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Object Expressions and Declarations

Sometimes we need to create an object of a slight modification of some class, without explicitly declaring a new subclass for it. Java handles this case with anonymous inner classes. Kotlin slightly generalizes this concept with object expressions and object declarations.

Object expressions

To create an object of an anonymous class that inherits from some type (or types), we write:

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 may be specified as a comma-separated 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
}

If, by any chance, we need "just an object", with no nontrivial supertypes, we can simply say:

fun foo() {
    val adHoc = object {
        var x: Int = 0
        var y: Int = 0
    }
    print(adHoc.x + adHoc.y)
}

Note that anonymous objects can be used as types only in local and private declarations. If you use an anonymous object as a return type of a public function or the type of a public property, the actual type of that function or property will be the declared supertype of the anonymous object, or Any if you didn't declare any supertype. Members added in the anonymous object will not be accessible.

class C {
    // Private function, so the return type is the anonymous object type
    private fun foo() = object {
        val x: String = "x"
    }

    // Public function, so the return type is Any
    fun publicFoo() = object {
        val x: String = "x"
    }

    fun bar() {
        val x1 = foo().x        // Works
        val x2 = publicFoo().x  // ERROR: Unresolved reference 'x'
    }
}

Just like Java's anonymous inner classes, code in object expressions can access variables from the enclosing scope. (Unlike Java, this is not restricted to final variables.)

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 is a very useful pattern, 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.

To refer to the object, we use its name directly:

DataProviderManager.registerDataProvider(...)

Such objects can have supertypes:

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

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

NOTE: 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.

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

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()
    }
}

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