: String = javaMethod() // May crash if null
val str: String? = javaMethod() // Safe

File Names

Kotlin file

Utils.kt

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fun square(x: Int) = x * x

Java sees it as:

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UtilsKt.square(5);

Customize with

@file:JvmName

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@file:JvmName("MathUtils")
package com.example

fun square(x: Int) = x * x

javaCopy
MathUtils.square(5);

Key Point: Seamless interop allows gradual migration from Java to Kotlin.