Object Expressions in Kotlin

Introduction

Object Expressions is a fundamental concept every Kotlin developer should understand. Object declarations and companion objects provide singletons, factory methods, and static-like members without the ceremony of Java static blocks.

Object expressions create anonymous objects on the fly. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• Object expressions create anonymous objects on the fly.
• They can implement interfaces or extend classes without declaring a new type.
• Useful for one-time callbacks.

Syntax

val listener = object : Clickable { override fun click() {} }

Object Expressions in Kotlin Example Program in Kotlin

interface Task {
    fun run()
}

fun main(args: Array<String>) {
    val task = object : Task {
        override fun run() = println("Anonymous task running")
    }
    task.run()
}

Sample Output

Anonymous task running

When to use

Use object for true singletons; use companion object for factory methods and constants tied to a class.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. val task = object : Task { assigns or updates a value used later in the program.

3. The println("Anonymous task running") statement writes a line to the console — this produces part of the sample output below.

4. Object expressions create anonymous objects on the fly.

5. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

Common Mistakes

Key Points

• Object expressions create anonymous objects on the fly.
• They can implement interfaces or extend classes without declaring a new type.
• Useful for one-time callbacks.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Semicolons at the end of statements are optional in Kotlin.