Interface method implementation

[sumi rankan]

This is from enthuware:


Consider these two interfaces:


interface I1
{
void m1() throws IOException;
}
interface I2
{
void m1() throws FileNotFoundException;
}


What methods have to be implemented by a class that says it implements I1 and I2 ?

The answer given is
public void m1() throws FileNotFoundException {}

I thought the answer would be public void m1() throws IOException {} ,since the implementing method should be able to handle both the exceptions .Did I miss something here?
ok obviously I did but cannot figure it out...

[Zdenek Pine]

Hi,
You must remember:
Implementing class must not declare any new or broader checked exceptions for declared interface method. It is possible to throw narrower or uncheck exceptions.
(BTW: constructor of subclass must declare all the checked exceptions declared in the base constructor or may add other exception. This is exactly opposite.)

Because FileNotFoundException is subclass of IOException, FileNotFoundException is ok for both method.

[Zdenek Pine]

p.s. and it is also possible to throw no exception.

[marc weber]

Look at it this way: Your implementing class IS-AN I2, in which the method is declared to throw a FileNotFoundException. So to fulfill the contract with I2, the following must work:

try {
  myImplementation.m1();
}
catch(FileNotFoundExeception e) {
  System.err.println(e);
}

If your method implementation throws an IOException, this will not work.

On the other hand, if your method implementation throws a FileNotFoundException, then your class satisfies both contracts, because it can be treated as either an I1 or an I2.

[sumi rankan]

Thanks..

[Vijitha Kumara]

Just to add to above posts..

If you use a reference of I1 to refer the implementation object then when you call the method m1() you should either catch IOException or declared to be thrown by that method since compiler checks the reference I1.
[ October 21, 2008: Message edited by: Vijitha Kumara ]