Kotlin - class extension

Add open in front of class to extend class in kotlin.

class KotlinPlayGroundActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_kotlin_play_ground)

        val people = People("jinhan", 30)
        println("name : ${people.name}, age : ${people.age}")

    }

    class People(name: String, age: Int) : Person(name, age) {
    }

    open class Person(val name: String, val age: Int) {

    }
}