What is the equivalent of Java static methods in Kotlin?

Question

There is no static keyword in Kotlin.

What is the best way to represent a static Java method in Kotlin?

Answer 1

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 companion keyword:

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

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

val instance = MyClass.create()

...

However, on the JVM you can have members of companion objects generated as real static methods and fields, if you use the @JvmStatic annotation. 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 from both Java and Kotlin as Foo.a().

If it is just disliked for the Companion name, then you can also provide 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, but from java like Foo.Blah.a() (which will also work in Kotlin).