Objects eligible for GC

Hi,

Can anyone pls help. How many objects are eligible for GC when System.gc() is invoked() in the following code:

class MyClass{
public void finalize()
{System.out.println("finalize");}

class Test{
private static void f (MyClass[] array){
array[0]=null; //line 1
array[1]=array[0]; //line 2
array[2]=array[1]; //line 3
array=null; /line 4
}

public static void main(String agrs[]){
MyClass[] array = new MyClass[4];
for (int i=0; i<array.length;i++)
array[i]=new MyClass();
f(array);
System.gc();
}

}//end class

correct answer is 3 whereas i feel it should be 4( the objects at line 1,2,3 and 4). In the method f() 3 out of 4 elements are made null and the array refernce itself is made null (at line4) then how come only 3 objects are eligible for GC.

thanks,
Sarika.

That's because Java is 'pass-by-value' NOT 'pass-by-reference'.

When array variable main() is passed into method f(), a new local array variable is created. They point to same array but are two different reference variables.

Hence doing array = null in f() will only change what local array variable points to and does not change the array variable in main().

Hope this helps.

HI,

this is exactly what i understand and this is what is causing confusion.
When a copy of array is passed to f() then i believe there are totol 5 objects. 1 array reference itself and 4 its internal objects. Now when 3 objecs and the array ref itself is set to null - that makes the count 4 for objects eligible for collection.

I understand that the main() array is still there and still pointing to the original array.

Please explain!

Originally posted by Sarikaa Bhatnagar:
HI,

this is exactly what i understand and this is what is causing confusion.
When a copy of array is passed to f() then i believe there are totol 5 objects.

Please explain!

Sorry maybe I wasn't clear enough.

The whole array does NOT get copied across. It's only a reference variable
that is getting copied. So there is only ever one array existing and this is created in main().

The array variable arrayVar is only a reference or handle to the
array object. The variable contains some sort of
address that identifies where the actual array exist in memory.

Hope this helps.

In main method we have created totally 5 objects.
1 array object and 4 MyClass objects.
After the call to f(array);, 3 of the MyClass objects doesn't have any reference.
Only two objects have references. The array object itself(through the reference 'array') and one MyClass object(through the reference 'array[3]').
So those 3 objects will be (eligible for) garbage collected after the call to f(array);.