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Subject: Re: Why can one invoke setters and getters from a (class) constructor?
Date: Sat, 23 Apr 2011 11:00:59 -0700
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On 4/23/2011 10:36 AM, Merciadri Luca wrote:
> But the object is only defined once the
> constructor has been parsed totally, isn't it?


Not exactly.  There still must be some thread of execution that 
initializes an object instance.  That's what you're doing in the 
constructor.

Conceptually, I think of objects (instances) in two parts.  The first 
part is a block of contiguous memory that holds the instance fields and 
resides in the heap.

The second part is the methods of the class.  The methods of a class 
always exist.  They're initialized when the class itself is loaded, and 
there's only one copy of the methods that are shared between all 
instances (objects) of that class.  This part does not live in the heap, 
it lives in the text segment (text here means "program code," not ASCII; 
  it's called "text" for historical reasons).

So you have one class, with the methods, and then 0 or more instances 
(objects) of that class.  Since the methods exist FIRST, when the class 
itself is loaded, before any objects of that class exist, and since the 
methods are separate from the objects, it should be easy to see how the 
methods could be used during initialization.

Conceptually, internally, each method has a reference to a block of 
memory.  Once that memory is allocated, each method just refers to that 
block of memory.  Your memory block is allocated before your constructor 
starts, so voila, there's no problem access your not yet defined 
objects, because the memory is there already.

This method:
   int getValue() {
     return value;
   }

becomes something like this

   int getValue( Object block ) {
     return block.offset[4];
   }

So here the field "value" was probably the 2nd field defined and it 
lives at offset 4 in the memory block your object uses to hold its 
instance fields.

All of this should be very familiar to you from your systems programming 
courses.  I think I did my work in my second or third year of college, 
in a course on compilers and linkers.  Question for you:  what year are 
you at in your Com Sci courses, and have you had compilers yet?