It is frequently desirable to have a Map (or Set) implementation with
    performance comparable to HashMap (or HashSet) that "preserves the
    order of insertion" of keys (or elements). In other words, when
    iterating over the Map's collection-views, or the set, the
    keys/values/entries/elements should appear in the order than they were
    inserted into the Map (or Set).

    This is useful, e.g., when the user of an API passes keys (or elements) in
    a certain order, and expects to get them back in the same order.