What is an object and a companion object in Kotlin?

An object in Kotlin is a way to create a singleton. It is declared using the object keyword and is the only instance of the class, initialized upon first access.

object Singleton {
    fun doSomething() {
        // Singleton logic
    }
}

A companion object is tied to a class and is used to declare static elements (fields and methods) accessible by the class name. Only one companion object can exist per class. It is initialized when the class is loaded.

class MyClass {
    companion object {
        const val CONSTANT = 123 // Static constant
        fun staticMethod() { // Static method
            // Static method logic
        }
    }
}

// Using static elements
val constantValue = MyClass.CONSTANT
MyClass.staticMethod()

Comparison:

Property	Object (object)	Companion object
Purpose	Singleton (single instance)	Static elements of the class (fields, methods)
Access	By object name	By class name
Number per class	Not tied to the class (can be standalone)	One per class
Initialization	On first access to the object	When the class is loaded