SSHJ: How to use StatefulSFTPClient

I'm confused... I am using com/hierynomus/sshj SFTP-lib and following code gives me ClassCastException:

SSHClient ssh = new SSHClient();
//... connect etc
StatefulSFTPClient client = (StatefulSFTPClient) ssh.newSFTPClient();

I need the StatefulSFTPClient but I don't know how to use it... Do I need to extend the SSHClient class with:

public StatefulSFTPClient newStatefulSFTPClient()
        throws IOException {
    checkConnected();
    checkAuthenticated();
    return new StatefulSFTPClient(new SFTPEngine(this).init());
}

Oh my... not the brightest moment - it can be done like this:

StatefulSFTPClient client = new StatefulSFTPClient(new SFTPEngine(ssh).init());

And to mimic the SSHClient.newSFTPClient():

            StatefulSFTPClient client;          
            if(ssh.isConnected()){
                if(ssh.isAuthenticated()){
                    client = new StatefulSFTPClient(new SFTPEngine(ssh).init());
                } else {
                    throw new IllegalStateException("Not authenticated");
                }
            } else {
                throw new IllegalStateException("Not connected");
            }