Why Java Collection Framework doesn't contain Tree and Graph

Question

I am familiar with Java Collection Framework which contains basic interfaces: Collection and Map. I am wondering why the Framework doesn\'t contain

Answer 1

I am afraid the answer is: it's too troublesome to design and maintain a generic tree structure and interfaces (see answer to this post). Most users will need the performance of tree-based implementations of lists and sets, but will not be concerned with the internals, so most of them are hidden.