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.
A method overrided in subclass can throw the checked exceptions or its subclasses thrown by the method in the super classes or it need not throw any exception. This rule is not applicable to unchecked exception.
In your code classnotfoundexception is a checked exception and runtime exception is unchecked exception.