Number of Objects eligible for Garbage Collection

[Santiago Bravo]

Hi All,

Question from Whizlabs SCJP5:

    
class MyClass{
 private MyClass z;
 public void other(MyClass c){z=c;}
 protected void finalize(){System.out.println("called");}
 
}
 
class Test{
 private static void f()
}
 
public static void main (String [] args){
 MyClass c1 = new MyClass();
 MyClass c2 = new MyClass();
 c1.other(c2); //LINE1
 c2.other(c1); //LINE2
 c1=c3;
 c2=c3;
 f();
 }
}

I thought the answer would be zero as at LINE1 and LINE2 the other() method
assigns the 'z' reference of each object to the objects referred to by
c1 and c2. So the original objects of c1 and c2 are being refered to by its
'z' variable

However the answer given was that the two objects referred to by c1 and c2
are eligble for garbage collection even thought the have references to each other

Can anyone please explain why they are eligible for garbage collection?


Thanks

[marc weber]

Answer is "Fails to compile."

What is c3?

And f() is missing an opening brace (if not more).

[Santiago Bravo]

whoops...this is the correct code (i copied it wrong the first time)

    
class MyClass{ 
 private MyClass z; 
 
public void other(MyClass c)
 {
 z=c;
} 
 
protected void finalize(){System.out.println("called");} 
} 
 
class Test{ 
private static void f(){}
 
public static void main (String [] args){ 
 MyClass c1 = new MyClass(); 
 MyClass c2 = new MyClass(); 
 MyClass c3 = new MyClass(); 
 
 c1.other(c2); //LINE1 
 c2.other(c1); //LINE2 
 c1=c3; 
 c2=c3; 
 f(); 
 }
}

[Harvinder Thakur]

Assuming the following is the corrected code that you want an answer to:

class MyClass{
 private MyClass z;
 public void other(MyClass c){z=c;}
 protected void finalize(){System.out.println("called");}
 
}
 
class Test{
 private static void f(){
}
 
public static void main (String [] args){
 MyClass c1 = new MyClass();
 MyClass c2 = new MyClass();
 c1.other(c2); //LINE1
 c2.other(c1); //LINE2
 //c1=c3;
 c1 = null;
 // c2=c3;
 c2 = null;
 //System.gc();
 f();
 }
}

At the point when you make c2 = null; you have lost any reachable reference to the objects c1 and c2 even though they are being referenced from within c1.z and c2.z. This is a classical case of circular reference leading to "islands of isolation". The garbage collector is intelligent enough to realize that these two objects cannot be accessed by any program from the outside world. Hence the two objects will be garbage collected. On uncommenting the call to System.gc(); you may see the finalize() method being called twice and printing (it did in my case):

called
called

[Harvinder Thakur]

Well you posted the correct code while i was writing the modified code

Anyways, the answer is the same. Here your actual reference variables c1 and c2 are no more pointing to the actual objects but are pointing to a third object referenced by c3.
It has the same effect as setting c1 = c2 = null;
The rest of the explanation is the same.

[Lukasz Picur]

I think the correct explanations is like this:

object referenced by c1 and c2 (before assingment to c3) formed so called "island", which means a group of object referencing one to another, but none of them cannot be reached out of the "island".

JVM treats this object group as eligible to garbage collection, so the answer is 2.

Correct me if I'm wrong

[meera kanekal]

Here is what I think happens ( following Henry Wong's graph method)

After 
 
  MyClass c1 = new MyClass(); 
  MyClass c2 = new MyClass(); 
  MyClass c3 = new MyClass(); 
 
 c1 ---> OC1 --> null
 c2 ---> OC2 --> null
 c3 ---> OC3 --> null
 
 where OC1,2,3 are objects. Then after
 
 c1.other(c2); //LINE1 
 
   c1 ---> OC1
         |
         |
         \/
   c2 ---> OC2
 
 After 
    c2.other(c1); //LINE2 
 
   c1 ---> OC1<-
          |      |
          |      |
          \/     |
   c2 ---> OC2 --
 
Then after
  c1=c3; 
  c2=c3;
 
 c1 ---> OC3
 c2 ---> OC3
 
 and an unreferenced "island of objects"
 
                  OC1<-
                  |    |
                  |    |
                  \/   |
                  OC2 --

Hence there are two objects that will be GCed.

Meera


Edit by mw: Added Code Tags to preserve formatting.
[ November 23, 2008: Message edited by: marc weber ]

[meera kanekal]

The figures have come out garbled after posting but while typing in it was ok. So I apologize for inconvenience.
Thanks,
Meera

[Harvinder Thakur]

To supplement Meera's efforts at giving a pictorial representation i would refer you to Corey McGlone's article on garbage collection. It's pretty cool...

How Garbage Collection works

[marc weber]

Originally posted by meera kanekal:
The figures have come out garbled after posting but while typing in it was ok. So I apologize for inconvenience...

I tried to fix it by adding Code Tags. This keeps "extra" spaces from being removed.