This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Because neither the compiler nor the JVM can guarantee that the object is an ArrayList<HttpSession>, only that it's an ArrayList, you will never get this to work without a warning. So in cases like this you have to suck it up and use @SuppressWarnings("unchecked").