Why do we use "companion object" as a kind of replacement for Java static fields in Kotlin?

What is the intended meaning of "companion object"? So far I have been using it just to replace Java's static when I need it.

I am confused with:

• Why is it called "companion"?
• Does it mean that to create multiple static properties, I have to group it together inside companion object block?
• To instantly create a singleton instance that is scoped to a class, I often write

:

companion object {
    val singleton by lazy { ... }
}

which seems like an unidiomatic way of doing it. What's the better way?

Solution 1:

• 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 objects 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 like other objects.

• 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() { /* ... */ }
      }
  }
  

• 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 the 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 initializers

  You might also find useful ways of initializing singleton state.

Solution 2:

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.

Solution 3:

Why is it called "companion"?

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()

If you only use 'object' without 'companion', you have to do like this:

val instance = MyClass.Factory.create()

In my understanding, 'companion' means this object is companion with the outter class.

Solution 4:

When the classes/objects with related functionalities belong together, they are like companions of each other. A companion means a partner or an associate, in this case.

Reasons for companionship

Cleaner top-level namespace

When some independent function is intended to be used with some specific class only, instead of defining it as a top-level function, we define it in that particular class. This prevents the pollution of top-level namespace and helps with more relevant auto-completion hints by IDE.

Packaging convenience

It's convenient to keep the classes/objects together when they are closely related to each other in terms of the functionality they offer to each other. We save the effort of keeping them in different files and tracking the association between them.

Code readability

Just by looking at the companionship, you get to know that this object provides helper functionality to the outer class and may not be used in any other contexts. Because if it was to be used with other classes, it would be a separate top level class or object or function.

Primary purpose of companion object

Problem: companion class

Let's have a look at the kinds of problems the companion objects solve. We'll take a simple real world example. Say we have a class User to represent a user in our app:

data class User(val id: String, val name: String)

And an interface for the data access object UserDao to add or remove the User from the database:

interface UserDao {
    fun add(user: User)
    fun remove(id: String)
}

Now since the functionalities of the User and implementation of the UserDao are logically related to each other, we may decide to group them together:

data class User(val id: String, val name: String) {
    class UserAccess : UserDao {
        override fun add(user: User) { }
        override fun remove(id: String) { }
    }
}

Usage:

fun main() {
    val john = User("34", "John")
    val userAccess = User.UserAccess()
    userAccess.add(john)
}

While this is a good setup, there are several problems in it:

1. We have an extra step of creating the UserAccess object before we can add/remove a User.
2. Multiple instances of the UserAccess can be created which we don't want. We just want one data access object (singleton) for User in the entire application.
3. There is a possibility of the UserAccess class to be used with or extended with other classes. So, it doesn't make our intent clear of exactly what we want to do.
4. The naming userAccess.add() or userAccess.addUser() doesn't seem very elegant. We would prefer something like User.add().

Solution: companion object

In the User class, we just replace the two words class UserAccess with the two other words companion object and it's done! All the problems mentioned above have been solved suddenly:

data class User(val id: String, val name: String) {
    companion object : UserDao {
        override fun add(user: User) { }
        override fun remove(id: String) { }
    }
}

Usage:

fun main() {
    val john = User("34", "John")
    User.add(john)
}

The ability to extend interfaces and classes is one of the features that sets the companion objects apart from Java's static functionality. Also, companions are objects, we can pass them around to the functions and assign them to variables just like all the other objects in Kotlin. We can pass them to the functions that accept those interfaces and classes and take advantage of the polymorphism.

companion object for compile-time const

When the compile-time constants are closely associated with the class, they can be defined inside the companion object.

data class User(val id: String, val name: String) {
    companion object {
        const val DEFAULT_NAME = "Guest"
        const val MIN_AGE = 16
    }
}

This is the kind of grouping you have mentioned in the question. This way we prevent the top-level namespace from polluting with the unrelated constants.

companion object with lazy { }

The lazy { } construct is not necessary to get a singleton. A companion object is by default a singleton, the object is initialized only once and it is thread safe. It is initialized when its corresponding class is loaded. Use lazy { } when you want to defer the initialization of the member of the companion object or when you have multiple members that you want to be initialized only on their first use, one by one:

data class User(val id: Long, val name: String) {
    companion object {

        val list by lazy {
            print("Fetching user list...")
            listOf("John", "Jane")
        }

        val settings by lazy {
            print("Fetching settings...")
            mapOf("Dark Theme" to "On", "Auto Backup" to "On")
        }
    }
}

In this code, fetching the list and settings are costly operations. So, we use lazy { } construct to initialize them only when they are actually required and first called, not all at once.

Usage:

fun main() {
    println(User.list)      // Fetching user list...[John, Jane]
    println(User.list)      // [John, Jane]
    println(User.settings)  // Fetching settings...{Dark Theme=On, Auto Backup=On}
    println(User.settings)  // {Dark Theme=On, Auto Backup=On}
}

The fetching statements will be executed only on the first use.

companion object for factory functions

Companion objects are used for defining factory functions while keeping the constructor private. For example, the newInstance() factory function in the following snippet creates a user by generating the id automatically:

class User private constructor(val id: Long, val name: String) {
    companion object {
        private var currentId = 0L;
        fun newInstance(name: String) = User(currentId++, name)
    }
}

Usage:

val john = User.newInstance("John")

Notice how the constructor is kept private but the companion object has access to the constructor. This is useful when you want to provide multiple ways to create an object where the object construction process is complex.

In the code above, consistency of the next id generation is guaranteed because a companion object is a singleton, only one object will keep track of the id, there won't be any duplicate ids.

Also notice that companion objects can have properties (currentId in this case) to represent state.

companion object extension

Companion objects cannot be inherited but we can use extension functions to enhance their functionality:

fun User.Companion.isLoggedIn(id: String): Boolean { }

The default class name of the companion object is Companion, if you don't specify it.

Usage:

if (User.isLoggedIn("34")) { allowContent() }

This is useful for extending the functionality of the companion objects of third party library classes. Another advantage over Java's static members.

When to avoid companion object

Somewhat related members

When the functions/properties are not closely related but only somewhat related to a class, it is recommended that you use top-level functions/properties instead of companion object. And preferably define those functions before the class declaration in the same file as that of class:

fun getAllUsers() { }

fun getProfileFor(userId: String) { }

data class User(val id: String, val name: String)

Maintain single responsibility principle

When the functionality of the object is complicated or when the classes are big, you may want to separate them into individual classes. For example, You may need a separate class to represent a User and another class UserDao for database operations. A separate UserCredentials class for functions related to login. When you have a huge list of constants that are used in different places, you may want to group them in another separate class or file UserConstants. A different class UserSettings to represent settings. Yet another class UserFactory to create different instances of the User and so on.

That's it! Hope that helps make your code more idiomatic to Kotlin.