sql在java使用中的拼接

String sql="select uid,name from user";

Vector gifts = new Vector();
PreparedStatement stmt = null;

ResultSet rst = null;
StringBuffer sql = new StringBuffer(SQL_GET_JIFEN);

GiftForm form = null;
Vector args = new Vector();
try {
   if(!Utilities.changeNull(giftForm.getUserName()).equals("")){
     sql.append(" AND t3.username like ? ");
    args.add("%"+String.valueOf(giftForm.getUserName())+"%");
   }
   if (!Utilities.changeNull(giftForm.getStartDay()).equals("")
     && !Utilities.changeNull(giftForm.getEndDay()).equals("")) {
    sql.append(" AND t2.submitdate >= ? ");
    sql.append(" AND t2.submitdate <= ? ");
    args.add(giftForm.getStartDay());
    args.add(giftForm.getEndDay());
   }