Object Expressions and Declarations

Sometimes you need to create an object that is a slight modification of some class, without explicitly declaring a new subclass for it.

Kotlin can handle this with object expressions and object declarations.

Object Expressions

Object expressions create objects of anonymous classes, that is, classes that aren't explicitly declared with the class declaration. Such classes are useful for one-time use.

You can define them from scratch, inherit from existing classes, or implement interfaces.

Instances of anonymous classes are also called anonymous objects because they are defined by an expression, not a name.

Creating anonymous objects from scratch

Object expressions start with the object keyword.

If you just need an object that doesn't have any nontrivial supertypes, write its members in curly braces after object:

val helloWorld = object {
    val hello = "Hello"
    val world = "World"
    // object expressions extend Any, so `override` is required on `toString()`
    override fun toString() = "$hello $world"
}

print(helloWorld)

// Output:
Hello World

Inheriting anonymous objects from supertypes

To create an object of an anonymous class that inherits from some type (or types), specify this type after object and a colon (:).

Then implement or override the members of this class as if you were inheriting from it:

window.addMouseListener(object : MouseAdapter() {
    override fun mouseClicked(e: MouseEvent) { /*...*/ }

    override fun mouseEntered(e: MouseEvent) { /*...*/ }
})

If a supertype has a constructor, pass appropriate constructor parameters to it. Multiple supertypes can be specified as a comma-delimited list after the colon:

open class A(x: Int) {
    public open val y: Int = x
}

interface B { /*...*/ }

val ab: A = object : A(1), B {
    override val y = 15
}

Using anonymous objects as return and value types

When an anonymous object is used as a type of a local or private but not inline declaration (function or property), all its members are accessible via this function or property:

class C {
    private fun getObject() = object {
        val x: String = "x"
    }

    fun printX() {
        println(getObject().x)
    }
}

If this function or property is public or private inline, its actual type is:

Any if the anonymous object doesn't have a declared supertype
The declared supertype of the anonymous object, if there is exactly one such type
The explicitly declared type if there is more than one declared supertype

In all these cases, members added in the anonymous object are not accessible. Overridden members are accessible if they are declared in the actual type of the function or property:

interface A {
    fun funFromA() {}
}
interface B

class C {
    // The return type is Any; x is not accessible
    fun getObject() = object {
        val x: String = "x"
    }

    // The return type is A; x is not accessible
    fun getObjectA() = object: A {
        override fun funFromA() {}
        val x: String = "x"
    }

    // The return type is B; funFromA() and x are not accessible
    fun getObjectB(): B = object: A, B { 
        override fun funFromA() {}
        val x: String = "x"
    }
    // explicit return type is required
}

Accessing variables from anonymous objects

The code in object expressions can access variables from the enclosing scope:

fun countClicks(window: JComponent) {
    var clickCount = 0
    var enterCount = 0

    window.addMouseListener(object : MouseAdapter() {
        override fun mouseClicked(e: MouseEvent) {
            clickCount++
        }

        override fun mouseEntered(e: MouseEvent) {
            enterCount++
        }
    })
    // ...
}

Object declarations

The Singleton pattern can be useful in several cases, and Kotlin makes it easy to declare singletons:

object DataProviderManager {
    fun registerDataProvider(provider: DataProvider) {
        // ...
    }

    val allDataProviders: Collection<DataProvider>
        get() = // ...
}

This is called an object declaration, and it always has a name following the object keyword.

Just like a variable declaration, an object declaration is not an expression, and it cannot be used on the right-hand side of an assignment statement.

The initialization of an object declaration is thread-safe and done on first access.

To refer to the object, use its name directly:

DataProviderManager.registerDataProvider(...)

Such objects can have supertypes:

object DefaultListener : MouseAdapter() {
    override fun mouseClicked(e: MouseEvent) { ... }

    override fun mouseEntered(e: MouseEvent) { ... }
}

Data objects

When printing a plain object declaration in Kotlin, the string representation contains both its name and the hash of the object:

object MyObject

fun main() {
    println(MyObject) // MyObject@1f32e575
}

Just like data classes, you can mark an object declaration with the data modifier.

This instructs the compiler to generate a number of functions for your object:

toString() returns the name of the data object
equals()/hashCode() pair

The toString() function of a data object returns the name of the object:

data object MyDataObject {
    val x: Int = 3
}

fun main() {
    println(MyDataObject) // MyDataObject
}

The equals() function for a data object ensures that all objects that have the type of your data object are considered equal.

In most cases, you will only have a single instance of your data object at runtime (after all, a data object declares a singleton). However, in the edge case where another object of the same type is generated at runtime (for example, by using platform reflection with java.lang.reflect or a JVM serialization library that uses this API under the hood), this ensures that the objects are treated as being equal.

The generated hashCode() function has behavior that is consistent with the equals() function, so that all runtime instances of a data object have the same hash code.

Differences between data objects and data classes

While data object and data class declarations are often used together and have some similarities, there are some functions that are not generated for a data object:

No copy() function. Because a data object declaration is intended to be used as singleton objects, no copy() function is generated.
- The singleton pattern restricts the instantiation of a class to a single instance, which would be violated by allowing copies of the instance to be created.
No componentN() function. Unlike a data class, a data object does not have any data properties. Since attempting to destructure such an object without data properties would not make sense, no componentN() functions are generated.

Using data objects with sealed hierarchies

Data object declarations are particularly useful for sealed hierarchies like sealed classes or sealed interfaces, since they allow you to maintain symmetry with any data classes you may have defined alongside the object.

In this example, declaring EndOfFile as a data object instead of a plain object means that it will get the toString() function without the need to override it manually:

sealed interface ReadResult
data class Number(val number: Int) : ReadResult
data class Text(val text: String) : ReadResult
data object EndOfFile : ReadResult

fun main() {
    println(Number(7)) // Number(number=7)
    println(EndOfFile) // EndOfFile
}

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 simply by using the class name as the qualifier:

val instance = MyClass.Factory.create()
// OR
val instance = MyClass.create()

The name of the companion object can be omitted, in which case the name Companion will be used:

class MyClass {
    companion object { }
}

val x = MyClass.Companion

Class members can access the private members of the corresponding companion object.

The name of a class used by itself (not as a qualifier to another name) acts as a reference to the companion object of the class (whether named or not):

class MyClass1 {
    companion object Named { }
}

val x = MyClass1

class MyClass2 {
    companion object { }
}

val y = MyClass2

Note that even though the members of companion objects look like static members in other languages, at runtime those are still instance members of real objects, and can, for example, implement interfaces:

interface Factory<T> {
    fun create(): T
}

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

fun main() {
    val f: Factory<MyClass> = MyClass
    println( f.create())
    println( f.create())
}

// Output:
MyClass@13221655
MyClass@2f2c9b19

However, on the JVM you can have members of companion objects generated as real static methods and fields if you use the @JvmStatic annotation.

Semantic difference

Object expressions are executed (and initialized) immediately, where they are used.
Object declarations are initialized lazily, when accessed for the first time.

Last modified: 07 March 2024