Is there a case where a companion object (singleton) for a class is needed? Why would I want to create a class, say Foo and also create a companion object for it?
The companion object basically provides a place where one can put "static-like" methods. Furthermore, a companion object, or companion module, has full access to the class members, including private ones.
Companion objects are great for encapsulating things like factory methods. Instead of having to have, for example, Foo and FooFactory everywhere, you can have a class with a companion object take on the factory responsibilities.
Companion objects are useful for storing state and methods that are common to all instances of a class but they do not use static methods or fields. They use regular virtual methods which can be overridden through inheritance. Scala truly has nothing static. There are lots of ways you can use this but here's a simple example.
abstract class AnimalCounter
{
var animals = 0
def name: String
def count()
{
animals += 1
println("%d %ss created so far".format(animals, name))
}
}
abstract class Animal
{
def companion: AnimalCounter
companion.count()
}
object Dog extends AnimalCounter
{
val name = "dog"
}
class Dog extends Animal
{
def companion = Dog
}
object Cat extends AnimalCounter
{
val name = "cat"
}
class Cat extends Animal
{
def companion = Cat
}
Which produces this output:
scala> new Dog
1 dogs created so far
scala> new Cat
1 cats created so far
scala> new Dog
2 dogs created so far
scala> new Cat
2 cats created so far
...and it's a good place to store static factory methods (not that DP) for accompanied classes. If you name those overloaded factory methods apply(/.../) you will be able to create/initialize you class
without 'new' (not really that important)
with different possible sets of parameters (compare to what Bloch writes in Effective Java about telescoping constructor)
with the ability to to decide which derived class you want to create instead of the abstract (accompanied) one
Example code:
abstract class AbstractClass;
class RealThing(s: String) extends AbstractClass;
class AlternativeThing(i: Int) extends AbstractClass;
object AbstractClass {
def apply(s: String) = {
new RealThing(s)
}
def apply(i: Int) = {
new AlternativeThing(i)
}
}
// somewhere else you can
val vs = AbstractClass("asdf") // gives you the RealThing wrapped over string
val vi = AbstractClass(123) // gives you AlternativeThing wrapped over int
I wouldn't call the object/base class AbstractXxxxx because it doesn't looks bad: like creating something abstract. Give those names a real meaning.
Consider using immutable, method less, case classes and seal the abstract base class.
In addition to the things Saem said in his reply, the Scala compiler also looks for implicit conversions of types in the corresponding companion objects (of either the source or the target), so the conversions don't need to be imported.
About the reason for singleton objects in general Programming in Scala says:
As mentioned in Chapter 1, one way in which Scala is more object-oriented than Java is that classes in Scala cannot have static members. Instead, Scala has singleton objects (p. 65).
I always see companion objects as a bridge to write both functional and object oriented code in Scala. Many times we just need pure functions which take some input and provide a processing result. Putting those relevant functions in the companion object makes it easy to look up and use, for myself as well as some one building on top of my code.
Moreover, it is a language provided feature to write the singleton pattern without doing anything. This is especially useful when you need a singleton to encapsulate a delegator for the life of JVM. For example, writing a simple HTTP client library in Scala where you can encapsulate an underlying Java implementation based delegator and let consumers of your API live in pure world.
If you define class and object in same file with same name, they known as companion class and object. Scala don't have static as JAVA keyword, You can take as replacement of static with companion class and object in Scala.
For more detail information please check article
class and object keyword in scala programming
At first, it provides a clear separation of static vs non static methods methods.Also provide a simple way to create singleton class.
It also can inherit methods from other classes and/or traits, which cannot be done with Java static methods.and can be passed as a parameter.
Related
The Code A is from the project android/architecture-components-samples.
The author place the code of instance a class DefaultServiceLocator in the interface ServiceLocator.
In my mind , normally a interface should not include any implement code.
Is it a good idea to place the code of instance a class in a interface in Kotlin?
Code A
interface ServiceLocator {
companion object {
private val LOCK = Any()
private var instance: ServiceLocator? = null
fun instance(context: Context): ServiceLocator {
synchronized(LOCK) {
if (instance == null) {
instance = DefaultServiceLocator(
app = context.applicationContext as Application,
useInMemoryDb = false)
}
return instance!!
