Companion object
A companion object is a singleton object associated with a class or type that provides members associated with the class rather than its instances.
Kotlin in Action, 2E
A companion object is the specially designated object declaration inside a Kotlin class whose members can be accessed through the containing class name.
- How it is used
- Provides class-associated factories and operations that need private-class access without requiring an instance.
- How deep it goes
- Defines class-name access, private-constructor factories, naming, interface implementation, and extensions; defers generic details used by the factory example.
- Examples
- Demonstrates MyClass.callMe(), User creation factories, Person JSON loading, JSONFactory implementation, and a Person.Companion extension.
- What you can do after
- Apply it via Kotlin class declarations to expose factories, implement interfaces, and attach class-associated operations.
Functional Programming in Scala, Second Edition
A companion object is a Scala singleton object declared with the same name as a class or other data type and granted access to its private and protected members.
- How it is used
- Organizes construction and operations for a data type beside its constructors.
- How deep it goes
- Defines naming, access privileges, variadic construction, and recursive operations without detailing compiler implementation.
- Examples
- Uses List.apply for construction and List.sum or List.product for recursive pattern-matching operations.
- What you can do after
- Build it from scratch by defining a matching object with constructors, variadic apply, and operations for a functional data type.
Read how Functional Programming in Scala, Second Edition explains companion object →
Related concepts
- object declarationkind of · An object declaration is a Kotlin declaration that defines a singleton object and creates its sole instance as part of the declaration.
- static methodalternative to · A static method is a function associated with a type or enclosing scope rather than an object instance, and callable without an instance receiver.
- constructor methoduses · A constructor is a type-associated operation or callable that creates or initializes an instance and establishes its initial state.
- static fieldalternative to · A named variable whose storage is associated with a class or has static duration, rather than being allocated separately for each object instance or function call.
- factoryimplements · A factory is a function, method, class, or component that creates and returns objects or other values while encapsulating construction details or selection.
- objectkind of · A computational object is a runtime entity or structured value representing data, state, or a resource, with properties or operations defined by its programming model.
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