About Question com.enthuware.ets.scjp.v6.2.530 :

[KevinC]

Why is class B {} not a correct answer in this case? The explanation discusses the fact that class A does not have a no-arg constructor, but nowhere in the question does it say that class B needs to extend class A. It asks "Which of the following would be a valid inner class for this class?".

This seems to compile just fine. Am I misreading the question?

Code: Select all

class A
{
    protected int i;

    public A(int i)
    {
        this.i = i;
    }
    
    class B{}
}

[patpat09]

In Teststudio v3.35 build 20130831 this question/answer ist fixed.
The current answers are:

Code: Select all

[1.] right:
class B {}

[2.] wrong:
class B extends A {}
//Note that A doesn't have any no-args constructor, so there is no way B's object can be created.

[3.] right:
class B {  B()  {  System.out.println("i = " + i); }  }

[4.] wrong:
class B { class A {} }
//Inner class cannot have same name as the enclosing class.

[5.] wrong:
class A {}
//Inner class cannot have same name as enclosing class.

But the explanation is still wrong, because there is no Inheritance involved.
//Now, since class A does not define a no-argument constructor the above code will not compile.

[admin]

Hi,
Not sure which explanation you think is wrong. Can you please more specific so that we can help?
Option 2 does involve inheritance.

-Paul.

[piotrkmiotczyk]

It is interesting to know that inner class B can extend its enclosing class A (option 2 is wrong, but another reason is given). Because (haven't tested yet, but thinking aloud) if that were the case it this should make the following possible:

B myB = new B.new B.new B.new B.new B(); //...arbitrary number of recursions

Is exteding enclosing class by an inner class really possible and how would that be useful in java?

[admin]

Is exteding enclosing class by an inner class really possible

Yes, you might want to try it out

and how would that be useful in java?

That is subjective. I can't think of any but I am sure the designers of the language had their reasons for allowing it.

HTH,
Paul.

[piotrkmiotczyk]

Yup. Just to follow up, apart from basic errors I made in the post above, the following is possible.:

A.B myB = new A().new B().new B().new B().new B().new B().new B(); //... etc.

I ran out of patience after a few thousand columns of nesting, though it does throw an error at some point (after extensive copy paste use).

A.B.B.B.B.B.B myB2 = new A().new B();

Also possible. The compiler just seems to ignore all the generations of B in between enclosing class A and whichever B the reference points to.