Does PG's JDBC support prepared statements at all?

There is a bunch of documentation for prepared statements in PG's
JDBC, it seems that the only thing prepared statements do is throw
exceptions.

Here's some code I'm trying:

String newvalue = "This is a new value";
int accountnumber = 54;
String qstring = "UPDATE foo SET message = '?' WHERE number = ?";
PreparedStatement st = db.prepareStatement(qstring);
st.setString(1, newvalue);
st.setInt(2, accountnumber);
st.execute();
st.clearParameters();
st.close();

and I always get a Parameter index out of range error, which seems
impossible. Any idea what's going on?