This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma 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).