Implementing a concurrent LinkedHashMap

Question

I\'m trying to create a concurrent LinkedHashMap for a multithreaded architecture.

If I use Collections#synchronizedMap(), I would have to use synchroni

Answer 1

Um, simple answer would be to use a monotonically increasing key provider that your Comparator operates on. Think AtomicInteger, and every time you insert, you create a new key to be used for comparisons. If you pool your real key, you can make an internal map of OrderedKey.

class OrderedKey implements Comparable> {
  T realKey;
  int index;
  OrderedKey(AtomicInteger source, T key) {
    index = source.getAndIncrement();
    realKey = key;
  }
  public int compareTo(OrderedKey other) {
    if (Objects.equals(realKey, other.realKey)) {
      return 0;
    }
    return index - other.index;
  }


}

This would obviate the need for a custom comparator, and give you a nice O(1) method to compute size (unless you allow removes, in which case, count those as well, so you can just subtract "all successful removes" from "all successful adds", where successful means an entry was actually created or removed).