object / companion object

// Defining a singleton object
object ObjectName {
    val propertyName = value
    fun methodName() { }
}

// companion object (static members inside a class)
class ClassName {
    companion object {
        val propertyName = value
        fun methodName() { }
    }
}

// Defining and using a singleton object
object AppConfig {
    val appName = "MyApp"
    val version = "1.0.0"
    fun printInfo() = println("$appName v$version")
}

// A class with a companion object
class Counter {
    companion object {
        private var count = 0
        fun increment() { count++ }
        fun getCount() = count
    }
}

fun main() {
    // Use the object directly without instantiation.
    AppConfig.printInfo()
    println(AppConfig.version)

    // companion object members are called via the class name.
    Counter.increment()
    Counter.increment()
    println(Counter.getCount()) // 2
}

An object declaration is a concise way to implement the singleton pattern. A single instance is created at the time of declaration and shared throughout the entire program. It is commonly used to group constants and utility functions.

A companion object defines static members tied to a class. Because Kotlin has no static keyword, a companion object is used to achieve the same effect as Java's static members. It is well suited for defining factory methods and constants.