type casting

[Anonymous]

What should be kept in mind regarding the comple and run without error for the following classes and references:
class One extends Object implements Runnable
class Two extends One implements Observer
Given the references:
One aone = new One();
Two atwo = new Two();
can someone explain which will give compile and run time error and why?
1. Object obj = aone;
Runnable rn = obj;
2. Object obj = aone;
Runnable rn = (Runnable)obj;
3. Object obj = aone;
Observer ob = (Observer)aone;
4. Object obj = atwo;
Observer ob2 = obj;

[Suresh]

faizaharis,
here I am listing the thumb rules for casting and conversion given by RHE.

For object Reference Conversion,

• An Interface may only be converted to its super interface or to Object
  
• A class type may be converted to its superclass type or its superinterface type.
  
• An Array may be converted to an Object type , a Cloneable interface type or an array of Superclass type
  

For Object Reference Casting
Let's say the casting is like this


OldType ot = new OldType();
NewType nt = (NewType) ot;





1. Compile time
   
   
   
   • When both OldType and NewType are classes, one class must be a subclass of the other.
     
   • When both OldType and NewType are arrays, it must be legal to cast an element of OldType to an element of NewType (Refer to the prev. rule).
     
   • We can always cast between an interface and a non-final object.
     
   • Run Time
     
     
     
     • If the NewType is a class, the underlying object in ot must be of NewType or must have inherited from NewType
       
     • If the NewType is an interface, the underlying object in ot, must implement NewType.
       
     
     
     

Based on these rules,
1. Runnable rn = obj;
will not compile since obj is a reference to Object and Object does not implement Runnable.

2. Runnable rn = (Runnable)obj;
will compile and run, since you can cast to any interface type, and the underlying object in obj implements Runnable.

3.Observer ob = (Observer)aone;
will have runtime error, since the class of aone, One does not implement Observer.

4.Observer ob2 = obj;
will have compile time error, since you cannot cast an Object reference to Observer, since Object does not implement Observer.

One more thing to be noted while casting is,

• Compile time rules apply to the references
  
• Runtime rules apply to the underlying objects.
  


Hope, I am right,
If there's anything wrong, please point it!!
Thanks and regards,
Suresh.

[This message has been edited by Suresh (edited May 21, 2000).]

[Anonymous]

thanx for the reply...i got it!
wana test?? give a few examples if u can?
thanx alot