Kotlin Help

Calling Java from Kotlin

Kotlin is designed with Java interoperability in mind. Existing Java code can be called from Kotlin in a natural way, and Kotlin code can be used from Java rather smoothly as well. In this section, we describe some details about calling Java code from Kotlin.

Pretty much all Java code can be used without any issues:

import java.util.* fun demo(source: List<Int>) { val list = ArrayList<Int>() // 'for'-loops work for Java collections: for (item in source) { list.add(item) } // Operator conventions work as well: for (i in 0..source.size - 1) { list[i] = source[i] // get and set are called } }

Getters and setters

Methods that follow the Java conventions for getters and setters (no-argument methods with names starting with get and single-argument methods with names starting with set) are represented as properties in Kotlin. Such properties are also called synthetic properties. Boolean accessor methods (where the name of the getter starts with is and the name of the setter starts with set) are represented as properties which have the same name as the getter method.

import java.util.Calendar fun calendarDemo() { val calendar = Calendar.getInstance() if (calendar.firstDayOfWeek == Calendar.SUNDAY) { // call getFirstDayOfWeek() calendar.firstDayOfWeek = Calendar.MONDAY // call setFirstDayOfWeek() } if (!calendar.isLenient) { // call isLenient() calendar.isLenient = true // call setLenient() } }

calendar.firstDayOfWeek above is an example of a synthetic property.

Note that, if the Java class only has a setter, it isn't visible as a property in Kotlin because Kotlin doesn't support set-only properties.

Java synthetic property references

Starting from Kotlin 1.8.20, you can create references to Java synthetic properties. Consider the following Java code:

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 int getAge() { return age; } }

Kotlin has always allowed you to write person.age, where age is a synthetic property. Now, you can also create references to Person::age and person::age. The same applies for name, as well.

val persons = listOf(Person("Jack", 11), Person("Sofie", 12), Person("Peter", 11)) persons // Call a reference to Java synthetic property: .sortedBy(Person::age) // Call Java getter via the Kotlin property syntax: .forEach { person -> println(person.name) }

How to enable Java synthetic property references

To enable this feature, set the -language-version 2.1 compiler option. In a Gradle project, you can do so by adding the following to your build.gradle(.kts):

tasks .withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompilationTask<*>>() .configureEach { compilerOptions .languageVersion .set( org.jetbrains.kotlin.gradle.dsl.KotlinVersion.KOTLIN_2_1 ) }
tasks .withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompilationTask.class) .configureEach { compilerOptions.languageVersion = org.jetbrains.kotlin.gradle.dsl.KotlinVersion.KOTLIN_2_1 }

Methods returning void

If a Java method returns void, it will return Unit when called from Kotlin. If by any chance someone uses that return value, it will be assigned at the call site by the Kotlin compiler since the value itself is known in advance (being Unit).

Escaping for Java identifiers that are keywords in Kotlin

Some of the Kotlin keywords are valid identifiers in Java: in, object, is, and other. If a Java library uses a Kotlin keyword for a method, you can still call the method escaping it with the backtick (`) character:


Null-safety and platform types

Any reference in Java may be null, which makes Kotlin's requirements of strict null-safety impractical for objects coming from Java. Types of Java declarations are treated in Kotlin in a specific manner and called platform types. Null-checks are relaxed for such types, so that safety guarantees for them are the same as in Java (see more below).

Consider the following examples:

val list = ArrayList<String>() // non-null (constructor result) list.add("Item") val size = list.size // non-null (primitive int) val item = list[0] // platform type inferred (ordinary Java object)

When you call methods on variables of platform types, Kotlin does not issue nullability errors at compile time, but the call may fail at runtime, because of a null-pointer exception or an assertion that Kotlin generates to prevent nulls from propagating:

item.substring(1) // allowed, throws an exception if item == null

Platform types are non-denotable, meaning that you can't write them down explicitly in the language. When a platform value is assigned to a Kotlin variable, you can rely on the type inference (the variable will have an inferred platform type then, as item has in the example above), or you can choose the type you expect (both nullable and non-nullable types are allowed):

val nullable: String? = item // allowed, always works val notNull: String = item // allowed, may fail at runtime

If you choose a non-nullable type, the compiler will emit an assertion upon assignment. This prevents Kotlin's non-nullable variables from holding nulls. Assertions are also emitted when you pass platform values to Kotlin functions expecting non-null values and in other cases. Overall, the compiler does its best to prevent nulls from propagating far through the program although sometimes this is impossible to eliminate entirely, because of generics.

