Re: Simple question about instantiating
Roy Gourgi wrote:
Hi,
I am a little bit confused with how OOP works with respect to instantiating.
For example take the following code below. When each of the 10 c1 objects
are instantiated from the CountTest() class, do they not each have their own
instanceCount class variable??? It does not seem to be so from this code
that I took from a book. In my thinking every time an object is created it
initializes the instanceCount class variable to 0 and therefore the count
would always be 1 after the constructor is run and the instanceCount is
incremented. Can someone clarify what actually happens behind the scenes
when an object is instantiated with the class variables.
TIA
Roy
public class CountTestApp
{
public static void main(String[] args)
{
printCount();
for (int i =0; i < 10; i++)
{
CountTest c1 = new CountTest();
printCount();
}
}
private static void printCount()
{
System.out.println("There are now " + CountTest.getInstanceCount() +
" instances");
}
}
class CountTest
{
private static int instanceCount = 0;
public CountTest()
{
instanceCount++;
}
public static int getInstanceCount()
{
return instanceCount;
}
}
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).
Since instanceCount belongs to the class it is initialized
when the class is initialized. Since it does not belong to
any particular object, it does not get re-initialized when
an object is created. (The object should initialize all its
parts, but instanceCount is not part of an object: it is part
of the class.)
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.
I hope this helps.
--
Eric Sosman
esosman@acm-dot-org.invalid