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Casting question

Tony Smith
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Joined: Jul 07, 2007
Posts: 229
If this has been asked recently feel free to direct me to the right post...

I understand it is possible to downcast a parent object to a child object. For example:

Asuusming child extends parent
Parent p = new Parent();
Child c = new Child();

c = (Child)p;

When would this be necessary? And when would this be a bad idea? Thanks.
marc weber
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Joined: Aug 31, 2004
Posts: 11343

I posted an example of this yesterday under this topic. See if that makes sense, and come back with any questions.


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Rob Spoor
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Joined: Oct 27, 2005
Posts: 19543
    
  16

Originally posted by Tony Smith:
When would this be necessary? And when would this be a bad idea? Thanks.

You should do this if:
1) you are 100% sure the object is a Child, or a subclass of Child (p instanceof Child returns true). Your object inheritance tree might ensure that.
2) you need the extra methods / fields that Child provides and Parent does not.

If 1) does not hold you may get a ClassCastException if the is not a Child. If 2) does not hold then there is no reason to cast.


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Stan James
(instanceof Sidekick)
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Joined: Jan 29, 2003
Posts: 8791
Following on from Rob's post, this kind of thing would work:

The compiler assures us the parameter is a Parent or some extension of it, but it can't guarantee that the parameter is a Child. If we left out the instanceof test and somebody passed in a Parent, the (Child)x cast would throw an exception.

This kind of test and cast is often a sign of a design that we should improve, but sometimes it's hard to avoid.


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Adam Nace
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Joined: Jul 17, 2006
Posts: 117
Originally posted by Tony Smith:

Parent p = new Parent();
Child c = new Child();

c = (Child)p;


Note that your code, as written, is not valid. We know that the run-time type of p is Parent, not Child (because we can see that you instantiated it that way). Casting is appropriate where the run-time type is actually a child, and it is not, here.

Also, note that you are creating a new Child object and assigning it to c, only to be overwritten in the next line. Are you sure you understand the difference between defining a variable, and initializing it?

- Adam
 
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