k&b book chapter 3 -exercise 2 - clarification needed

[m prabhu]

Hi,

   
class CardBoard {
Short story = 5;
CardBoard go(CardBoard cb) {
cb = null;
return cb;
}
public static void main(String[] args) {
CardBoard c1 = new CardBoard();       //one object created
CardBoard c2 = new CardBoard();       //second object created
CardBoard c3 = c1.go(c2);             //the method sets cb as null
                                      //so c2 becomes null
c1 = null;                             //c1 becomes null
// do Stuff
} }
 
 

so as per my understanding there should be 3 objects eligible for GC.
c1,c2 and thier associated Short object.

Please explain

thank you..

[Nikos Pougounias]

CardBoard c3 = c1.go(c2);

Is c2 itself actually passed into method go() or a copy of c2?

Thanks

[m prabhu]

we are passing the refrence copy of c2.
c2 is a object reference variable.
So both 'cb' and 'c2 ' wpointed to same object refernce only know.
hence when 'cb' is set to null,the 'c2' will also be 'null'

Thats the reason i think c2 will also become 'null'.

Please explain with an example.

thank you.

[Nikos Pougounias]

Correct, both 'cb' and 'c2' point to the same object.

So, what happens when 'cb' is set to null?

cb = null;

It stops pointing the object and points to null. This does not affect 'c2' at all! This does not affect the real object also!

Example: Assume object1, object2, ..., object100 of type Object that all reference the same object on the heap.

Object object1 = new Object();
Object object2 = object1;
...
Object object100 = object1;

If one of them is set to null,

object1 = null;

all the others still reference the same live object!

[Siri Naray]

cb and c3 are just references but not objects.
Lets see the stack and heap here.

stack|heap
c1 | cardboardobj1,storyobj1
c2 | cardboardobj2,storyobj2
c3 is reference variable.
Now cb ref vble refers to c2.
when cb = null, cb ref vble is made null but c2 object is still intact.
c3 ref vble is also null.
But remember ref vbles live on the stack and only objects live on the heap.
so cb and c3 are not on heap at all.
Now when c1 = null, the 2 objects associted with c1 are available for garbage collection as no references are referring to them anymore.
so 2 objects are available for GC

[m prabhu]

thank you..