Kotlin static methods and variables

Question

I want to be able to save a class instance to a public static variable but I can't figure out how to do this in Kotlin.

class Foo {      public static Foo instance;     public Foo() {         instance = this;     }  } 

Answer 1

The closest thing to Java's static fields is a companion object. You can find the documentation reference for them here: https://kotlinlang.org/docs/reference/object-declarations.html#companion-objects

Your code in Kotlin would look something like this:

class Foo {      companion object {         lateinit var instance: Foo     }      init {         instance = this     }  } 

If you want your fields/methods to be exposed as static to Java callers, you can apply the @JvmStatic annotation:

class Foo {      companion object {         @JvmStatic lateinit var instance: Foo     }      init {         instance = this     }  } 

Answer 2

It looks that you want to define a singleton object. It is supported in Kotlin as a first-class concept:

object Foo {   ...  } 

All the boilerplate code with static field and constructor is taken care by the Kotlin automatically. You don't have to write any of that.

From the Kotlin code you can refer to the instance of this object simply as Foo. From the Java code you can referer to the instance of this object as Foo.INSTANCE, because the Kotlin compiler automatically creates the corresponding static field named INSTANCE.