HashMap
implementations in general are un-ordered for Iteration
.
LinkedHashMap
is predictablely ordered for Iteration
( insertion order ) but does not expose the List
interface and a LinkedList
( which is what mirrors the key set insertion order ) does not track index position itself either, it is very in-efficient to find the index as well. The LinkedHashMap
doesn't expose the reference to the internal LinkedList
either.
The actual "Linked List" behavior is implementation specific. Some
may actually use an instance of LinkedList
some many just have
Entry
track a previous and next Entry
and use that as its
implementation. Don't assume anything without looking at the source.
The KeySet
that contains the keys does not guarantee order as well because of the hashing algorithms used for placement in the backing data structure of the inherited HashMap
. So you can't use that.
The only way to do this, without writing your own implementation, is to walk the Iterator
which uses the mirroring LinkedList
and keep a count where you are, this will be very in-efficient with large data sets.
Solution
What it sounds like you want is original insertion order index positions, you would have to mirror the keys in the KeySet
in something like an ArrayList
, keep it in sync with updates to the HashMap
and use it for finding position. Creating a sub-class of HashMap
, say IndexedHashMap
and adding this ArrayList
internally and adding a .getKeyIndex(<K> key)
that delegates to the internal ArrayList
.indexOf()
is probably the best way to go about this.
This is what LinkedHashMap
does but with a LinkedList
mirroring the KeySet
instead of an ArrayList
.
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