Some more basic quries

Question

Hi All,
Thanx for the great responses from you all last time. I am back with some more doubts, please solve them:
1. Why there can be only one public class per source file?
2. Why the name of the public class must match the name of the source file/compilation unit?
3. Why private/protected access modifiers not applicable to classes? Is there any means by which we can apply these modifiers to a class.
4. The result of 5.0%0 is NaN : What does "NaN" means/signifies?
5. The non-access modifier "static" not applicable to classes? Or Y cant we have static classes?
6. Please follow the below hierarchy:
public class Parent
public void parentmethod()
System.out.println(�parentmethod�);
public class Child extends Parent
public void childemethod()
System.out.println(�childmethod�);
public static void main(String args[])
Parent obj = new Child();
obj.parentmethod(); // prints parentmethod
If I add the below statement and then compile it gives the compilation error
obj.childmethod();
Why the compiler throws an error when the above 'obj' [parent] reference is used to access the Child specific methods, even though the Child object has them? but if I access the parent class methods it executes perfectly.
Thanks & Regards
-Sandeep

Answer 1

For your last question, Java compiler only knows that obj has only one method, .parentmethod(). If you need to call .childmethod(), obj must be cast to Child type and then access to .childmethod()
(Child)obj.childmethod();