Simple question about instantiating

[Roy Gourgi]

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;
}

}

 

[Eric Sosman]

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.

 

[Roy Gourgi]

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,

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.

 

[Lew]

(Top-posting re-arranged)

> 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.

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:
>
> 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?

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

 

[Patricia Shanahan]

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.

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:

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?

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

 

[Roy Gourgi]

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

 

[Tor Iver Wilhelmsen]

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.