Exceptions - To check or uncheck

To decide whether to throw a checked exception or an unchecked runtime exception, you must look at the abnormal condition you are signalling. If you are throwing an exception to indicate an improper use of your class, you are signalling a software bug. The class of exception you throw probably should descend from RuntimeException, which will make it UNCHECKED.
Otherwise, if you are throwing an exception to indicate not a software bug but an abnormal condition that client programmers should deal with every time they use your method, your exception should be CHECKED.