Merge two lists in constant time in Java

Question

Does anyone know if it\'s possible to merge two lists (or any collection) in constant time in Java ?

http://www.cppreference.com/wiki/stl/list/splice

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Answer 1

The classes in the JDK library don't support this, as far as I know.

It's possible if you build your own implementation of List - which you're free to do, it's perfectly legal. You could use LinkedLists and recognize the special case that the collection to be added is also a LinkedList.

In documenting your class, you'd need to point out that the added object becomes part of the new object, in other words a lot of generality is lost. There's also lots of potential for error: Altering either of the original lists (if they're mutable) after joining would allow you to create a list with a gap in it, or with two tails. Also, most other operations wouldn't benefit from your hacked-up class. In other words, at first blush it seems like a crazy idea.

Note that "merging" lists usually has different connotations; when merging sorted lists, for example, one would expect the resultant list to have the same ordering. What you're talking about with joining two Linked Lists is really better termed as "splicing". Or maybe just "joining."