Reference variable casting w.r.t interfaces

[Rekha Srinath]

Source: Get Certified Exam Lab / Practice Exam 1, Q No. 29
Which of the lines cause exception?
Answer: None of the line cause exception.

1.  interface Min{}
       2.
       3.  class A{}
       4. 
       5.  class B extends A implements Min{}
       6.
       7.  class C extends B{}
       8.
       9.  public class CCExp{
     10.      public static void main(String args[]){
     11.           A a=new A();
     12.           B b=new B();
     13.           C c=new C();
     14.           Min m=null;
     15.           b=(C)m;
     16.           m=(Min)b;
     17.           A a2=(A)m;
     18.           B b2=(B)a2;
     19.           Min m2=(C)b2;
     20.           C c2=(C)m2;
     21.      }
     22. }

I am able to understand till line 16.

But in line 17, there is a casting done on m with reference variable of type A. My doubt is: When there is no relation between A and Min, how is this reference variable casting working? What are the rules behind this? And beyond line 17 too, I am getting confused

I am having a tough time understanding this. If anyone could explain this concept (casting with respect to interfaces specifically) in a simple way, I would be pleased.

Thanks in advance.
[ November 20, 2008: Message edited by: Rekha Srinath ]

[Ankit Garg]

Well line 17 works as A and Min have a common sub type B. So the object represented by m might point to an object of class B this is why the assignment worked. Even the cast is not neccesary. If however m points to an object of type other than B, then there will be a ClassCastException at runtime but in the question we are concerned about compilation only. This problem that I posted when I first came to javaranch is also a similar problem but it is related to instanceof...
[ November 20, 2008: Message edited by: Ankit Garg ]

[Rekha Srinath]

Thanks Ankit..I think I am getting it. Will get back in case of further doubts.