}
}
/**
* Allows tests to replace the default implementations.
*/
#VisibleForTesting
fun swap(locator: ServiceLocator) {
instance = locator
}
}
...
}
open class DefaultServiceLocator(val app: Application, val useInMemoryDb: Boolean) : ServiceLocator {
...
}
In my mind , normally a interface should not include any implement code.
Welcome back from hibernation ;) Yes, you could achieve the same with interface + abstract class but you can have default implementation also as part of the interface for some time now in many languages. Which way you go is up to you, but if you have only one abstract class implementing your interface then it is often handy to be able to merge this into one file for sake of ease of future maintenance.
As per kotlin interfaces documentation:
Interfaces in Kotlin can contain declarations of abstract methods, as well as method implementations. What makes them different from abstract classes is that interfaces cannot store state. They can have properties but these need to be abstract or to provide accessor implementations.
So... there's no problem in using method implementations on the interfaces. That feature might offer you extra power (if you like and need to use it).
#37 Kotlin Tutorial | Companion Object | Factory Pattern says you can use a Kotlin companion object when creating a class when it is complicated.
Kotlin Primary Constructor and Initializer Blocks shows how to use a Kotlin initializer block in a class.
fun main(args: Array<String>) {
val person1 = Person("joe", 25)
}
class Person(fName: String, personAge: Int) {
val firstName: String
var age: Int
// initializer block
init {
firstName = fName.capitalize()
age = personAge
println("First Name = $firstName")
println("Age = $age")
}
}
Why would you use the companion object to do this rather than the initializer block?
Regarding this question, I have no interest in Java.
This practice of considering static factory methods (aka companion objects in Kotlin) over constructors (or init blocks in Kotlin) has been borrowed from the Java world.
The differences between static factory methods and constructors in Java are pretty much the same as the differences between companion objects and init blocks in Kotlin.
Here's a Medium article which describes the advantages of companion objects neatly. To summarize:
Unlike constructors, they have names
Unlike constructors, they do not require to create a new object each time they’re invoked
Unlike constructors, they can return an object of any subtype of their return type
They reduce verbosity of creating parameterized type instances
EDIT: Added AjahnCharles' comment as it's a valid use-case.
They can return a null object, thereby indicating invalid params being passed for creating the object. ()
As with all design patterns, there are also disadvantages with companion objects:
They cannot be used in subclasses construction
They are not readily distinguishable from other static methods
The general idea behind factory methods is that you have some input to the factory method, execute some logic, and then have "output" that is the necessary parameters for the object constructor.
When you have only a single factory method, the main advantage of using it is that it lets your class expose only a single primary constructor that shows everything your class needs and everything your class retains. The logic to derive all of these properties is elsewhere. I personally think that this is nice, but it is admittedly not an extremely strong reason to prefer a factory method.
However, as soon as you have two different factory methods, the plan of using an initializer block becomes impossible. Even if you create a secondary constructor, any constructor of your class will execute all init blocks... so there's no way to have two "separate" factories using initializers.
The following is a very contrived example, but it does demonstrate behavior that would be impossible with only constructors and init blocks:
class Example(val text: String) {
companion object {
fun firstFactory(): Example {
println("you called the first factory method")
return Example("first")
}
fun secondFactory(): Example {
println("you called the second factory method")
return Example("second")
}
}
}
I've bumped into this code and I'm not sure why would anyone do this. Basically the author decided for making the class constructor private so that it cannot be instantiated outside the file, and added a public method to a companion object in the class that creates a new instance of this class. What is the benefit of this approach?
This is what I found:
class Foo private constructor(private val arg1: Any) {
//more code here..
companion object {
fun newFoo(arg1: Any) = Foo(arg1 = arg1)
}
}
Why is it better than this?
class Foo(private val arg1: Any) {
//more code here..
}
There are several benefits to providing a factory method instead of a public constructor, including:
It can do lots of processing before calling the construstor. (This can be important if the superclass constructor takes parameters that need to be calculated.)
