Object expression
An object expression is a language construct that creates an object without requiring a separately declared named type.
Kotlin in Action, 2E
An object expression is Kotlin syntax that creates an anonymous object, optionally implementing a class or interfaces, at the point where it is written.
- How it is used
- Creates local interface or class implementations for event listeners and other ad hoc behaviors.
- How deep it goes
- Explains syntax, instance creation, interface combinations, variable capture, mutation, and the boundary with SAM conversion.
- Examples
- Uses MouseListener event handling, stored listeners, and a mutable click counter as concrete examples.
- What you can do after
- Apply it to implement multi-method listeners, capture local state, and choose between anonymous objects and lambdas.
F# in Action: A Revised Edition of Get Programming with F#
An object expression is an F# expression that creates an object implementing specified class or interface members without requiring a separately declared type.
- How it is used
- Uses inline implementations to satisfy interfaces for C# interoperability, runtime-pluggable APIs, and function-shaped contracts.
- How deep it goes
- Defines syntax and application patterns, but stops short of broader advanced alternatives such as active patterns and statically resolved type parameters.
- Examples
- Demonstrates multi-member interfaces, time-display adapters, and selectable printer implementations.
- What you can do after
- Build it from scratch by mapping functions into interface instances without declaring concrete implementation types.
Read how F# in Action: A Revised Edition of Get Programming with F# explains object expression →
Related concepts
- interfaceimplements · A software interface is a contract or boundary that specifies the operations, properties, or capabilities through which types or components interact, without requiring a particular implementation.
- natural language interfaceimplements · A natural language interface enables interaction with a system through human-language text or speech.
- object declarationcontrasts with · An object declaration is a Kotlin declaration that defines a singleton object and creates its sole instance as part of the declaration.
- anonymous structequivalent to · An anonymous struct is a structured type declared without an explicit source-level type name.
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