Inheritence

[Bishal P]

This is from Dan's site

 1. interface I1 {}
 2. interface I2 {}
 3. class Base implements I1 {}
 4. class Sub extends Base implements I2 {}
 5.
 6. class Red {
 7.   public static void main(String args[]) {
 8.     Sub s1 = new Sub();
 9.     I2 i2 = s1;
10.     I1 i1 = s1;
11.     Base base = s1;
12.     Sub s2 = (Sub)base;
13.   }
14. }
What is the result of attempting to compile and run the above program? 
a.  Compiler error at line 9. 
b.  Compiler error at line 10. 
c.  Compiler error at line 11. 
d.  Compiler error at line 12. 
e.  Runtime error. 
f.  Compiles and runs without error. 

And the answer is f, and the reason given is that

Although line 12 appears to be the source of a compiler error it is not. In line 12, the reference named base actually refers to an object of type Sub. Therefore, the reference to the object may be cast to type Sub.

which is quite logical. But i guessed the answer to be line 10: I1 i1 = s1;
because i thought that i1 and s1 have no direct hierarchy between them.
But seems like since base implements i1 and then sub extends base, s1 can be assigned to i1.
Is this correct?

[Chitra Jay]

Yes its correct.
Sub extends Base which implements interface I1.
Hence Sub indirectly implements I1,and is therefore a supertype.
A supertype reference can be assigned a subtype reference and so no compiler error.
Chitra