It can return cached values instead of new instances where appropriate.
It can return a subclass. (This allows you to make the top class an interface, as noted in another answer.) The exact class can differ between calls, and can even be an anonymous type.
It can have a name (as noted in another answer). This is especially important if you need multiple methods taking the same parameters. (E.g. a Point object which could be constructed from rectangular or polar co-ordinates.) However, a factory method doesn't need a specific name; if you implement the invoke() method in the companion object, you can call it in exactly the same way as a constructor.
It makes it easier to change the implementation of the class without affecting its public interface.
It also has an important drawback:
It can't be used by subclass constructors.
Factory methods seem to be less used in Kotlin than Java, perhaps due to Kotlin's simpler syntax for primary constructors and properties. But they're still worth considering — especially as Kotlin companion objects can inherit.
For much deeper info, see this article, which looks at the recommendation in Effective Java and how it applies to Kotlin.
If you want to change Foo into an interface in the future the code based on the method will keep working, since you can return a concrete class which still implements Foo, unlike the constructor which no longer exists.
An example specific to android is, that Fragments should be constructed with an empty constructed, and any data you'd like to pass through to them should be put in a bundle.
We can create a static/companion function, which takes in the arguments we need for that fragment, and this method would construct the fragment using the empty constructor and pass in the data using a bundle.
There are many useful cases, for example what Kiskae described. Another good one would be to be able to "give your constructors names":
class Foo<S: Any, T: Any> private constructor(private val a: S, private val b: T) {
//more code here...
companion object {
fun <S: Any> createForPurposeX(a: S) = Foo(a = a, b = "Default value")
fun createForPurposeY() = Foo(a = 1, b = 2)
}
}
Call site:
Foo.createForPurposeX("Hey")
Foo.createForPurposeY()
Note: You should use generic types instead of Any.
I'm aware of two differences between Abstract classes and Interfaces in Kotlin:
An abstract class can have state (e.g. var...)
A class can implement multiple interfaces, but not multiple abstract classes.
Since Kotlin is a rather fresh language, I wonder why Abstract Classes were not abandoned? Interfaces seem superior tool, with a very little need for Abstract Classes.
To elaborate: Kotlin does support concrete function implementation in interfaces, e.g.:
interface Shiny {
fun shine(amount : Int) // abstract function
fun reflect(s : String) { print ("**$s**") } // concrete function
}
Can someone provide a strong practical example of the need for Abstract Classes?
The practical side of abstract classes is that you can encapsulate a part of implementation that works with the state, so that it cannot be overridden in the derived classes.
In an interface, you can only define a property without a backing field, and an implementation class must override that property (with either a backing field or custom accessors).
Given that, you cannot define logic that stores some state in an interface in a reliable way: an implementation class might override the properties in an unexpected way.
Example:
interface MyContainer {
var size: Int
fun add(item: MyItem) {
// ...
size = size + 1
}
}
Here, we provide a default implementation for add that increments size. But it might break if an implementing class is defined like this:
class MyContainerImpl : MyContainer {
override val size: Int
get() = 0
set(value) { println("Just ignoring the $value") }
}
On contrary, abstract classes support this use case and thus allow you to provide some guarantees and contract for all their implementations: they can define some state and its transitions that will stay the same in a derived class.
Apart from that, abstract classes can have non-public API (internal, protected) and final members, whereas interfaces cannot (they can only have private members, which can be used in the default implementations), and all their default implementations can be overridden in the classes.
Abstract classes exist essentially for a hierarchy of classes. For example, if the abstract parent class had a concrete function that was also defined in the child class which extends the parent class, then in certain cases it would be necessary to call the parent's function. When you use an interface it is impossible to do so due to the entirely abstract nature of the class.
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?
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.
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.
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:
We have an extra step of creating the UserAccess object before we can add/remove a User.
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.
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.
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.
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.
We can say that companion is same as "Static Block" like Java, But in case of Kotlin there is no Static Block concept, than companion comes into the frame.
How to define a companion block:
class Example {
companion object {
fun display(){
//place your code
}
}
}
Calling method of companion block, direct with class name
Example.Companion.display