Why can't overriding methods throw exceptions broader than the overridden method?

Question

I was going through SCJP 6 book by Kathe sierra and came across this explanations of throwing exceptions in overridden method. I quite didn\'t get it. Can any one explain it

Answer 1

say you have super class A with method M1 throwin E1 and class B deriving from A with method M2 overriding M1. M2 can not throw anything DIFFERENT or LESS SPECIALIZED than E1.

Because of polymorphism, the client using class A should be able to treat B as if it were A. Inharitance ===> Is-a (B is-a A). What if this code dealing with class A was handling exception E1, as M1 declares it throws this checked exception, but then different type of exception was thrown? If M1 was throwing IOException M2 could well throw FileNotFoundException, as it is-a IOException. Clients of A could handle this without a problem. If the exception thrown was wider, clients of A would not have a chance of knowing about this and therefore would not have a chance to catch it.