Object expression
- Kotlin course
- Intermediate
- Lesson 32/116
- 5 min
Learn to create anonymous objects with object:.
object expression
An object expression is an anonymous object that implements an interface or class without giving it a name. It is written as object : Type { ... }. It is convenient for one-off implementations.
Example: val listener = object : ClickListener { override fun onClick() = "clicked" }. Here an unnamed object implementing ClickListener is created. It is assigned to a variable and used later. It provides a quick implementation without a class definition; similar to anonymous inner classes in Java.
The ClickListener interface
An object expression is most often used to implement an interface. All methods of the interface must be overridden.
Example: interface ClickListener { fun onClick(): String }. To implement it: val l = object : ClickListener { override fun onClick() = "clicked" }. Calling l.onClick() returns clicked. An object expression gives a one-off implementation without opening a new class file. If the interface has multiple methods, you must override all of them.
Practice An anonymous object implementing ClickListener
Define an interface ClickListener with a fun onClick(): String method. In main create an anonymous object as val listener = object : ClickListener { override fun onClick() = "clicked" } and print the result with println(listener.onClick()).
Starter code
// Define the ClickListener interface
// implement it with an anonymous object in main
// call onClick and printSample solution
interface ClickListener {
fun onClick(): String
}
fun main() {
val listener = object : ClickListener {
override fun onClick() = "clicked"
}
println(listener.onClick())
}Expected output
clicked
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the object expression
val listener = ____ : ClickListener {
override fun onClick() = "clicked"
}val listener = object : ClickListener {
override fun onClick() = "clicked"
}Explanation: object : Type creates an anonymous object.
2 Match the object expression parts
| Term | Meaning |
|---|---|
object : ClickListener |
Anonymous object |
override fun onClick |
Method implementation |
val l = object |
Assignment |
"clicked" |
Return value |
3 When is an <strong>object expression</strong> preferred?
- When a one-off anonymous implementation is needed Correct answer
- When you need to create multiple instances
- When you want to open a new class file
- When you want to open an abstract class
Explanation: An object expression provides a one-off anonymous implementation.