This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
Java is strongly typed language and the compiler error is a testament to that.
You could also try this, just for learning's sake - Now that your Set can only accept Items and since any class in Java automatically is a subclass of Object, you could use type casting and introduce an instance of Object into the Set. See what happens then.