You place the function in the "companion object".

So the java code like this:

class Foo {  public static int a() { return 1; }}

will become

class Foo {  companion object {     fun a() : Int = 1  }}

You can then use it from inside Kotlin code as

Foo.a();

But from within Java code, you would need to call it as

Foo.Companion.a();

(Which also works from within Kotlin.)

If you don't like having to specify the Companion bit you can either add a @JvmStatic annotation or name your companion class.

From the docs:

Companion Objects

An object declaration inside a class can be marked with the companionkeyword:

class MyClass {   companion object Factory {       fun create(): MyClass = MyClass()   }}

Members of the companion object can be called by using simply the classname as the qualifier:

val instance = MyClass.create()

...

However, on the JVM you can have members of companion objects generatedas real static methods and fields, if you use the @JvmStaticannotation. See the Java interoperability section for more details.

Adding the @JvmStatic annotation looks like this

class Foo {  companion object {    @JvmStatic    fun a() : Int = 1;  }}

and then it will exist as a real Java static function, accessible fromboth Java and Kotlin as Foo.a().

If it is just disliked for the Companion name, then you can alsoprovide an explicit name for the companion object looks like this:

class Foo {  companion object Blah {    fun a() : Int = 1;  }}

which will let you call it from Kotlin in the same way, butfrom java like Foo.Blah.a() (which will also work in Kotlin).

Docs recommends to solve most of the needs for static functions with package-level functions. They are simply declared outside a class in a source code file. The package of a file can be specified at the beginning of a file with the package keyword.

Declaration

package foofun bar() = {}

Usage

import foo.bar

Alternatively

import foo.*

You can now call the function with:

bar()

or if you do not use the import keyword:

foo.bar()

If you do not specify the package the function will be accessible from the root.

If you only have experience with java, this might seem a little strange. The reason is that kotlin is not a strictly object-oriented language. You could say it supports methods outside of classes.

Edit: They have edited the documentation to no longer include the sentence about recommending package level functions. This is the original that was referred to above.

A. Old Java Way :

1. Declare a companion object to enclose a static method / variable

   class Foo{companion object {    fun foo() = println("Foo")    val bar ="bar"      }}

2. Use :

   Foo.foo()        // Outputs Foo    println(Foo.bar) // Outputs bar

B. New Kotlin way

1. Declare directly on file without class on a .kt file.

   fun foo() = println("Foo")val bar ="bar"

2. Use the methods/variables with their names. (After importing them)

   Use :

   foo()        // Outputs Foo          println(bar) // Outputs bar