select col1, col2, ... colN from table_name where col1 = ? ... ...

0 -> TABLE_1
1 -> TABLE_2
2 -> TABLE_3
...

...
String usertypeCode = request.getParameter("usertype_code");
String driver = ("com.mysql.jdbc.Driver");
Class.forName(driver); // MySQL database connection
Connection conn = DriverManager.getConnection("jdbc:mysql://localhost/ebooking" + "?user=user&password=user");

if (usertypeCode.equals("0") {
    usertype = "TABLE_0";
} else if (usertypeCode.equals("1") {
    usertype = "TABLE_1";
} else if (usertypeCode.equals("2") {
    usertype = "TABLE_2";
} else {
    // handle invalid input
}

PreparedStatement pst = conn.prepareStatement("Select * from ? where stu_username=? and stu_password=?");
pst.setString(1, usertype);
pst.setString(2, username);
pst.setString(3, password);
ResultSet rs = pst.executeQuery();
...