ExamLab Question

[Sagar Shroff]

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;  // How this works ??  :confused: 
     18.           B b2=(B)a2;
     19.           Min m2=(C)b2;
     20.           C c2=(C)m2;
     21.      }
     22. }

I am not able to understand how Line 17 works.There is no relationship between the two.Then why is it compiling.I think i am having some wrong concepts here please can someone solve it ???

[Piyush Joshi]

If you look closely, you'll find that from line 14 onwards you are casting only null.
null can be cast to anything but you can assign the casted reference to only a reference falling in same hierarchy. So everything is legal from line 14 onwards as we are in a single hierarchy.

[Sagar Shroff]

My Understanding of casting says that you can cast i.e upcast or downcast if there is some relationship between the two.I mean Parent child relationship.Now in this problem we are assigning "m" to "a",there is no relationship between the two.Then also it is compiling.I know i am having wrong concepts here that's why this is compiling so please HELP ??

[shuba gopal]

During compilation: Min implements B. So m can be upcast to A for the compilation to work do you agree? Instead of null if you had Min m = new B(), compilation would succeed for A a2 = (A) m. But there will be ClassCastException during runtime.

During runtime: Since m is null ClassCastException is not thrown as m is not found to be pointing at any inappropriate object.

[Piyush Joshi]

Sagar Shroff wrote:My Understanding of casting says that you can cast i.e upcast or downcast if there is some relationship between the two.I mean Parent child relationship.Now in this problem we are assigning "m" to "a",there is no relationship between the two.Then also it is compiling.I know i am having wrong concepts here that's why this is compiling so please HELP ??

lets explore what can be done in casting:
we start with some modifications in your example:

interface Min{}  

class A{}  
 
class B extends A implements Min{}  

class C extends B{}

class D implements Min{}

class E{}
final class F{}

public class CCExp{
public static void main(String args[]){  
       A a=new A();  
       B b=new B();  
       C c=new C();  
       Min m=null;  
       b=(C)m;  // ok, because its similar to saying b = (C)null; and C is a B
       b=(D)m; // not ok, similar to saying b = (D)null; but D is not a B so inconvertible types
       m=(Min)b;//ok, similar to m = (Min)null; and same type Min
       A a2=(A)m;//ok, similar to A a2 = (A)null; and same type A
       B b2=(B)a2;//ok, similar to  B b2=(B)null; and same type B
       Min m2=(C)b2;//ok, similar to Min m2=(C)null; and C is a Min
       C c2=(C)m2;  //ok, similar to  C c2=(C)null; and same type C

       //lets explore something more
       Min mn = new D();
       E e1 = (E)mn; //surprisingly ok!! but what's happening here E does not implement Min!! how the cast is compiling?
       // answer is: checking is deferred to runtime. At runtime there may be a posibility that mn may refer to a subtype of E which implements Min
       // but here in this code it will fail at runtime, a ClassCasException will be thrown
       F f1 = (F)mn; //not ok, can not cast from Min to F
       //the posibility that mn may refer to a subtype of F is eliminated at compile time because F class is final
  }  
}

I think it solves your confusion. Does'nt it?
If you want to learn more about type conversions read Conversions and Promotions

[Sagar Shroff]

shuba gopal wrote:During compilation: Min implements B. So m can be upcast to A for the compilation to work do you agree? Instead of null if you had Min m = new B(), compilation would succeed for A a2 = (A) m. But there will be ClassCastException during runtime.

During runtime: Since m is null ClassCastException is not thrown as m is not found to be pointing at any inappropriate object.

Thanks shuba i got it !!

[Sagar Shroff]

 Consider Min m = new B();
compilation would succeed for A a2 = (A) m. But there will be ClassCastException during runtime.

So based on this i have analyzed 1 thing is that casting is based on the object because m is referring to b object and there is a relationship between class B and A...
Am i right ??.....Correct me if i am wrong.....

[Piyush Joshi]

Sagar Shroff wrote:

 Consider Min m = new B();
compilation would succeed for A a2 = (A) m. But there will be ClassCastException during runtime.

So based on this i have analyzed 1 thing is that casting is based on the object because m is referring to b object and there is a relationship between class B and A...
Am i right ??.....Correct me if i am wrong.....

There is no need for a relationship between class B and A, because Min is an Interface.
If you remove B extends A from your code then also this cast will work, try to do it. (it will compile successfully).
Just imagin for a moment that B and A are unrelated, and change your code to this:

class A{}
interface Min{}
class B implements Min{}

now A and B are unrelated, now also you can write:

Min m = new B();
A a2 = (A)m;

and compiler will not complain. Just try to write the above code and see for yourself.

Why? because during runtime m could be reassigned to a subclass of A which implements Min.
Therefore at runtime there is a possibility of this cast to succeed.
So compiler intelligently defers type checking to runtime, and you do not get any compile error.

But if Min was a class

class A{}
class Min{}
class B extends Min{}

and you try to do the same:

Min m = new B();
A a2 = (A)m;

now compilation will fail. because compiler knows that for this cast to work m should be reassigned to a subclass of A which also extends Min.
but compiler also knows that this is not going to happen, because any class can extend only one class.

So there is difference how compiler handles interface casting. Do you now get my point?
I suggest you do the code changes and try to experiment.

[Ikpefua Jacob-Obinyan]

Hello Piyush... Your analysis is perfect and the Java Language Specification explains it like that. Here is a caption: "The cast is always legal at compile time (because even if S does not implement T, a subclass of S might)."

HtH

Ikpefua

P.S If you click on the link, you need to scroll down the page to a sub-title called; 'CASTING CONVERSION'

[Sagar Shroff]

Thanks Piyush you banged it !!!...i have understood what you were trying to tell !!.....But just 1 thing you said at runtime m could be reassigned to a subclass of A which can implement Min.....now all assignments are made at compile time......so how can you assign sometime at runtime can you give me 1 example Please...???.....

[Piyush Joshi]

At compile time we can only be sure of the type of a variable, but the actual object referenced by that variable is only decided at runtime.

For example:

class A{}  
interface Min{}  
class B implements Min{} 
class C extends A implements Min{}
public class Test{
  public static void main(String args[]){
    Min m = new B();
    if(args.length>0){
    m = new C();
    }
    A a2 = (A)m;
  }
}

at compile time it is certain that type of variable m is Min and type of variable a2 is A. but the type of object referenced by variable m depends on runtime.

So for casting conversions, compile time rules only check on the basis of the type of variables,
if compilation succeeds then in some cases a run time check is required to be performed on the actual objects referenced by the variables.

So when you are checking that a cast will compile or not: use the types of variables not the type of objects referred by them.

[Sagar Shroff]

Thanks Piyush !!!