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how can a child class be instanciated using it's parent class?

omerejaz
Greenhorn

Joined: May 31, 2001
Posts: 3
hi!
I was trying to instanciate the child class object which inherit the properties of the parent class and an error of incompatible type is shown ,
i did it like this

class child cobj= (cobj) new parent ();
i explicitely type cast the object but still there the problem
i am compiling this program under jdk 1.3 , if there's any way out?
Frank Carver
Sheriff

Joined: Jan 07, 1999
Posts: 6920
"omerejaz",
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Rosie Vogel
Ranch Hand

Joined: Jan 31, 2001
Posts: 228
class child cobj= (cobj) new parent ();
Omerajaz,
Maybe I misunderstand what you are trying to do, but I'm not sure I get what the word "class" is doing here. Also, it seems that you use cobj as an object reference but also as a cast, which doesn't make sense because you normally use class names as casts.
My guess is that you need to write:
child cobj = (child) new parent();
Hope this helps.
Junilu Lacar
Bartender

Joined: Feb 26, 2001
Posts: 4782
    
    8

Originally posted by Rosie Vogel:
> child cobj = (child) new parent();
This will be illegal as well. You can never downcast an object, only object references. Once created, an object can never change its type. You can refer to it as a different type but only if it is substitutable with the object's actual type.
The object created with new parent() is a parent object and can never be thought of as a child object. A child object, however, can be referred to as a parent object.

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Cindy Glass
"The Hood"
Sheriff

Joined: Sep 29, 2000
Posts: 8521
What you have to understand is that when you instantiate an object, the JVM runs out and gets space on the heap and creates a place to put all of the fields (member variables) for the object. This is all set in stone before the fields get initialized. A subclass is very likely to have MORE fields than it's parent class. If you create an instance of the parent, all the casting in the world is NOT going to make additional fields in the object out on the heap.
However if you create a child instance, you can cast it to the parent, because those additional fields will just get ignored.


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