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.
if it's some 3rd party class or java class (i.e. anything other than your own class), you might want to make sure you have all the required jars in the classpath... (I know really generic answer, but can't make it out unless I know for what class are you getting this error).