How to .resolve*() and .relativize() Paths which are not issued from the same FileSystem?

[Francis Galiegue]

Hello,

I am writing a generic java.nio.file API to allow users to implement
FileSystem over various stuff (for instance, DropBox and Google Drive
come to mind) but I have trouble with the methods mentioned in the
subject.

Let us suppose DropBox. I link the provider to URI scheme "dropbox".

Now, I create two FileSystems out of the provider, on two different
accounts. If I have a path issued from one provider and another issued
from the second, at a first glance it does not make any sense to
resolve/relativize one against the other.

However, the javadoc of java.nio.file only covers the case of
different _providers_ for two different paths: "Unless otherwise
noted, invoking a method of any class or interface in this package
created by one provider with a parameter that is an object created by
another provider, will throw ProviderMismatchException."

And there is no such exception as FileSystemMismatchException defined;
so, how do I handle this case? Do I define this (unchecked) exception?
Should the documentation be updated to account for "incompatible"
FileSystems? Do I throw ProviderMismatchException anyway?

-- 
Francis Galiegue, fgaliegue at gmail.com, https://github.com/fge
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