Calling Kotlin from Java

A Kotlin property is compiled to the following Java elements:

• a getter method, with the name calculated by prepending the get prefix

• a setter method, with the name calculated by prepending the set prefix (only for var properties)

• a private field, with the same name as the property name (only for properties with backing fields)

For example, var firstName: String compiles to the following Java declarations:

private String firstName;

public String getFirstName() {
    return firstName;
}

public void setFirstName(String firstName) {
    this.firstName = firstName;
}

If the name of the property starts with is, a different name mapping rule is used: the name of the getter will be the same as the property name, and the name of the setter will be obtained by replacing is with set. For example, for a property isOpen, the getter will be called isOpen() and the setter will be called setOpen(). This rule applies for properties of any type, not just Boolean.

All the functions and properties declared in a file app.kt inside a package org.example, including extension functions, are compiled into static methods of a Java class named org.example.AppKt.

// app.kt
package org.example

class Util

fun getTime() { /*...*/ }

// Java
new org.example.Util();
org.example.AppKt.getTime();

To set a custom name to the generated Java class, use the @JvmName annotation:

@file:JvmName("DemoUtils")

package org.example

class Util

fun getTime() { /*...*/ }

// Java
new org.example.Util();
org.example.DemoUtils.getTime();

Having multiple files with the same generated Java class name (the same package and the same name or the same @JvmName annotation) is normally an error. However, the compiler can generate a single Java facade class which has the specified name and contains all the declarations from all the files which have that name. To enable the generation of such a facade, use the @JvmMultifileClass annotation in all such files.

// oldutils.kt
@file:JvmName("Utils")
@file:JvmMultifileClass

package org.example

fun getTime() { /*...*/ }

// newutils.kt
@file:JvmName("Utils")
@file:JvmMultifileClass

package org.example

fun getDate() { /*...*/ }

// Java
org.example.Utils.getTime();
org.example.Utils.getDate();

If you need to expose a Kotlin property as a field in Java, annotate it with the @JvmField annotation. The field will have the same visibility as the underlying property. You can annotate a property with @JvmField if it:

• has a backing field

• is not private

• does not have open, override or const modifiers

• is not a delegated property

class User(id: String) {
    @JvmField val ID = id
}

// Java
class JavaClient {
    public String getID(User user) {
        return user.ID;
    }
}

Late-Initialized properties are also exposed as fields. The visibility of the field will be the same as the visibility of lateinit property setter.

Kotlin properties declared in a named object or a companion object will have static backing fields either in that named object or in the class containing the companion object.

Usually these fields are private but they can be exposed in one of the following ways:

• @JvmField annotation

• lateinit modifier

• const modifier

Annotating such a property with @JvmField makes it a static field with the same visibility as the property itself.

class Key(val value: Int) {
    companion object {
        @JvmField
        val COMPARATOR: Comparator<Key> = compareBy<Key> { it.value }
    }
}

// Java
Key.COMPARATOR.compare(key1, key2);
// public static final field in Key class

A late-initialized property in an object or a companion object has a static backing field with the same visibility as the property setter.

object Singleton {
    lateinit var provider: Provider
}

// Java
Singleton.provider = new Provider();
// public static non-final field in Singleton class

Properties declared as const (in classes as well as at the top level) are turned into static fields in Java:

// file example.kt

object Obj {
    const val CONST = 1
}

class C {
    companion object {
        const val VERSION = 9
    }
}

const val MAX = 239

In Java:

int constant = Obj.CONST;
int max = ExampleKt.MAX;
int version = C.VERSION;

As mentioned above, Kotlin represents package-level functions as static methods. Kotlin can also generate static methods for functions defined in named objects or companion objects if you annotate those functions as @JvmStatic. If you use this annotation, the compiler will generate both a static method in the enclosing class of the object and an instance method in the object itself. For example:

class C {
    companion object {
        @JvmStatic fun callStatic() {}
        fun callNonStatic() {}
    }
}

Now, callStatic() is static in Java while callNonStatic() is not:

C.callStatic(); // works fine
C.callNonStatic(); // error: not a static method
C.Companion.callStatic(); // instance method remains
C.Companion.callNonStatic(); // the only way it works

Same for named objects:

object Obj {
    @JvmStatic fun callStatic() {}
    fun callNonStatic() {}
}

In Java:

Obj.callStatic(); // works fine
Obj.callNonStatic(); // error
Obj.INSTANCE.callNonStatic(); // works, a call through the singleton instance
Obj.INSTANCE.callStatic(); // works too

Starting from Kotlin 1.3, @JvmStatic applies to functions defined in companion objects of interfaces as well. Such functions compile to static methods in interfaces. Note that static method in interfaces were introduced in Java 1.8, so be sure to use the corresponding targets.

interface ChatBot {
    companion object {
        @JvmStatic fun greet(username: String) {
            println("Hello, $username")
        }
    }
}

@JvmStatic annotation can also be applied on a property of an object or a companion object making its getter and setter methods static members in that object or the class containing the companion object.

note

Default methods are available only for targets JVM 1.8 and above.

Starting from JDK 1.8, interfaces in Java can contain default methods. To make all non-abstract members of Kotlin interfaces default for the Java classes implementing them, compile the Kotlin code with the -Xjvm-default=all compiler option.

Here is an example of a Kotlin interface with a default method:

// compile with -Xjvm-default=all

interface Robot {
    fun move() { println("~walking~") }  // will be default in the Java interface
    fun speak(): Unit
}

The default implementation is available for Java classes implementing the interface.

//Java implementation
public class C3PO implements Robot {
    // move() implementation from Robot is available implicitly
    @Override
    public void speak() {
        System.out.println("I beg your pardon, sir");
    }
}

C3PO c3po = new C3PO();
c3po.move(); // default implementation from the Robot interface
c3po.speak();

Implementations of the interface can override default methods.

//Java
public class BB8 implements Robot {
    //own implementation of the default method
    @Override
    public void move() {
        System.out.println("~rolling~");
    }

    @Override
    public void speak() {
        System.out.println("Beep-beep");
    }
}

note

Prior to Kotlin 1.4, to generate default methods, you could use the @JvmDefault annotation on these methods. Compiling with -Xjvm-default=all in 1.4+ generally works as if you annotated all non-abstract methods of interfaces with @JvmDefaultand compiled with -Xjvm-default=enable. However, there are cases when their behavior differs. Detailed information about the changes in default methods generation in Kotlin 1.4 is provided in this post on the Kotlin blog.

The Kotlin visibility modifiers map to Java in the following way:

• private members are compiled to private members

• private top-level declarations are compiled to private top-level declarations. Package-private accessors are also included, if accessed from within a class.

• protected remains protected (note that Java allows accessing protected members from other classes in the same package and Kotlin doesn't, so Java classes will have broader access to the code)

• internal declarations become public in Java. Members of internal classes go through name mangling, to make it harder to accidentally use them from Java and to allow overloading for members with the same signature that don't see each other according to Kotlin rules

• public remains public

Sometimes you need to call a Kotlin method with a parameter of type KClass. There is no automatic conversion from Class to KClass, so you have to do it manually by invoking the equivalent of the Class<T>.kotlin extension property:

kotlin.jvm.JvmClassMappingKt.getKotlinClass(MainView.class)

Sometimes we have a named function in Kotlin, for which we need a different JVM name in the bytecode. The most prominent example happens due to type erasure:

fun List<String>.filterValid(): List<String>
fun List<Int>.filterValid(): List<Int>

These two functions can not be defined side-by-side, because their JVM signatures are the same: filterValid(Ljava/util/List;)Ljava/util/List;. If we really want them to have the same name in Kotlin, we can annotate one (or both) of them with @JvmName and specify a different name as an argument:

fun List<String>.filterValid(): List<String>

@JvmName("filterValidInt")
fun List<Int>.filterValid(): List<Int>

From Kotlin they will be accessible by the same name filterValid, but from Java it will be filterValid and filterValidInt.

The same trick applies when we need to have a property x alongside with a function getX():

val x: Int
    @JvmName("getX_prop")
    get() = 15

fun getX() = 10

To change the names of generated accessor methods for properties without explicitly implemented getters and setters, you can use @get:JvmName and @set:JvmName:

@get:JvmName("x")
@set:JvmName("changeX")
var x: Int = 23

Normally, if you write a Kotlin function with default parameter values, it will be visible in Java only as a full signature, with all parameters present. If you wish to expose multiple overloads to Java callers, you can use the @JvmOverloads annotation.