Notation for platform types

As mentioned above, platform types can't be mentioned explicitly in the program, so there's no syntax for them in the language. Nevertheless, the compiler and IDE need to display them sometimes (for example, in error messages or parameter info), so there is a mnemonic notation for them:

  • T! means "T or T?",

  • (Mutable)Collection<T>! means "Java collection of T may be mutable or not, may be nullable or not",

  • Array<(out) T>! means "Java array of T (or a subtype of T), nullable or not"

Nullability annotations

Java types that have nullability annotations are represented not as platform types, but as actual nullable or non-nullable Kotlin types. The compiler supports several flavors of nullability annotations, including:

  • JetBrains (@Nullable and @NotNull from the org.jetbrains.annotations package)

  • JSpecify (org.jspecify.nullness)

  • Android (com.android.annotations and android.support.annotations)

  • JSR-305 (javax.annotation, more details below)

  • FindBugs (edu.umd.cs.findbugs.annotations)

  • Eclipse (org.eclipse.jdt.annotation)

  • Lombok (lombok.NonNull)

  • RxJava 3 (io.reactivex.rxjava3.annotations)

You can specify whether the compiler reports a nullability mismatch based on the information from specific types of nullability annotations. Use the compiler option -Xnullability-annotations=@<package-name>:<report-level>. In the argument, specify the fully qualified nullability annotations package and one of these report levels:

  • ignore to ignore nullability mismatches

  • warn to report warnings

  • strict to report errors.

See the full list of supported nullability annotations in the Kotlin compiler source code.

Annotating type arguments and type parameters

You can annotate the type arguments and type parameters of generic types to provide nullability information for them as well.

Type arguments

Consider these annotations on a Java declaration:

@NotNull Set<@NotNull String> toSet(@NotNull Collection<@NotNull String> elements) { ... }

They result in the following signature in Kotlin:

fun toSet(elements: (Mutable)Collection<String>) : (Mutable)Set<String> { ... }

When the @NotNull annotation is missing from a type argument, you get a platform type instead:

fun toSet(elements: (Mutable)Collection<String!>) : (Mutable)Set<String!> { ... }

Kotlin also takes into account nullability annotations on type arguments of base classes and interfaces. For example, there are two Java classes with the signatures provided below:

public class Base<T> {}
public class Derived extends Base<@Nullable String> {}

In the Kotlin code, passing the instance of Derived where the Base<String> is assumed produces the warning.

fun takeBaseOfNotNullStrings(x: Base<String>) {} fun main() { takeBaseOfNotNullStrings(Derived()) // warning: nullability mismatch }

The upper bound of Derived is set to Base<String?>, which is different from Base<String>.

Learn more about Java generics in Kotlin.

Type parameters

By default, the nullability of plain type parameters in both Kotlin and Java is undefined. In Java, you can specify it using nullability annotations. Let's annotate the type parameter of the Base class:

public class Base<@NotNull T> {}

When inheriting from Base, Kotlin expects a non-nullable type argument or type parameter. Thus, the following Kotlin code produces a warning:

class Derived<K> : Base<K> {} // warning: K has undefined nullability

You can fix it by specifying the upper bound K : Any.

Kotlin also supports nullability annotations on the bounds of Java type parameters. Let's add bounds to Base:

public class BaseWithBound<T extends @NotNull Number> {}

Kotlin translates this just as follows:

class BaseWithBound<T : Number> {}

So passing nullable type as a type argument or type parameter produces a warning.

Annotating type arguments and type parameters works with the Java 8 target or higher. The feature requires that the nullability annotations support the TYPE_USE target (org.jetbrains.annotations supports this in version 15 and above). Pass the -Xtype-enhancement-improvements-strict-mode compiler option to report errors in Kotlin code that uses nullability which deviates from the nullability annotations from Java.

JSR-305 support

The @Nonnull annotation defined in JSR-305 is supported for denoting nullability of Java types.

If the @Nonnull(when = ...) value is When.ALWAYS, the annotated type is treated as non-nullable; When.MAYBE and When.NEVER denote a nullable type; and When.UNKNOWN forces the type to be platform one.

A library can be compiled against the JSR-305 annotations, but there's no need to make the annotations artifact (e.g. jsr305.jar) a compile dependency for the library consumers. The Kotlin compiler can read the JSR-305 annotations from a library without the annotations present on the classpath.

Custom nullability qualifiers (KEEP-79) are also supported (see below).

