Why LinkedHashSet has boolean accessOrder set to false

Question

In Java LinkedHashSet is created with backing HashSet creating LinkedHashMap with following LinkedHashMap constructor

map = new LinkedHashMap<>(initialCapacity, loadFactor);

Now in LinkedHashMap, the above constructor in turn calls

 public LinkedHashMap(int initialCapacity, float loadFactor) {
    super(initialCapacity, loadFactor);
    accessOrder = false;
}

so is there any way to have a LinkedHashSet with accessOrder set to true? This can help create LRU cache implementation with LinkedHashSet.

Answer 1

LinkedHashSet doesn't support access order, since you are not accessing elements of a LinkedHashSet.

You add elements to the LinkedHashSet, and you can iterate over them in insertion order.

When you check if an element is a member of the LinkedHashSet, you are not accessing it. You check membership via boolean contains(Object o), calls map.containsKey(o) for the backing map. However, containsKey() doesn't affect the access order of the Map.

On the other hand, the get(Object key) method of LinkedHashMap does affect access order, but it is never used by LinkedHashSet.

Answer 2

As you can see from the source code, a LinkedHashSet is backed by a LinkedHashMap with the accessOrder set to false.

Furthermore, there is no public constructor for LinkedHashSet to change the access order from insertion order to access order.

You could try this (just an example):

LinkedHashSet<E> set = (LinkedHashSet<E>) Collections.newSetFromMap(new LinkedHashMap<>(16, 0.75f, true));