object
- Kotlin course
- Intermediate
- Lesson 30/116
- 5 min
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.
Practice 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").
Starter code
// Define the Logger object
// add a log method to print the message
// call it by object name in mainSample solution
object Logger {
fun log(message: String) {
println("Log: $message")
}
}
fun main() {
Logger.log("Started")
}Expected output
Log: Started
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does an <strong>object</strong> declaration guarantee?
- An abstract class that cannot be instantiated
- A new instance per call
- A single instance (singleton) Correct answer
- Multiple instances
Explanation: An object declares a singleton with a single instance.
2 Complete the object definition
object ____ {
fun log(message: String) {
println("Log: $message")
}
}object Logger {
fun log(message: String) {
println("Log: $message")
}
}Explanation: object Logger declares a singleton.
3 Match the object parts
| Term | Meaning |
|---|---|
object Logger |
Singleton declaration |
fun log(message) |
Method |
Logger.log(...) |
Call |
single instance |
Guarantee |