Kotlin Design Patterns HappyMap.kt

/**
 * Using <code>by</code> keyword you can delegate all but overridden methods
 */
class HappyMap<K, V>(val map : MutableMap<K, V> = mutableMapOf()) : MutableMap<K, V> by map{
    override fun put(key: K, value: V): V? {
        return map.put(key, value).apply {
            if (this == null) {
                println("Yay! $key")
            }
        }
    }
}