Inner classes must extend?

[Christoffer Jeffson]

Just did John Hunt's Mock exam and stumbled over this:

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.

The correct answer is C. But must inner classes really extend any class or interface?

[Robert Ziel]

Hi Cristoffer,
an inner class my extend another class or implement an interface
or nothing (implicit Object)
robert

[Corey McGlone]

Actually, I find the wording to be rather innacurate. The question states, "What class must an inner class extend?" For one thing, interfaces aren't classes and a class doesn't extend an interface, it implements the interface. Only interfaces can extend interfaces. So, the mention of interfaces is rather hokey with the way the question is worded.
Any class that you create in Java will necessarily extend some object. You might specify a class that it extends or, if you don't, your class will implicitly extend Object. Therefore, every inner class, like any other class, must extend some class. Of course, that can be any class you like.
Even though I dislike the wording, an inner class can also implement any interface (or number of interfaces), but it is not required to do so.
I hope that helps,
Corey

[Thiru Thangavelu]

Is that true? I thought Inner class can extend one class or implement only one interface. Also, I assumed, if it is not extending or implementing, Object class is extended.

[Jose Botella]

An inner class can extend another AND implement many interfaces. An anonymous class only can extend another OR implement an interface.

[Corey McGlone]

Originally posted by Thiru Thangavelu:
Is that true? I thought Inner class can extend one class or implement only one interface. Also, I assumed, if it is not extending or implementing, Object class is extended.

Perhaps you're thinking of an anonymous class. When you use the following syntax to create an anonymous class:

Runnable r = new Runnable()
   {
      public void run() {}
   };

You only have room to identify a single class or interface. However, if you make a simple inner class, like this:

public class A
{
   private class Inner extends Object implements Runnable, Serializable
   {
      public void run() {}
   }
 
   public static void main(String[] args)
   {
   }
}

You can see here that I can extend a class and also implement as many interfaces as I'd like. Of course, as with any class, if no class is extended, the Object class is extended implicitly.
I hope that helps,
Corey