Type qualifier nicknames

If an annotation type is annotated with both @TypeQualifierNickname and JSR-305 @Nonnull (or its another nickname, such as @CheckForNull), then the annotation type is itself used for retrieving precise nullability and has the same meaning as that nullability annotation:

@TypeQualifierNickname @Nonnull(when = When.ALWAYS) @Retention(RetentionPolicy.RUNTIME) public @interface MyNonnull { } @TypeQualifierNickname @CheckForNull // a nickname to another type qualifier nickname @Retention(RetentionPolicy.RUNTIME) public @interface MyNullable { } interface A { @MyNullable String foo(@MyNonnull String x); // in Kotlin (strict mode): `fun foo(x: String): String?` String bar(List<@MyNonnull String> x); // in Kotlin (strict mode): `fun bar(x: List<String>!): String!` }

Type qualifier defaults

@TypeQualifierDefault allows introducing annotations that, when being applied, define the default nullability within the scope of the annotated element.

Such annotation type should itself be annotated with both @Nonnull (or its nickname) and @TypeQualifierDefault(...) with one or more ElementType values:

  • ElementType.METHOD for return types of methods

  • ElementType.PARAMETER for value parameters

  • ElementType.FIELD for fields

  • ElementType.TYPE_USE for any type including type arguments, upper bounds of type parameters and wildcard types

The default nullability is used when a type itself is not annotated by a nullability annotation, and the default is determined by the innermost enclosing element annotated with a type qualifier default annotation with the ElementType matching the type usage.

@Nonnull @TypeQualifierDefault({ElementType.METHOD, ElementType.PARAMETER}) public @interface NonNullApi { } @Nonnull(when = When.MAYBE) @TypeQualifierDefault({ElementType.METHOD, ElementType.PARAMETER, ElementType.TYPE_USE}) public @interface NullableApi { } @NullableApi interface A { String foo(String x); // fun foo(x: String?): String? @NotNullApi // overriding default from the interface String bar(String x, @Nullable String y); // fun bar(x: String, y: String?): String // The List<String> type argument is seen as nullable because of `@NullableApi` // having the `TYPE_USE` element type: String baz(List<String> x); // fun baz(List<String?>?): String? // The type of `x` parameter remains platform because there's an explicit // UNKNOWN-marked nullability annotation: String qux(@Nonnull(when = When.UNKNOWN) String x); // fun baz(x: String!): String? }

Package-level default nullability is also supported:

// FILE: test/package-info.java @NonNullApi // declaring all types in package 'test' as non-nullable by default package test;

@UnderMigration annotation

The @UnderMigration annotation (provided in a separate artifact kotlin-annotations-jvm) can be used by library maintainers to define the migration status for the nullability type qualifiers.

The status value in @UnderMigration(status = ...) specifies how the compiler treats inappropriate usages of the annotated types in Kotlin (e.g. using a @MyNullable-annotated type value as non-null):

  • MigrationStatus.STRICT makes annotation work as any plain nullability annotation, i.e. report errors for the inappropriate usages and affect the types in the annotated declarations as they are seen in Kotlin

  • MigrationStatus.WARN: the inappropriate usages are reported as compilation warnings instead of errors, but the types in the annotated declarations remain platform

  • MigrationStatus.IGNORE makes the compiler ignore the nullability annotation completely

A library maintainer can add @UnderMigration status to both type qualifier nicknames and type qualifier defaults:

@Nonnull(when = When.ALWAYS) @TypeQualifierDefault({ElementType.METHOD, ElementType.PARAMETER}) @UnderMigration(status = MigrationStatus.WARN) public @interface NonNullApi { } // The types in the class are non-nullable, but only warnings are reported // because `@NonNullApi` is annotated `@UnderMigration(status = MigrationStatus.WARN)` @NonNullApi public class Test {}

If a default type qualifier uses a type qualifier nickname and they are both @UnderMigration, the status from the default type qualifier is used.

Compiler configuration

The JSR-305 checks can be configured by adding the -Xjsr305 compiler flag with the following options (and their combination):

  • -Xjsr305={strict|warn|ignore} to set up the behavior for non-@UnderMigration annotations. Custom nullability qualifiers, especially @TypeQualifierDefault, are already spread among many well-known libraries, and users may need to migrate smoothly when updating to the Kotlin version containing JSR-305 support. Since Kotlin 1.1.60, this flag only affects non-@UnderMigration annotations.

  • -Xjsr305=under-migration:{strict|warn|ignore} to override the behavior for the @UnderMigration annotations. Users may have different view on the migration status for the libraries: they may want to have errors while the official migration status is WARN, or vice versa, they may wish to postpone errors reporting for some until they complete their migration.

