Computational complexity of TreeSet operations in Java?

Question

I am trying to clear up some things regarding complexity in some of the operations of TreeSet. On the javadoc it says:

\"This implementation provides

Answer 1

It is not possible to perform merging of trees or join sets like in Disjoint-set data structures because you don't know if the elements in the 2 trees are disjoint. Since the data structures have knowledge about the content in other trees, it is necessary to check if one element exists in the other tree before adding to it or at-least trying to add it into another tree and abort adding it if you find it on the way. So, it should be O(MlogN)