//First, create an uri to connect with
URI uri = URI.create("ssh://root@localhost");
//Then, you can use it as any URI
Files.newDirectoryStream(uri).forEach(file -> System.out.println(file));

//By default, it searches for an id_rsa and a known_hosts in ~/.ssh. If you'd like to change these parameters, you can pass them via a map
Map<String, ?> parameters = new HashMap<>();
parameters.put(SSHFileSystem.CONFIG_KNOWN_HOSTS, "/some/other/known_hosts");
parameters.put(SSHFileSystem.CONFIG_IDENTITY, "/some/other/id_rsa");
URI uri = URI.create("ssh://root@localhost");
FileSystems.newFileSystem(uri, parameters);