The Ultimate Guide to Kotlin Programming/

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Companion Objects and Class Members

Companion Objects and Class Members

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The classes we created so far had properties and instance methods. If a property or a method is needed at the class level and not on a specific instance of the class, we can’t drop them into the class. Instead, place them in a companion object. In Singleton with Object Declaration, we created singletons. Companion objects are singletons defined within a class—they’re singleton companions of classes. In addition, companion objects may implement interfaces and may extend from base classes, and thus are useful with code reuse as well.

Class-level members

In the next example, a MachineOperator class needs a property and a method at the class level; we achieve that using a companion object:

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class MachineOperator(val name: String) {

  fun checkin() = checkedIn++

  fun checkout() = checkedIn--

  

  companion object {

    var checkedIn = 0

    

    fun minimumBreak() = "15 minutes every 2 hours"

  }

}

Within the class, the companion object, literally defined using those keywords, is nested. The property checkedIn within the companion object becomes the class-level property of MachineOperator. Likewise, the method minimumBreak doesn’t belong to any instance; it’s part of the class.

The members of the companion object of a class can be accessed using the class name as reference, like so:

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MachineOperator("Mater").checkin()



println(MachineOperator.minimumBreak()) //15 minutes every 2 hours



println(MachineOperator.checkedIn) //1

Use caution: placing mutable properties ...