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casting

nupur dhawan
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Joined: Jun 26, 2002
Posts: 71
code:
1. interface I1 {}
2. interface I2 {}
3. class Base implements I1 {}
4. class Sub extends Base implements I2 {}
5.
6. class Orange {
7. public static void main(String args[]) {
8. Base base = new Base();
9. I1 i1 = base;
10. Sub sub = (Sub)base;
11. }
12. }
this gives a runtime error on line 10 when compiled.
isnt downcasting permitted with an explicit cast??


Nupur. <br />SCJP2.
Dave Vick
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Joined: May 10, 2001
Posts: 3244
Nupur
the thing to remeber here is the is a principle. You can cast one thing to another when the object being cast is a object of the same type it's being cast to.
In your example Base implments I1 so every Base object is a I1 object. Class Sub extends Base and implements I2, so every Sub object is a Base and is a I2.
In your code you create a Base object called base. The assignment on line 9:
I1 i1 = base;
is OK because you are upcasting the Base object to a I1. An upcast, or widening, is accepted here becauase the obejct being cast is a I1,the type being cast to. You can do implicitly because it is a widening cast.
The cast on line 10:
Sub sub = (Sub)base;
will not compile even though it is an explicit cast because the obejct being cast, a Base object, is not a Sub object.
You could do this if you wanted to:
Base b = (Base)i1;
Here it is ok because the i1 variable refers to an I1 object. That I1 object initially came from a Base object so it is a Base object and can be downcast to one.
hope that helps


Dave
nupur dhawan
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Joined: Jun 26, 2002
Posts: 71
Thank you for the explanation Dave .
It helped clarify a lot of my queries .
Thomas Paul
mister krabs
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Joined: May 05, 2000
Posts: 13974
Originally posted by nupur dhawan:
isnt downcasting permitted with an explicit cast??

downcasting with an explicit cast is permitted only when the object you are casting from is the type you are casting to.


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nupur dhawan
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Joined: Jun 26, 2002
Posts: 71
Thank you Thomas ,it is clear to me now .
 
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