  • -Xjsr305=@<fq.name>:{strict|warn|ignore} to override the behavior for a single annotation, where <fq.name> is the fully qualified class name of the annotation. May appear several times for different annotations. This is useful for managing the migration state for a particular library.

The strict, warn and ignore values have the same meaning as those of MigrationStatus, and only the strict mode affects the types in the annotated declarations as they are seen in Kotlin.

For example, adding -Xjsr305=ignore -Xjsr305=under-migration:ignore -Xjsr305=@org.library.MyNullable:warn to the compiler arguments makes the compiler generate warnings for inappropriate usages of types annotated by @org.library.MyNullable and ignore all other JSR-305 annotations.

The default behavior is the same to -Xjsr305=warn. The strict value should be considered experimental (more checks may be added to it in the future).

Mapped types

Kotlin treats some Java types specifically. Such types are not loaded from Java "as is", but are mapped to corresponding Kotlin types. The mapping only matters at compile time, the runtime representation remains unchanged. Java's primitive types are mapped to corresponding Kotlin types (keeping platform types in mind):

Java type

Kotlin type

















Some non-primitive built-in classes are also mapped:

Java type

Kotlin type



















Java's boxed primitive types are mapped to nullable Kotlin types:

Java type

Kotlin type

















Note that a boxed primitive type used as a type parameter is mapped to a platform type: for example, List<java.lang.Integer> becomes a List<Int!> in Kotlin.

Collection types may be read-only or mutable in Kotlin, so Java's collections are mapped as follows (all Kotlin types in this table reside in the package kotlin.collections):

Java type

Kotlin read-only type

Kotlin mutable type

Loaded platform type

























Map<K, V>

Map<K, V>

MutableMap<K, V>

(Mutable)Map<K, V>!

Map.Entry<K, V>

Map.Entry<K, V>


(Mutable)Map.(Mutable)Entry<K, V>!

Java's arrays are mapped as mentioned below:

Java type

Kotlin type




kotlin.Array<(out) String!>!

Java generics in Kotlin

Kotlin's generics are a little different from Java's (see Generics). When importing Java types to Kotlin, the following conversions are done:

  • Java's wildcards are converted into type projections:

    • Foo<? extends Bar> becomes Foo<out Bar!>!

    • Foo<? super Bar> becomes Foo<in Bar!>!

  • Java's raw types are converted into star projections:

    • List becomes List<*>! that is List<out Any?>!

Like Java's, Kotlin's generics are not retained at runtime: objects do not carry information about actual type arguments passed to their constructors. For example, ArrayList<Integer>() is indistinguishable from ArrayList<Character>(). This makes it impossible to perform is-checks that take generics into account. Kotlin only allows is-checks for star-projected generic types:

if (a is List<Int>) // Error: cannot check if it is really a List of Ints // but if (a is List<*>) // OK: no guarantees about the contents of the list

Java arrays

Arrays in Kotlin are invariant, unlike Java. This means that Kotlin won't let you assign an Array<String> to an Array<Any>, which prevents a possible runtime failure. Passing an array of a subclass as an array of superclass to a Kotlin method is also prohibited, but for Java methods this is allowed through platform types of the form Array<(out) String>!.

Arrays are used with primitive datatypes on the Java platform to avoid the cost of boxing/unboxing operations. As Kotlin hides those implementation details, a workaround is required to interface with Java code. There are specialized classes for every type of primitive array (IntArray, DoubleArray, CharArray, and so on) to handle this case. They are not related to the Array class and are compiled down to Java's primitive arrays for maximum performance.

Suppose there is a Java method that accepts an int array of indices:

public class JavaArrayExample { public void removeIndices(int[] indices) { // code here... } }

To pass an array of primitive values, you can do the following in Kotlin:

val javaObj = JavaArrayExample() val array = intArrayOf(0, 1, 2, 3) javaObj.removeIndices(array) // passes int[] to method

When compiling to the JVM bytecode, the compiler optimizes access to arrays so that there's no overhead introduced:

val array = arrayOf(1, 2, 3, 4) array[1] = array[1] * 2 // no actual calls to get() and set() generated for (x in array) { // no iterator created print(x) }

Even when you navigate with an index, it does not introduce any overhead:

for (i in array.indices) { // no iterator created array[i] += 2 }

Finally, in-checks have no overhead either:

if (i in array.indices) { // same as (i >= 0 && i < array.size) print(array[i]) }

Java varargs

Java classes sometimes use a method declaration for the indices with a variable number of arguments (varargs):

public class JavaArrayExample { public void removeIndicesVarArg(int... indices) { // code here... } }

In that case you need to use the spread operator * to pass the IntArray:

val javaObj = JavaArrayExample() val array = intArrayOf(0, 1, 2, 3) javaObj.removeIndicesVarArg(*array)


Since Java has no way of marking methods for which it makes sense to use the operator syntax, Kotlin allows using any Java methods with the right name and signature as operator overloads and other conventions (invoke() etc.) Calling Java methods using the infix call syntax is not allowed.

