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 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.
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:
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"
}
print(helloWorld)
//sampleEnd
}{kotlin-runnable="true"}
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
}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:
Anyif 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 { // explicit return type is required
override fun funFromA() {}
val x: String = "x"
}
}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++
}
})
// ...
}{id="object-declarations-overview"}
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) { ... }
}Object declarations can't be local (that is, they can't be nested directly inside a function), but they can be nested into other object declarations or non-inner classes.
{type="note"}
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()pairYou can't provide a custom
equalsorhashCodeimplementation for adata object.{type="note"}
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.
Make sure that you only compare
data objectsstructurally (using the==operator) and never by reference (using the===operator). This helps you to avoid pitfalls when more than one instance of a data object exists at runtime.
{type="warning"}
import java.lang.reflect.Constructor
data object MySingleton
fun main() {
val evilTwin = createInstanceViaReflection()
println(MySingleton) // MySingleton
println(evilTwin) // MySingleton
// Even when a library forcefully creates a second instance of MySingleton, its `equals` method returns true:
println(MySingleton == evilTwin) // true
// Do not compare data objects via ===.
println(MySingleton === evilTwin) // false
}
fun createInstanceViaReflection(): MySingleton {
// Kotlin reflection does not permit the instantiation of data objects.
// This creates a new MySingleton instance "by force" (i.e. Java platform reflection)
// Don't do this yourself!
return (MySingleton.javaClass.declaredConstructors[0].apply { isAccessible = true } as Constructor<MySingleton>).newInstance()
}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.
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 adata objectdeclaration is intended to be used as singleton objects, nocopy()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 adata class, adata objectdoes not have any data properties. Since attempting to destructure such an object without data properties would not make sense, nocomponentN()functions are generated.
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
}{kotlin-runnable="true" id="data-objects-sealed-hierarchies"}
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.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.CompanionClass 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 = MyClass2Note 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> = MyClassHowever, 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 detail.
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) that matches the semantics of a Java static initializer.