About Question enthuware.ocajp.i.v7.2.1244 :

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Javier
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Re: About Question enthuware.ocajp.i.v7.2.1244 :

Post by Javier »

Hi Admin!

I want to ask just one question:

a1=a; // the reference a1 now is pointing to the a object, right?
a=b;// the reference a now is pointing to the b object, right?

I would like to know after these to statements, why is not pointing a1 to the b object??
Why is there not conexion between a1 and b??

It is just because the references are pointing to objects and not to other references??

Thank you so much!!

admin
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Re: About Question enthuware.ocajp.i.v7.2.1244 :

Post by admin »

You have a fundamental problem in your understanding. a, a1, and b are not objects. They are references pointing to objects. (References don't point to other references. A reference can only point to an object or nothing i.e. null).

It's like this - let's say you have a TV (call it, TV1) and a remote (call it, a) that is pointing to that TV. The TV is the object and the remote is a reference pointing to that object.

You now buy another remote (a1) and set this remote to point to the same TV as your previous remote by doing a1 = a. Now you have two remotes pointing to the same TV i.e. TV1.

Now, you change your first remote(a) to point to another TV (TV2) that is pointed to by some other remote b by doing a = b. So now, you have remote b as well as remote a pointing to TV2.

But what happens to the remote a1? Nothing. The second remote will still be pointing to the same TV1 that it was set to point to earlier. Setting a to point to another remote doesn't affect what b is pointing to.
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Sergey
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Re: About Question enthuware.ocajp.i.v7.2.1244 :

Post by Sergey »

it seems it might be helpful to you if you read about the difference between the type of a variable and the type of the object pointed to by a variable.
A a = new B();
A - type of a variable
B - type of the object

Am i correct?

admin
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Re: About Question enthuware.ocajp.i.v7.2.1244 :

Post by admin »

Sergey wrote:
it seems it might be helpful to you if you read about the difference between the type of a variable and the type of the object pointed to by a variable.
A a = new B();
A - type of a variable
B - type of the object

Am i correct?
Correct.
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crazymind
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Re: About Question enthuware.ocajp.i.v7.2.1244 :

Post by crazymind »

a1 fails at run time since a1 hold an instance of A not B ? but a hold an instance of B therefore it is valid.

Dreamweaver
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Re: About Question enthuware.ocajp.i.v7.2.1244 :

Post by Dreamweaver »

can it be corrected like this? (I try it and run)

The line //1 will be allowed during compilation, since assignment is done from a subclass reference to a superclass reference.

The line //1 will be allowed during compilation and run time, since assignment is done from a subclass reference to a superclass reference.

admin
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Re: About Question enthuware.ocajp.i.v7.2.1244 :

Post by admin »

Yes, it will work even at run time.
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