Is there an insertion order preserving Set that also implements List?

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TreeSet is sorted by element order; LinkedHashSet retains insertion order. Hopefully one of those is what you were after.

You’ve specified that you want to be able to insert at an arbitrary location, I suspect you’ll have to write your own – just create a class containing a HashSet<T> and an ArrayList<T>; when adding an item, check whether or not it’s in the set before adding it to the list.

Alternatively Apache’s commons-collections4 offers ListOrderedSet and SetUniqueList, which behave similarly and should meet the given requirements.

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