The default file system provider returns URI's with trailing slashes if the URI is known to refer to a directory. This only way to know for sure if a URI refers to a directory or a file is if the directory actually exists on disk. For example:

    Path rootPath = Paths.get(URI.create("file:/"));
    System.out.println(rootPath.toUri());
    Path dir1 = rootPath.resolve("dir1");
    System.out.println(dir1.toUri());
    Path dir1Path = Files.createDirectory(dir1);
    System.out.println(dir1.toUri());
    System.out.println(dir1Path.toUri());

If C:\dir1\ does not exist before running this code, the result will be:

file:///C:/
file:///C:/dir1
file:///C:/dir1/
file:///C:/dir1/

Notice that dir1.toUri() does not contain a trailing slash before the directory is created, but it does contain a trailing slash afterward.

I have verified this same behavior on OSX as well.