object-declarations.md

Sometimes you need to create an object of a slight modification of some class, without explicitly declaring a new subclass for it. Kotlin handles this case 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 handy 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 need just an object with no nontrivial supertypes, write its members in curly braces after object:

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

{kotlin-runnable="true"}

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 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, appropriate constructor parameters must be passed to it. Many 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 object 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. Overriden 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 { // explicit return type is required
        override fun funFromA() {}
        val x: String = "x"
    }
}

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

Singleton can be useful in several cases, and Kotlin (after Scala) 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 cannot be used on the right hand side of an assignment statement.

Object declaration's initialization is thread-safe and done at 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) { ... }
}

Object declarations can't be local (i.e. be nested directly inside a function), but they can be nested into other object declarations or non-inner classes.

{type="note"}

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 by using simply the class name as the qualifier:

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 fields of a 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()
    }
}

val f: Factory<MyClass> = MyClass

However, on the JVM you can have members of companion objects generated as real static methods and fields, if you use the @JvmStatic annotation. See the Java interoperability section for more details.

Semantic difference between object expressions and declarations

There is one important semantic difference between object expressions and object declarations:

• Object expressions are executed (and initialized) immediately, where they are used.
• Object declarations are initialized lazily, when accessed for the first time.
• A companion object is initialized when the corresponding class is loaded (resolved), matching the semantics of a Java static initializer.