You have been tasked with writing a program that expects user input and, as always users do not always enter the right sort of information. As a consequence some of the code you have written will cause some methods calls to throw exceptions. Some of these methods throw unchecked exceptions whilst others return an invalid value (e.g. -1 or null). What is the differences between a checked and unchecked exception? Why do some methods return invalid values rather than throwing an exception?以上测试