Inheritance with extended classes

[Jason Attin]

Hi guys, I'm having some problems understanding this code.

class A{ 
	public int i = 10; 
	private int j = 20; 
} 
class B extends A{ 
	private int i = 30; // 1 
	public int k = 40; 
} 
class C extends B{ } 
public class TestClass{ 
	public static void main( String args[]){ 
		C c = new C(); 
		System.out.println( c.i); // 2 
		System.out.println( c.j); // 3 
		System.out.println( c.k); } 
	}
}

A. The code will print 10 and 40 if // 3 is commented.
B. The code will print 40 if // 2 and // 3 are commented.
C. The code will not compile because of // 1.
D. The code will compile if the line marked // 2 is commented out.
E. None of these.

Apparently the correct answer is B. Now, I would have thought that C could access i (so c.i was valid) because C is A anyway and even if B's i is private, surely C inherits A's i. But apparently if we want to call i from C we have to do

( (A) c). i

and what I don't get is the reason for the cast.
ANy idea?
thanks

[Henry Wong]

Please QuoteYourSources.

Henry

[praveen kumaar]

Hi! jason,"i" is a private variable in class B so you have not inherit it during sub-classing B and by doing like this-
((A) C).i you are actually casting in accordance for the heirarchy,compiler interpret it as--->((A)((B) C)).i,this is all about how java compiler is intelligent.remember in java a class can only have a single parent for ex-

Class Heirarchy	Sub-classing	IS-A Relationship
Class A	sub-class of Object	A is-a Object,A's only parent is object
Class B	sub-class of A	B is a A but A is-a Object =>B is a Object,B's only parent is A
Class C	sub-class of B	C is a B but B is-a A =>C is a A & A is a Object => C is a object,C's only parent is B

the only parent of C is B,B is A,A is Object.But due to is-a relationship and polymorphism,that holds according to the class heirarchy,we can cast class C to A which is up in the heirarchy(acc. to C).and one more thing you have quote-

Jason Attin wrote:surely C inherits A's i

this is incorrect,c only inherits its super class B member(though it cannot inherit B's private member acc. to the inheritance rule).
so the only reason for casting is a multiple-level inheritance(Acc.to the class heirarchy).

Hope this will help!

kind regards,
Praveen.

[Jason Attin]

Please QuoteYourSources.

Right, sorry it's Glenn, Mitchell. OCAJP Oracle Certified Associate Java SE 8 Programmer Practice Exams (Kindle Location 4738). Enthuware. Kindle Edition.

"i" is a private variable in class B so you have not inherit it during sub-classing B

OK I understand now thanks.
A couple of other points though:
1)Usually if you go up the hierarchy, say you want to access from a subclass variables or methods of a superclass you don't need to cast because if A is superclass of B then B is A. But you need to cast if you go down the hierarchy, correct?
2)In a slightly different scenario, say we have again superclass A, subclass B extending A and subclass C extending B, and let's say that all the variables are all coderanch, then all the members in A are also inherited by B and by C, so C effectively has members in A. Correct?

[Henry Wong]

Jason Attin wrote:
1)Usually if you go up the hierarchy, say you want to access from a subclass variables or methods of a superclass you don't need to cast because if A is superclass of B then B is A. But you need to cast if you go down the hierarchy, correct?

For variables (including ones that hides other variables), that is correct. If you have an A reference that is pointing to a B instance, then you need to cast in order to use B instance variables.

For instance methods (that correctly overrides), casting is not necessary. Instance methods are polymorphic, meaning the overridden method is called.

Henry

[praveen kumaar]

remember this rule while casting-
--->(class you are casting) IS-A (class to which you are casting)=>if this statement holds cast is permissible or other wise you will get a ClassCastException,you can always check for it by using instanceof operator before casting to avoid this kind of error.

Jason Attin wrote:1)Usually if you go up the hierarchy, say you want to access from a subclass variables or methods of a superclass you don't need to cast because if A is superclass of B then B is A. But you need to cast if you go down the hierarchy, correct?

i am not pretty sure what you intended to say but it is not possible to perform a cast down the heirarchy.for-ex-
suppose their is  class A,B is a sub-class of A and C is a sub-class of B.

A toCast =new A();

B castingPerformed=(B) toCast; // ClassCastException because A is not B.

Jason Attin wrote:2)In a slightly different scenario, say we have again superclass A, subclass B extending A and subclass C extending B, and let's say that all the variables are all coderanch, then all the members in A are also inherited by B and by C, so C effectively has members in A. Correct?

Yes all the members of A are inherited in C indirectly and due to the fact that all members are publically accessible,indirectly means via multiple-level-inheritance(i.e.A to B,B to C) but not multiple inheritance.

Kind Regards,
Praveen.