Java Hashtable的put不能为null

public java.util.ArrayList<java.util.Hashtable<String, String>> UserList() {

  ArrayList<Hashtable<String, String>> list = new ArrayList<Hashtable<String, String>>();

  String sql = "Select * from user where Deleted=0 order by orderby";

  Connection con = null;
  Statement stmt = null;
  ResultSet rs = null;

  try {
   con = DBConnection.getConnection();
   stmt = con.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,
     ResultSet.CONCUR_READ_ONLY);
   rs = stmt.executeQuery(sql);

   while (rs.next()) {
    Hashtable<String, String> user = new Hashtable<String, String>();
    
    user.put("autoID", String.valueOf(rs.getInt("id")));
    user.put("UserName", rs.getString("UserName")+""); //如果数据为null的时候就会出错，所以加+""
    user.put("realName", rs.getString("RealName")+"");
    user.put("email", ""+rs.getString("Email"));
    user.put("qq", ""+rs.getString("QQ"));
    user.put("addr", ""+rs.getString("Addr"));
    user.put("tel", ""+rs.getString("tel"));
    
    list.add(user);
   }
  } catch (final Exception e) {
  } finally {
   try {
    if (rs != null)
     rs.close();

    if (stmt != null)
     stmt.close();

    DBConnection.freeConnection(con);
   } catch (final Exception ex) {
   }

   rs = null;
   stmt = null;
   con = null;
  }

  return list;

 }