What is the intended meaning of “companion object”? Why is it called “companion”?
First, Kotlin doesn’t use the Java concept of static
members because Kotlin has its own concept of object
s for describing properties and functions connected with singleton state, and Java static
part of a class can be elegantly expressed in terms of singleton: it’s a singleton object that can be called by the class’ name. Hence the naming: it’s an object that comes with a class.
Its name used to be class object
and default object
, but then it got renamed to companion object
which is more clear and is also consistent with Scala companion objects.
Apart from naming, it is more powerful than Java static
members: it can extend classes and interfaces, and you can reference and pass it around just as other objects.
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Does it mean that to create multiple static properties, I have to group it together inside
companion object
block?Yes, that’s the idiomatic way. Or you can even group them in non-companion objects by their meaning:
class MyClass { object IO { fun makeSomethingWithIO() { /* ... */ } } object Factory { fun createSomething() { /* ... */ } } }
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To instantly create a singleton instance that is scoped to a class, I often write
/*...*/
which seems like an unidiomatic way of doing it. What’s the better way?It depends on what you need in each particular case. Your code suits well for storing state bound to a class which is initialized upon first call to it.
If you don’t need it to be connected with a class, just use object declaration:
object Foo { val something by lazy { ... } }
You can also remove
lazy { ... }
delegation to make the property initialize on first class’ usage, just like Java static initializersYou might also find useful ways of initializing singleton state.
Why is it called “companion”?
This object is a companion of the instances. IIRC there was lengthy discussion here: upcoming-change-class-objects-rethought
Does it mean that to create multiple static properties, I have to group it together inside companion object block?
Yes. Every “static” property/method needs to be placed inside this companion.
To instantly create a singleton instance that is scoped to a class, I often write
You do not create the singleton instance instantly. It is created when accessing singleton
for the first time.
which seems like an unidiomatic way of doing it. What’s the better way?
Rather go with object Singleton { }
to define a singleton-class. See: Object Declarations You do not have to create an instance of Singleton
, just use it like that Singleton.doWork()
Just keep in mind that Kotlin offers other stuff to organize your code. There are now alternatives to simple static functions e.g. you could use Top-Level-Functions instead.