While reviewing FileChannel#map API after merging the Panama Foreign I noticed some inconsistencies between implementation and documentation:

The new method returning MemorySegment, which was added to FileChannel class couldn't be made abstract (backwards compatibility). Therefore the default implementation is just throwing UnsupportedOperationException.

In the documentation it says:
     * @throws UnsupportedOperationException
     * If an unsupported map mode is specified.

This statement is incomplete, because it also throws this exception when a custom FileSystem implementation returns a FileChannel implementation that does not implement that method.

For the already existing abstract method, the same statement is given, but I'd suggest to change the spec here, too, so a custom FileSystem implementation that does not support mmap could implement the abstract method also with UnsupportedOperationException. I think jrtfs does this (not checked).