New to Java

I was trying to use Runtime.getRuntime().exec() to run a very simple scirpt that only contains one line:
su $1

In my program, i was trying to feed in the password by :
PrintWriter pw = new PrintWriter(new OutputStreamWriter(p.getOutputStream()));
pw.println(password);
pw.flush();

and I am getting this error when I read from p.getErrorStream()

I do not have much experience with Linux and scripting. Any suggestions will be highly appreciated!