
Signature
Eric Sosman
esosman@acm-dot-org.invalid
Hi Eric,
Is the constructor also only for the class (almost like a static method)
that is run each time an object is instantiated? And if that is the case,
then all the 10 objects that are created really have nothing in them because
the class variable belongs to the class, the constructor also belongs to the
class and finally also the public static int getInstanceCount() method?
Correct me if I am wrong.
One final question is in main there is the instantiation statement:
CountTest c1 = new CountTest();
because this statement is in a for loop that runs 10 times, would the c1
reference variable not point to only the last object (i.e. 10th object)
because each time that it is initialized it points to the last object?
Thanks
Roy
>> Hi,
>>
[quoted text clipped - 68 lines]
>
> I hope this helps.
Lew - 01 Oct 2006 17:26 GMT
(Top-posting re-arranged)
>> instanceCount is a `static' variable, meaning that it
>> belongs to the CountTest class as a whole and not to any
>> particular instance of a CountTest object. To look at it
>> another way, the single instanceCount (there is only one)
>> is "shared" by all the CountTest objects (however many you
>> decide to create).
-- snip --
>> However, the CountTest constructor is written in such a
>> way that it increments instanceCount once each tim a CountTest
>> object is constructed. This isn't really part of initializing
>> the new CountTest, but a sort of side-effect. The constructor
>> is doing two things: Initializing the new CountTest and also
>> incrementing the single shared class variable.
> Hi Eric,
>
[quoted text clipped - 4 lines]
> class and finally also the public static int getInstanceCount() method?
> Correct me if I am wrong.
The constructor does not belong to the class as static elements do, but
constructs the individual instance. Constructors never "belong" to a method,
static or instance. Check the definition of the keyword "static". Study the
tutorials.
> One final question is in main there is the instantiation statement:
>
[quoted text clipped - 3 lines]
> reference variable not point to only the last object (i.e. 10th object)
> because each time that it is initialized it points to the last object?
Correct.
I recommend Bruce Eckel's book, "Thinking in Java",
http://www.mindview.net/Books/TIJ/
Chapter 4 deals with constructors in detail.
Check the index for the various references to the "static" keyword.
- Lew
Roy Gourgi - 01 Oct 2006 18:06 GMT
Hi,
The reason I was asking about the constructor was because of the class
variable instanceCount that is initialized as:
private static int instanceCount=0;
I thought that class variables such as instanceCount could only be altered
from static methods. Is that not the case?
Thanks
Roy
> (Top-posting re-arranged)
>
[quoted text clipped - 46 lines]
>
> - Lew
Tor Iver Wilhelmsen - 01 Oct 2006 18:10 GMT
> I thought that class variables such as instanceCount could only be altered
> from static methods. Is that not the case?
No. A non-final variable can be altered wherever it is visible, and
static variables are visible to both static and non-static (instance)
code.
Patricia Shanahan - 01 Oct 2006 17:27 GMT
> Hi Eric,
>
[quoted text clipped - 4 lines]
> class and finally also the public static int getInstanceCount() method?
> Correct me if I am wrong.
Every class, directly or indirectly, extends java.lang.Object, so every
object, including each CountTest, has all the instance methods declared
for Object. That said, as the name implies, a CountTest does not seem to
have any use of its own. As the name implies, it appears to exist only
to demonstrate the ability to count instance creation events.
When a constructor runs it is associated with a particular instance, and
"this" means the same as it would in a non-static method. In that way,
it is more like a non-static method than a static method. On the other
hand, unlike a static method, you don't need an existing instance of the
class to invoke it.
It is best to think of a constructor as just being a constructor,
without the static/non-static distinction that applies to fields and method.
> One final question is in main there is the instantiation statement:
>
[quoted text clipped - 3 lines]
> reference variable not point to only the last object (i.e. 10th object)
> because each time that it is initialized it points to the last object?
At least in theory, a c1 appears each time the loop body is executed,
references the CountTest that was created by its initializer, and ceases
to exist at the end of that execution of the loop body. The c1 that
existed during the first iteration pointed to the first CountTest. The
c1 that existed during the second iteration pointed to the second
CountTest...
Patricia