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