The annotation also works for constructors, static methods, and so on. It can't be used on abstract methods, including methods defined in interfaces.

class Circle @JvmOverloads constructor(centerX: Int, centerY: Int, radius: Double = 1.0) {
    @JvmOverloads fun draw(label: String, lineWidth: Int = 1, color: String = "red") { /*...*/ }
}

For every parameter with a default value, this will generate one additional overload, which has this parameter and all parameters to the right of it in the parameter list removed. In this example, the following will be generated:

// Constructors:
Circle(int centerX, int centerY, double radius)
Circle(int centerX, int centerY)

// Methods
void draw(String label, int lineWidth, String color) { }
void draw(String label, int lineWidth) { }
void draw(String label) { }

Note that, as described in Secondary constructors, if a class has default values for all constructor parameters, a public constructor with no arguments will be generated for it. This works even if the @JvmOverloads annotation is not specified.

Kotlin does not have checked exceptions. So, normally the Java signatures of Kotlin functions do not declare exceptions thrown. Thus, if you have a function in Kotlin like this:

// example.kt
package demo

fun writeToFile() {
    /*...*/
    throw IOException()
}

And you want to call it from Java and catch the exception:

// Java
try {
    demo.Example.writeToFile();
} catch (IOException e) {
    // error: writeToFile() does not declare IOException in the throws list
    // ...
}

You get an error message from the Java compiler, because writeToFile() does not declare IOException. To work around this problem, use the @Throws annotation in Kotlin:

@Throws(IOException::class)
fun writeToFile() {
    /*...*/
    throw IOException()
}

When calling Kotlin functions from Java, nobody prevents us from passing null as a non-nullable parameter. That's why Kotlin generates runtime checks for all public functions that expect non-nulls. This way we get a NullPointerException in the Java code immediately.

When Kotlin classes make use of declaration-site variance, there are two options of how their usages are seen from the Java code. For example, imagine you have the following class and two functions that use it:

class Box<out T>(val value: T)

interface Base
class Derived : Base

fun boxDerived(value: Derived): Box<Derived> = Box(value)
fun unboxBase(box: Box<Base>): Base = box.value

A naive way of translating these functions into Java would be this:

Box<Derived> boxDerived(Derived value) { ... }
Base unboxBase(Box<Base> box) { ... }

The problem is that in Kotlin you can write unboxBase(boxDerived(Derived())) but in Java that would be impossible because in Java the class Box is invariant in its parameter T, and thus Box<Derived> is not a subtype of Box<Base>. To make this work in Java, you would have to define unboxBase as follows:

Base unboxBase(Box<? extends Base> box) { ... }

This declaration uses Java's wildcards types (? extends Base) to emulate declaration-site variance through use-site variance, because it is all Java has.

To make Kotlin APIs work in Java, the compiler generates Box<Super> as Box<? extends Super> for covariantly defined Box (or Foo<? super Bar> for contravariantly defined Foo) when it appears as a parameter. When it's a return value, wildcards are not generated, because otherwise Java clients will have to deal with them (and it's against the common Java coding style). Therefore, the functions from our example are actually translated as follows:

// return type - no wildcards
Box<Derived> boxDerived(Derived value) { ... }

// parameter - wildcards
Base unboxBase(Box<? extends Base> box) { ... }

note

When the argument type is final, there's usually no point in generating the wildcard, so Box<String> is always Box<String>, no matter what position it takes.

If you need wildcards where they are not generated by default, use the @JvmWildcard annotation:

fun boxDerived(value: Derived): Box<@JvmWildcard Derived> = Box(value)
// is translated to
// Box<? extends Derived> boxDerived(Derived value) { ... }

In the opposite case, if you don't need wildcards where they are generated, use @JvmSuppressWildcards:

fun unboxBase(box: Box<@JvmSuppressWildcards Base>): Base = box.value
// is translated to
// Base unboxBase(Box<Base> box) { ... }

note

@JvmSuppressWildcards can be used not only on individual type arguments, but on entire declarations, such as functions or classes, causing all wildcards inside them to be suppressed.

fun emptyList(): List<Nothing> = listOf()
// is translated to
// List emptyList() { ... }