About Question enthuware.ocpjp.v7.2.1168 :

[Ambiorix]

I don't follow the given explanation. It states, "the overriding declaration must have a throws clause that is compatible with ALL the overridden declarations".

By the overriding declaration, I assume that means the m1() method implemented in TestClass. In that case, shouldn't the correct answer be: public void m1() throws Exception?

[admin]

Compatible means the throws clause of the overriding method must not violate the throws clause of the overridden method.

throws Exception is neither compatible with throws IOException nor throws SQLException. So why should it be the right answer?

[The_Nick]

Hi, I tried the code out and I added a little line to test some ambiguity:

Code: Select all

public static void main(String args[])  throws Exception
	   {
	     IntefacesTricks tc = new IntefacesTricks();
	     I1 i1 = (I1) tc; //This is valid.
	     i1.m1();
	     tc.m1(); // added line

	     I2 i2 = (I2) tc; //This is valid too.
	     i2.m1();

	   }
	   public void m1()
	   {
	     System.out.println("Hi there");
	   }
	}

The output is 3 times "Hi there". Should not be given a runtimeexception due to ambiguity? How does tc,m1() (without having been cast to any of the interfaces created) know which one to choose? It's acceptable that 2 different interfaces refer to the same method? Cool I did not know that..
Could you provide me with an example of interface method ambiguity? Thanks in advance.

The_Nick.

[admin]

Interface methods never cause ambiguity issue because interface itself has no implementation. So no matter how many interfaces you implement with the same method, there can be only one implementation available in the target class. So the compiler invokes that method.

Try with interface fields to see issues with ambiguity.

[jordanglassman]

I think the question the OP meant to ask is why does the rule about overriding methods having equal to or narrower exceptions not apply here?

What do you mean in your reply by "not violate" and "compatible" in other words.

[admin]

I think the question the OP meant to ask is why does the rule about overriding methods having equal to or narrower exceptions not apply here?

It does apply here. Why do you think it does not?

What do you mean in your reply by "not violate" and "compatible" in other words.

If you throw a wider exception from an overriding method then that violates the throws clause of the overridden method. Not compatible is just another way of saying the same thing.

[jordanglassman]

I see, so valid answers are both:

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public void m1() throws FileNotFoundException {}

and

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public void m1() {}

[admin]

Why do you think public void m1() throws FileNotFoundException {} is valid? As the explanation says, it should be compatible with the declarations from both the interfaces? Do you think it satisfies both of them?

[gablegca]

Overriding methods can declare to throws the same exception, narrower exceptions, none or new runtime exceptions.

In this case m1 to throw no exception it's the only valid answer.

[zhengye1]

Just want to confirm

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public void m1() throws SQLException, IOException{}

will be also valid right?

[admin]

What happened when you tried it out?
-Paul.

[zhengye1]

Just try it, hopefully my understanding is correct.

if I implements the m1() by following

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public void m1() throws SQLException, IOException{}

the compiler will complain because he is trying to match the list of exception of base class(es), but actually m1() in interface I1 doesn't have SQLException, and m1() in interface I2 doesn't have IOException.

So in this case, the best way just not throws any exception in subclass, when the exception is actually occurred in the child class, let the parent to handle it, is that correct?

[admin]

That is correct.