Make private BTreeMap constructor public about btreemap

[batterseapower]

You have to edit these lines anyway if you are making the change so I don’t see it as particularly problematic that you need to switch to static methods at the same time.

[PoslavskySV]

The problem is actually is that it is not possible to replace inheritors of TreeMap. E.g. one has:

class MyMap extends TreeMap<String, String> {
    public MyMap(Comparator<String> cmp){
        super(cmp);
    }

    // some my methods goes below
    ...
}

This is not possible with BTreeMap because of the private constructor (not even protected), so one need to make a huge refactoring, which is inconvenient.

[batterseapower]

I think that good style with modern Java is to use composition over inheritance (Effective Java, item 16) and either design/document for inheritance or forbid it (item 17). I haven't taken the time to design this class for inheritance, so I've chosen to forbid it.

Other popular implementations of collection types also take this approach -- e.g. Guava's ArrayListMultiMap is final.

Sorry!

[PoslavskySV]

Now I see! Thank you!