I haven't debugged it to find the cause. I guess it is all the manual parsing
that's in this code? :-)

The problem occurs if you do

URL url = new URL("vfsmemory://MYID/directory);
URL sub = new URL(url, "sub");

The actual URL comes out incorrectly as:
vfsmemory://MYID/sub
instead of
vfsmemory://MYID/directory/sub

The obvious hack is to do

URL sub = new URL(url + "/sub");

but generic code won't be able to do this.