Question about throws exception in interface and realized class

Question

Hi,all
If I define a method In the interface like that-
public void Execute() throws NumberFormatException; In the realized class, I can define this method like that-
public void Execute() throws RuntimeException{
        System.out.println("test");       
}This is a right realization. But if I manage it to throws Exception or to throws Throwable in the class it leads a error. Why add this restrict only to Exception? What is the purpose of this way?
Greetings and thanks,
Jason

Answer 1

When you say "throws NumberFormatException" you are really saying:
When using classes implementing this interface, you have to be prepared to deal with them throwing a NumberFormatException. Other exceptions won't happen.
When you come to implement it, it might not be possible for it to throw a NumberFormatException, so there's no reason why you should have to lie about that. Code handling it directly will be fine (it doesn't throw a NFE) and code handling it via the interface will be fine (it will handle NFEs, they just don't happen).
If your concrete implementation throws other sorts of exception, such as Exception or IOException, and so on, it's incompatible with code that's manipulating the interface. The interface says "NFEs might get thrown, nothing else will." while your implementation says "IOExceptions might get thrown".
So that explains why you can't use arbitrary exceptions. The special case is RuntimeException. RuntimeExceptions don't have to be caught, and it's expected that they can happen at any time - so there's no point trying to catch and handle them as a general case.
The classic example of a RuntimeException is a NullPointerException. You don't need to declare your method as throwing a NullPointerException even though it might get thrown. And you don't have to add NullPointerException to the throws part of the interface definition, because you already know that the concrete implementation could throw a NPE at any time.
Hope that helps (a little).
I'd recommend reading the API documentation on RuntimeException and Exception to get a clearer insight into this.
Dave.