object

Learn to define singleton classes with object.

object declaration

In Kotlin an object declaration defines a singleton class with a single instance. The class definition and the instance creation happen at once; there is no new keyword. Example: object Logger { fun log(message: String) { println("Log: $message") } }. This object is called directly as Logger.log("..."). There is only one Logger instance for the whole application, which makes it suitable for global state and helper functions.

The Logger example

The body of an object declaration can hold functions and properties. You access them through the object name; no instance creation is needed.

Example: object Logger { fun log(message: String) { println("Log: $message") } }. Calling Logger.log("Started") prints Log: Started. Inside an object, this refers to the object itself; it stays the same for the whole application. It is suitable for configuration, logging and helper functions.

Exercise Define the Logger object

Define an object named Logger. Add a fun log(message: String) method whose body writes println("Log: $message"). In main call Logger.log("Started").

Show example solution

object Logger {
    fun log(message: String) {
        println("Log: $message")
    }
}

fun main() {
    Logger.log("Started")
}

Test yourself

What does an <strong>object</strong> declaration guarantee?
- An abstract class that cannot be instantiated
- A new instance per call
- A single instance (singleton)
- Multiple instances
Show answer

A single instance (singleton) — An object declares a singleton with a single instance.

Complete the object definition

object ____ {
    fun log(message: String) {
        println("Log: $message")
    }
}

Show answer

object Logger {
    fun log(message: String) {
        println("Log: $message")
    }
}

Match the object parts

object Logger

Singleton declaration
fun log(message)

Method
Logger.log(...)

Call
single instance

Guarantee

Next lesson: companion object

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