LRUCache.kt

import java.util.*

class LRUCache<K, T>(private val capacity: Int) {
  private val map: LinkedHashMap<K, T> = LinkedHashMap()

  operator fun get(key: K): T? {
    val value = map[key]
    if (value != null) {
      this[key] = value
    }
    return value
  }

  operator fun set(key: K, value: T?) {
    when {
      // Null value provided, remove the key-value from the cache.
      value == null -> {
        map.remove(key)
      }
      // The cache already contains the key, remove it before inserting, to rearrange to the top.
      map.containsKey(key) -> {
        map.remove(key)
        map[key] = value
      }
      // Max capacity reached, remove the oldest item before inserting the value.
      map.size == capacity -> {
        val it = map.keys.iterator()
        it.next()
        it.remove()
        map[key] = value
      }
      else -> {
        map[key] = value
      }
    }
  }
}