Checked exceptions

In Kotlin, all exceptions are unchecked, meaning that the compiler does not force you to catch any of them. So, when you call a Java method that declares a checked exception, Kotlin does not force you to do anything:

fun render(list: List<*>, to: Appendable) { for (item in list) { to.append(item.toString()) // Java would require us to catch IOException here } }

Object methods

When Java types are imported into Kotlin, all the references of the type java.lang.Object are turned into Any. Since Any is not platform-specific, it only declares toString(), hashCode() and equals() as its members, so to make other members of java.lang.Object available, Kotlin uses extension functions.


Methods wait() and notify() are not available on references of type Any. Their usage is generally discouraged in favor of java.util.concurrent. If you really need to call these methods, you can cast to java.lang.Object:

(foo as java.lang.Object).wait()


To retrieve the Java class of an object, use the java extension property on a class reference:

val fooClass = foo::class.java

The code above uses a bound class reference. You can also use the javaClass extension property:

val fooClass = foo.javaClass


To override clone(), your class needs to extend kotlin.Cloneable:

class Example : Cloneable { override fun clone(): Any { ... } }

Don't forget about Effective Java, 3rd Edition, Item 13: Override clone judiciously.


To override finalize(), all you need to do is simply declare it, without using the override keyword:

class C { protected fun finalize() { // finalization logic } }

According to Java's rules, finalize() must not be private.

Inheritance from Java classes

At most one Java class (and as many Java interfaces as you like) can be a supertype for a class in Kotlin.

Accessing static members

Static members of Java classes form "companion objects" for these classes. You can't pass such a "companion object" around as a value but can access the members explicitly, for example:

if (Character.isLetter(a)) { ... }

To access static members of a Java type that is mapped to a Kotlin type, use the full qualified name of the Java type: java.lang.Integer.bitCount(foo).

Java reflection

Java reflection works on Kotlin classes and vice versa. As mentioned above, you can use instance::class.java, ClassName::class.java or instance.javaClass to enter Java reflection through java.lang.Class. Do not use ClassName.javaClass for this purpose because it refers to ClassName's companion object class, which is the same as ClassName.Companion::class.java and not ClassName::class.java.

For each primitive type, there are two different Java classes, and Kotlin provides ways to get both. For example, Int::class.java will return the class instance representing the primitive type itself, corresponding to Integer.TYPE in Java. To get the class of the corresponding wrapper type, use Int::class.javaObjectType, which is equivalent of Java's Integer.class.

Other supported cases include acquiring a Java getter/setter method or a backing field for a Kotlin property, a KProperty for a Java field, a Java method or constructor for a KFunction and vice versa.

SAM conversions

Kotlin supports SAM conversions for both Java and Kotlin interfaces. This support for Java means that Kotlin function literals can be automatically converted into implementations of Java interfaces with a single non-default method, as long as the parameter types of the interface method match the parameter types of the Kotlin function.

You can use this for creating instances of SAM interfaces:

val runnable = Runnable { println("This runs in a runnable") }

...and in method calls:

val executor = ThreadPoolExecutor() // Java signature: void execute(Runnable command) executor.execute { println("This runs in a thread pool") }

If the Java class has multiple methods taking functional interfaces, you can choose the one you need to call by using an adapter function that converts a lambda to a specific SAM type. Those adapter functions are also generated by the compiler when needed:

executor.execute(Runnable { println("This runs in a thread pool") })

Using JNI with Kotlin

To declare a function that is implemented in native (C or C++) code, you need to mark it with the external modifier:

external fun foo(x: Int): Double

The rest of the procedure works in exactly the same way as in Java.

You can also mark property getters and setters as external:

var myProperty: String external get external set

Behind the scenes, this will create two functions getMyProperty and setMyProperty, both marked as external.

Using Lombok-generated declarations in Kotlin

You can use Java's Lombok-generated declarations in Kotlin code. If you need to generate and use these declarations in the same mixed Java/Kotlin module, you can learn how to do this on the Lombok compiler plugin's page. If you call such declarations from another module, then you don't need to use this plugin to compile that module.

Last modified: 24 June 2024