implementing and extending

[Willie Toma]

The answers are 1,2, and 4. I would think that #3 would be correct and #2 would be false.

interface IFace{}
class CFace implements IFace{}
class Base{}
public class ObRef extends Base{
public static void main(String argv[]){
        ObRef ob = new ObRef();
        Base b = new Base();
        Object o1 = new Object();
        IFace o2 = new CFace();
        }
}
1)o1=o2;
2)b=ob;
3)ob=b;
4)o1=b;

[This message has been edited by Willie Toma (edited August 29, 2001).]
[This message has been edited by Willie Toma (edited August 29, 2001).]

[Valentin Crettaz]

Hi Willie,
Let's go through the options:
1. o1 is a reference of type Object, o2 is a reference of type IFace pointing to an object of class CFace. Since CFace is implicitely an object the assignment is valid.
2. b is a reference of type Base, ob is a reference of type ObRef pointing to an object of class ObRef. Since ObRef is a subclass of Base the conversion in the assignment is allowed and thus the assignement is calid.
3. b and ob the same as in 2, but this time we try to have a conversion of b to ob, this is not valid since b (of class Base) is a superclass of ob (of class ObRef), thus a explicit cast is needed here otherwise there will be a compilation error.
4. o1 is a reference of type Object, b a reference of type Base pointing to an object of class Base, since Base is_a Object (i.e. Base is a subclass of Object) the assignment is valid.
Keep in mind the following
if you are trying to convert an object in an assignment like this
Oldtype o;
Newtype n = o;
then Oldtype must be a subclass of Newtype OR in the case Newtype is an interface, Oldtype must implement the interface Newtype.
Hopes that helps, let me know if something is wrong or unclear
Val

[Willie Toma]

"You can assign up the inheritance tree from a child to a parent but not the other way without an explicit casting."
I think I'm confusing the parent and the child. 'ObRef' extends 'Base', doesn't that make Base the parent?

[Willie Toma]

As is
o1 = ob // would be considered true
If this was written in the block of code:
IFace iF = new IFace();
CFace cF = new CFace();
Would these be true?
o1 = iF
o1 = cF
iF = cF

[Cameron Park]

I am confused by "I would think that #2 would be correct and #2 and 4 would be false." You said that #2 is correct but #2 and #4 are not? I don't understand what you don't understand.
Firstly, all objects can be assigned to an Object reference. That validates 2 choices. Then an instance can be assigned to the interface reference type that the instance implements.
Hope this helps.