ArrayList<DiskBean> ShowCart = new ArrayList<DiskBean>();
	HttpSession session;
ShowCart = DiskDAO.showCart(cart_id); // get checked disks from db 
	session = request.getSession(true);
		DiskBean ShowCart = (DiskBean) session.getAttribute("cart");
		if(ShowCart==null) {
			ShowCart = new DiskBean();
			session.setAttribute("cart", ShowCart);
		}
//а если уже создана сессия, то setAttribute перезапишет все данные, как их дописать?

DiskBean IsCart = (DiskBean) session.getAttribute("cart");
		if(IsCart==null) {
			IsCart = new DiskBean();
			session = request.getSession(true);
			session.setAttribute("cart", ShowCart);
		} else {
			for (Iterator itr=tempList.iterator(); itr.hasNext();) {
				tempList.add(IsCart);
			}
			session.setAttribute("cart", tempList);
		}

DiskBean ShowCart = (DiskBean) session.getAttribute("cart");

if(tempList==null) {
			session = request.getSession(true);
			session.setAttribute("cart", ShowCart);
		} 

ShowCart = DiskDAO.showCart(cart_id); // get checked disks from db 

Cart cart = session.getAttribute("cart")

List<DiskBean> cart = session.getAttribute("cart");

List<DiskBean> cart = (List<DiskBean>) session.getAttribute("cart");

tempList = (ArrayList<DiskBean>) session.getAttribute("cart");
		if(tempList==null) {
			
			session = request.getSession(true);
			session.setAttribute("cart", ShowCart);
		} else {
			for (Iterator itr=tempList.iterator(); itr.hasNext();) {
				tempList.addAll(ShowCart);
			}
			session.setAttribute("cart", tempList);
		}

public void addDiskBean(DiskBean disk...

	doPost ....
addDiskBean(ShowCart, session);
	}
	
	public void addDiskBean(ArrayList<DiskBean> disk, HttpSession session){
		ArrayList<DiskBean> cart = (ArrayList<DiskBean>) session.getAttribute("cart");
		   if(cart == null){
		      cart = new ArrayList<DiskBean>();
		      session.setAttribute("cart", cart); 
		   }
		   cart.addAll(disk);
		}

public static ArrayList<DiskBean> showCart(int[] cart_id) {
		ArrayList<DiskBean> ShowCart = new ArrayList<DiskBean>(); 
		PreparedStatement stmt = null;
		String showQuery = "select * from disk where disk_id=?";
		System.out.println("Query: "+showQuery);
		
		try {
			currentCon = ConnectionManager.getConnection();
			stmt = currentCon.prepareStatement(showQuery);
			for(int i=0; i<cart_id.length; i++){
				stmt.setInt(1,cart_id[i]);
				rs = stmt.executeQuery();
				while (rs.next()) {
					DiskBean disk = new DiskBean();
					disk.setDisk_id(rs.getInt("disk_id"));
					disk.setTitle(rs.getString("title"));
					ShowCart.add(disk);
				}
			}
			for(int i=0; i<ShowCart.size(); i++) {
			System.out.println("Here we are!"+ShowCart.get(i).getTitle()); 
			}
	}
		catch(Exception e) { System.out.println("Showing disks is failed: An Exception has occurred! " + e); }
		finally {
			if(rs !=  null) {
				try { rs.close(); } 
				catch (Exception e) {} rs = null; 
			}
			if (stmt != null) { 
				try { stmt.close(); } 
				catch (Exception e) {} stmt = null; } 
			if (currentCon != null) { try { currentCon.close(); }
			catch (Exception e) { } currentCon = null; } 
		}
		return ShowCart;
}

ArrayList<DiskBean> cart = (ArrayList<DiskBean>) session.getAttribute("cart");

doPost ...[color=red]session = request.getSession(true);[/color]
		addDiskBean(ShowCart, session);