Error in Question enthuware.ocajp.i.v7.2.899 :

[ETS User]

Interface in question enthuware.ocajp.i.v7.2.899 must be defined like this:

interface I { String toString(); }


without this definition we have compile error.

[admin]

Are you sure?

[admin]

Hint: Every object has a toString() method.

[adrian110288]

OK I can understand that every interface gets all the methods from the Object class. I read that on StackOverflow:

When an interface has no direct SuperInterface, it will create abstract public method for all those public methods present in the Object class.

SO please someone explain me how are we not forced to implement those methods in the class when its clearly said that those methods from Object are public static ??!

[admin]

I have no idea what stackoverflow says.
But since every object ultimately extends from Object, why would you need to implement any method from Object class in your class? Your class already has those methods because of inheritance.

I don't know what you mean by all methods from Object are public static. Where did you read that?

HTH,
Paul.

[adrian110288]

I am sorry, my mistake. I meant to say public abstract.
And also interfaces are not really Objects, are they? They dnt extend from Object class. I am sorry if i brought confusion, I just dnt see the reason in that interfaces get Object methods.

[admin]

I am sorry, my mistake. I meant to say public abstract.
And also interfaces are not really Objects, are they? They dnt extend from Object class.

No, they are not but every class does extend from Object. So it is immaterial what interface it implements. It will always have all the methods of Object class.

I just dnt see the reason in that interfaces get Object methods.

Again, I have no idea what the person at stackoverflow is saying. But an interface contains only the methods that are declared in that interface.

[admin]

I am sorry, my mistake. I meant to say public abstract.

That is not correct either.

[adrian110288]

OK thank you Paul. Not delving deeper into it , I will just stick to the fact that interfaces do have object methods.

[admin]

OK thank you Paul. Not delving deeper into it , I will just stick to the fact that interfaces do have object methods.

There is no such fact. As I mentioned above, an interface has just the method declarations that are declared in that interface.
-Paul.

[mehdihamlili]

Hello,

I still don't understand , if interface doesn't contains the methods of Object , how did the reference 'i' knows that there's a method called toString() in the object.

Normaly we take the signature from the reference?

Thanks

[admin]

Because every object in Java ultimately extends from Object class. Object class has toString method. That is why you can always call toString method on any reference.

[mehdihamlili]

Even if the reference doesn't contain the method?

For example :

interface I {
}

class C implements I{
public void test(){
}

public static void main(String[] args){
I i=new C();
i.test(); // this is valid even if i doesn't declare test method?
}
}

[admin]

Not sure why do you expect i.test() to work. Do all objects in Java have test() method?

When you call a method on a reference, the compiler has to make sure that it is a valid method call for the type of the reference. Here, i.test() will not compile because the compile knows that the type of i is I and I doesn't have test() method.

I would suggest you to go through a good java book before attempting these mock questions.

HTH,
Paul.

[OlgaAG]

What reason to cast System.out.println ((B) a); ? The heir is placed in the parent variable without cast.
A a = b; System.out.println (a); works fine with the same result

[admin]

No reason other than testing your knowledge

[enthunoob]

This question made me wonder the same as above, if interfaces implement Object methods (such as toString method) as they don't have Object as superclass. It thought this stackoverflow question explained it very well:

https://stackoverflow.com/questions/605 ... ss-in-java