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Re: Why can one invoke setters and getters from a (class) constructor?

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On 04/23/2011 01:36 PM, Merciadri Luca wrote:
> My question might appear stupid, and unfounded, but I'm wondering why
> I can invoke (i.e. use) setters and getters in a (class)
> constructor. Setters and getters are defined over the object whose
> type is defined by the class. But the object is only defined once the
> constructor has been parsed totally, isn't it?

Shorter answer: the object is defined before the constructor is called.

Short answer: The object is defined--but possibly not fully 
initialized--at the time the constructor is called.

Long answer: By the time constructor has been called, it has been 
guaranteed that the object's memory has been initialized to the default 
values (i.e., 0 or the equivalent of 0 for the appropriate object type). 
Therefore, it makes sense to refer to "this" within the constructor, so 
it is possible to call anything that refers to "this" (including 
instance methods). However, the subclasses' constructor will not have 
been called by this point in time, so calling a method that could be 
overridden by a subclass might prove problematic.

Longer answer:
So, the initialization of memory happens pretty much at the point you 
type in "new" (or the equivalent newInstance statement when doing 
reflection). This causes all of the local variables to be 
default-initialized to 0. Immediately thereafter (unless someone is 
playing with bytecode), the appropriate constructor is called, which 
then immediately (again, unless someone is playing with bytecode) calls 
the superclass's constructor, etc., until it reaches Object. At that 
point, the calls start to unwind. Note that subclasses do not get a 
chance to really run until after their superclass does, so although it 
is possible to call overridable instance methods, it is not recommended 
to do so because said instance method might expect more initialization 
than is the case. If you do do this, make sure that you emphasize this 
in documentation so implementors at least know that they need to be 
prepared for this case.

-- 
Beware of bugs in the above code; I have only proved it correct, not 
tried it. -- Donald E. Knuth

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Why can one invoke setters and getters from a (class) constructor? Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-23 19:36 +0200
  Re: Why can one invoke setters and getters from a (class) constructor? Lew <noone@lewscanon.com> - 2011-04-23 13:58 -0400
  Re: Why can one invoke setters and getters from a (class) constructor? markspace <-@.> - 2011-04-23 11:00 -0700
    Re: Why can one invoke setters and getters from a (class) constructor? Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-23 23:53 +0200
  Re: Why can one invoke setters and getters from a (class) constructor? Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-04-23 20:36 -0400
    Re: Why can one invoke setters and getters from a (class) constructor? Lew <noone@lewscanon.com> - 2011-04-23 22:37 -0400
  Re: Why can one invoke setters and getters from a (class) constructor? Roedy Green <see_website@mindprod.com.invalid> - 2011-04-24 06:53 -0700
    Re: Why can one invoke setters and getters from a (class) constructor? Fred <fred.l.kleinschmidt@boeing.com> - 2011-04-25 11:06 -0700
      Re: Why can one invoke setters and getters from a (class) constructor? Lew <noone@lewscanon.com> - 2011-04-25 15:14 -0400

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