Declarations and scoping

[Curtis Rutter]

I've looked for an answer to this question and haven't found anything. Hopefully I haven't missed a post somewhere.
Background: I'm studying for the SCJP at a technical school where, unfortunately, none of the instructors at my location are Java experts. I'm stumped by the answer to one of the questions about the following code snippet (commented lines are those I've added to get the code to compile, line numbers as in the test question):

1. class SuperClass {

// public class A {
2. public A getA() {
3. return new A();
4. }
// }
5. }
6. class SubClass extends SuperClass {

// public class B {
7. public B getA() {
8. return new B();
9. }
// }
10. }

The "correct" answer says: "Compilation will succeed if B extends A." Other, "incorrect" answers are that A. compilation will fail because of an error in line 3 and that B. compilation will fail because of an error in line 9.

Given that I had to add code to even get the original code to compile (and doing so without "B extending A"), any thoughts on how the code should look to get to the answer as stated? What concept are they trying to test here? NetBeans pukes on lines 2, 3, 7, & 8.

Thanks!

[Garrett Rowe]

You added the "wrong lines". I think you have to assume classes A and B exist. In that case the only way the code will compile is if B extends A (google java covariant return type).

class SuperClass {
  public A getA() {
    return new A();
  }
}

class Subclass extends Superclass {
  public B getA() {
    return new B();
  }
}

class A {}
class B /* extends A */ {}

[Curtis Rutter]

OK, sometime you miss the forest for the trees.

Great answer, and quick. And the Google suggestion was a good idea that helped.

Thanks!

CR

[Janeice DelVecchio]

Curtis,

Welcome to Java Ranch!!

Glad you got a response so quickly!

[K Abhijit]

this was simple but still useful post....

thanks Rowe, Curtis

[Campbell Ritchie]

The correct answer is "Compilation will succeed if B extends A and you are compiling on Java5 or Java6".

Covariant return was introduced in Java5.

[sharma pankaj]

sheriff is intelligent

[Campbell Ritchie]

sharma pankaj wrote:sheriff is intelligent

Flattery will get you anywhere

[Curtis Rutter]

Looks like the forums on JavaRanch include lots of folks who are willing to help everyone, including greenhorns like me. I'm impressed with the JavaRanch community. I'm running Java5, BTW.
Muchas gracias. CR

[Campbell Ritchie]

You're welcome but it was "GR" who provided 99% of the answer.