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
:
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:
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:
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:
If this function or property is public or private inline, its actual type is:
Any
if the anonymous object doesn't have a declared supertypeThe 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:
Accessing variables from anonymous objects
The code in object expressions can access variables from the enclosing scope:
Object declarations
The Singleton pattern can be useful in several cases, and Kotlin makes it easy to declare singletons:
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:
Such objects can have supertypes:
Data objects
When printing a plain object
declaration in Kotlin, the string representation contains both its name and the hash of the object:
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 objectequals()
/hashCode()
pair
The toString()
function of a data object returns the name of the object:
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 adata object
declaration 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 object
does not have any data properties. Since attempting to destructure such an object without data properties would not make sense, nocomponentN()
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:
Companion objects
An object declaration inside a class can be marked with the companion
keyword:
Members of the companion object can be called simply by using the class name as the qualifier:
The name of the companion object can be omitted, in which case the name Companion
will be used:
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):
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:
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 detail.
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) that matches the semantics of a Java static initializer.