public static int getBookID(String isbn){
		
		try {
			   if(conn == null || conn.isClosed()){
			    conn = DatabaseConnection.getConnection();
			   }
			  } catch (SQLException e1) {
			   // TODO Auto-generated catch block
			   e1.printStackTrace();
			  }
		
		int bookID = 0;
		
		try {
			PreparedStatement stmnt = conn.prepareStatement("SELECT book_id FROM Books WHERE isbn = ?");
			stmnt.setString(1, isbn);
			ResultSet res = stmnt.executeQuery();
			bookID = res.getInt(1);
			System.out.println("opgevraagde book id dao: " + bookID);
			}
		catch(SQLException e) {
			System.out.println(e);
		}
		
		
		return bookID;
	}