Devaka's Exam Lab Question

[Mansi Agarwal]

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.      } }

Dont understand how the conversion. Please help..

[Don Redd]

Hi,
First look at compile time evaluation of below statements

    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;  

line 15: b=(C) m; this line has two castings 1) from m to C(Since m is of Interface type compiler skips check, it passes)
2) from C to b ( since variable b is of Type B which can refer to subclass C,this test passes as well)

work similarly for rest,,,,

second at runtime, you have to check for incompatible assignments, as show in below example

ie: A a=new A();

    B b=(B)a;

the above piece compiles fine but fails at run time

[Don Redd]

one more hint:

the below code compiles and runs fine..........

A a=null;    
B b=(B)a;  

[gurpeet singh]

i'm posting this for the third consecutive time. the question that deals with casting can be confusing and under pressure one can mark wrong answer.the following rules are really helpful i solving casting questions. they are derived from concepts only but are much faster to work with.

consider the generalize scenario

A B = (C) D

1. C and D should have relationship, either C is-a D or D is-a C, otherwise COMPILER ERROR. e.g Dog d = new Dog() ; String s = (String)d;

2. A and C should be of the same type of C should be subtype of A, otherwise COMPILER ERROR.

3. D should refer to a type that is same as of C or subtype of D, otherwise Runtime error.

also if D is null then it can be typecasted to anything(since null can be assigned to anything, except primitives) and the result will be null.

i recommend you follow the above rules and try to solve the question once more and definitely you will be able to crack it. also these are not rules/official, they have been derived from the common concepts that we know. if you are still not able to understand then please post it . we'll help you out

[Mansi Agarwal]

Thanks Don and Gurpreet.

I will keep those points in my mind.