John Hunt exam #30

[John Towell]

http://www.javaranch.com/maha/_Mock_Exams/JohnJuntMockExam.htm
Q. 30
What class must an inner class extend:
A. The top level class
B. The Object class
C. Any class or interface
D. It must extend an interface
Select the most appropriate answer.
Answer C.
Isn't it true that a inner class cannot extend it's outer class? So the answer C. Any class or interface would be wrong. Also all objects extend from Oject either explicitly or implicitly so would correct answer be B?

[Corey McGlone]

Originally posted by John Towell:
Isn't it true that a inner class cannot extend it's outer class?

Where did you hear that? Try compiling this:

public class Test
{
   class InnerTest extends Test
   {
   }
}

I think you'll find that it compiles just fine. It is perfectly legal for an inner class to extend its enclosing class.

Also all objects extend from Oject either explicitly or implicitly so would correct answer be B?

It's true that, somewhere along the way, all classes obtain the attributes of Object, but that's not the normal way that we think about it. In Java, each class can only extend one class. There is no such thing as multiple inheritance in Java. Therefore, If class A extends class B and class B extends Object, we don't say that class A extends Object. True, class A does obtain the traits of the Object class, we just don't look at as if A extended Object directly. Rather, you say that A extends B and obtains all of the traits of class B, which happen to include the traits of Object, which class B inherited.
You can put up a pretty good argument about this, but the point of the question is simply to make you realize that an inner class can extend any class - there is no requirement as to what is can and can't extend.
I hope that helps,
Corey

[Dan Chisholm]

John,
I agree that the wording of the question could be improved. For example, the question could have asked for all correct answers or only the most correct answer. Also, I assume the author was referring to the enclosing class rather then the "top level" class in answer A.
Apparently, an inner class can extend the enclosing class.

class Orange {
  String s = "Orange.s";
  class Yellow extends Orange {
    String s = "Yellow.s";
    public String toString() {return this.s + "," + super.s;}
  }
  public static void main (String args[]) {
    Yellow y = new Orange().new Yellow();
    System.out.print(y);
  }
}

It would be interesting to see if anyone can suggest a good application for this option.

If you find yourself looking around for additional mock exams, then I hope that you will try mine at the URL contained in my signature.

[John Towell]

Thnaks for the replies guys. I didn't know that an inner class could extend it's outer class. That's cool. I will definately try out your practice exams Dan.
-John