Object expression

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.

Exercise 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()).

Show example solution

interface ClickListener {
    fun onClick(): String
}

fun main() {
    val listener = object : ClickListener {
        override fun onClick() = "clicked"
    }
    println(listener.onClick())
}

Test yourself

Complete the object expression

val listener = ____ : ClickListener {
    override fun onClick() = "clicked"
}

Show answer

val listener = object : ClickListener {
    override fun onClick() = "clicked"
}

Match the object expression parts

object : ClickListener

Anonymous object
override fun onClick

Method implementation
val l = object

Assignment
"clicked"

Return value
When is an <strong>object expression</strong> preferred?
- When a one-off anonymous implementation is needed
- When you need to create multiple instances
- When you want to open a new class file
- When you want to open an abstract class
Show answer

When a one-off anonymous implementation is needed — An object expression provides a one-off anonymous implementation.

Next lesson: companion and interface

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