Question about Exception!

Question

class A
public void f() throws Myexcption
class B extends A
public void f() throws [Myexception][devied exception from Myexception]
Why B.f() can only throw less exceptions than A.f()?
if A temp = new B();
temp.f() will perform the B.f()'s defination. And if I defined B.f() as
public void f() throws AnotherException
throw AnotherException;
It will has nothing to do with A.f().
Why did the designer design the less exception throwwing rules?
Thanks

Answer 1

class MyFirstException extends Exception {}
class MySecondException extends MyFirstException {}
class A {
  public void f() throws MyFirstException { ... }
class B extends A {
  public void f() throws MySecondException { ... }
Why B.f() can only throw less exceptions than A.f()?The reason is one of decoupling. If you write code to the contract of A but are actually using an instance of B and the compiler allowed new exceptions to be thrown (not inheriting from the exceptions declared on the original contract) - you would have no way of knowing which other exceptions you must catch. Therefore by enforcing that an overriding method must throw only exceptions within the inheritance hierarchy of the original contract you can be certain to have caught all possible exceptions.
EG
A a = new B();
try {
  a.f();
} catch(MyFirstException e) { ... }--
Talden