class CounterMap(val map: MutableMap<String, Int>): MutableMap<String, Int> by map {
    override fun put(key: String, value: Int): Int? {
        val previousValue = map[key]
        
        map[key] = (previousValue ?: 0 ) + value
        	
        return previousValue
    }
}

Map<String, exactly Any>

out Any

MutableMap

Map<String, Class<?>> hm = new HashMap<>();

private val myMap = mutableMapOf<String, Long>()
    private fun defaultValue(): Long = 5
    fun get(key: String) {
        myMap.getOrPut(key, ::defaultValue)
    }

fun main(args: Array<String>)

data class

data class

data class ManageUserHelper(val canKick: Boolean, val canBan: Boolean)

constructor(val actingUser: User, val targetUser: User, val relationship: Relationship) {
  val canKick = // do something complicated w/ users + relationship
  val canBan = // do something complicated w/ users + relationship, which may depend on the above
  this(canKick, canBan)
}

buildFrom

companion object

also

apply

source.retryWhen { errors ->  
  errors
    .zipWith(Observable.range(1, 3)) { n, i -> i }
    .flatMap { retryCount -> Observable.timer(Math.pow(5.0, retryCount.toDouble()).toLong(), TimeUnit.SECONDS) }
}

String

String?

String?

val s = buildSequence { while(true) yieldAll(list) }

(Boolean) -> String