That's not an error, but a warning. If you recompile with javac -Xlint:unchecked BeerExpert.java it will tell you what it thinks the problem is. What it means is that there is a risk of your putting something into the Lists and trying to get it out as a different type, in which case there will be a ClassCastException thrown. That sort of warning only started appearing when Java5 was introduced in Autumn 2004. It can be obviated by using generics, but that is probably too advanced a topic for the earlier pages of a Head First book. By the way: was that book printed before Autumn 2004?