Remove eldest entry from scala LinkedHashMap - scala

Question

I'm trying to use scala's LinkedHashMap as an LRU cache, but I'm not sure how to remove the oldest entry of such a map. I know java's LinkedHashMap has a method removeEldestEntry, but there does not seem to be a similar method for scala's implementation. I'd prefer not to convert to java's implementation just to have access to removeEldestEntry. How can I achieve this?

Answer 1

This will do what you want:
def removeOldestEntry[K](m: scala.collection.mutable.LinkedHashMap[K, _]): m.type =
m -= m.head._1
(Kudos to Jasper-M for pointing out that head will give the oldest entry)

Answer 2

You can do this in the following way:
object myApp {
def main(args: Array[String]) {
val myMap = new MyLinkedHashMap[Int,String]()
myMap.add(1, "a") // Map(1 -> a)
myMap.add(2, "b") // Map(1 -> a, 2 -> b)
myMap.add(3, "c") // Map(1 -> a, 2 -> b, 3 -> c)
myMap.add(4, "d") // Map(1 -> a, 2 -> b, 3 -> c, 4 -> d)
myMap.removeEldest // Map(2 -> b, 3 -> c, 4 -> d)
myMap.get(2) // Map(3 -> c, 4 -> d, 2 -> b)
myMap.removeEldest // Map(4 -> d, 2 -> b)
}
}
class MyLinkedHashMap[K,V] {
import scala.collection.mutable.LinkedHashMap
var map = new LinkedHashMap[K, V]()
/* adds an element to the HaskMap */
def add(key: K, value: V) {
map.put(key, value)
}
/* removes the LRU element from the HaskMap */
def removeEldest {
if (!map.isEmpty) {
map = map.drop(1)
}
}
/* gets the value for the given key and moves it to the top of the HashMap */
def get(key: K): Option[V] = {
val value = map.remove(key)
if (value != None) {
map.put(key, value.get)
}
return value
}
}