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.
This error occurs when there is a problem with Java SDK versions. For example, it could occur if you had compiled the class under SDK 1.5 or 1.6, and then tried to run it in an Application Server that is running under SDK 1.4.
You can still use the higher release compiler as long as you specify