Garbage collection output doubt

[Amit Batra]

This is a question about garbage collection. The output is 10,20,10. In the line which has an arrow, t1 is nulled before t2. so shldnt the output be something like 10,10,20 instead?

class Test9{
 int x;
 Test9(int i)
 { x=i; }
 
 protected void finalize()
 {
        System.out.println(x);
 }
 
 public static void main(String[] args) {
        Test9 t1=new Test9(10);
        t1.finalize();
        Test9 t2=new Test9(20);
        t1=null; t2=null;  //<- 
        System.gc();
 }
}

[Javier Sanchez Cerrillo]

No, the order is not guaranted in gc.

[Dave Reinhardt]

Somewhere in SCJP it says that calling the System.gc() is just a recommendation to the JVM to clean up some garbage. So, just like threads, my guess is there is no gaurantee that the objects will be garbage collected at all, let alone in any predictable order.

Is this a question on a mock exam?

I get the second output if I do the following:
t1=null;
System.gc();
t2=null; //<-
System.gc();

[wise owen]

12.6.2 Finalizer Invocations are Not Ordered

[Vaibhav Chauhan]

as said that the order of finalize method is not decided....
in the following code the result is same as 10,20,10 for all the iterations and it remains same for any number of runs of the program.
Shouldn't the answer get changed to 10,10,20 for even a single time ??

  
public class TestGC {
 
 int x;
 TestGC(int i) 
 { 
 x=i; 
 } 
 protected void finalize()
 {    
 System.out.println(x); 
 }  
 public static void main(String[] args)
 {     
 int x=10;
 
 TestGC[] array=new TestGC[10];
 
 for(TestGC a:array){
 a=new TestGC(x);    
 a.finalize();  
 x+=10;
 a=new TestGC(x); 
 x-=10;
 a=null; 
 a=null;
 System.gc(); 
 }
 }
 }  

[Sathish Nagappan]

Originally posted by Vaibhav Chauhan:
as said that the order of finalize method is not decided....
in the following code the result is same as 10,20,10 for all the iterations and it remains same for any number of runs of the program.
Shouldn't the answer get changed to 10,10,20 for even a single time ??

That again depends on the algorithm used for GC. Typical Mark and Sweep algorithms , use a generalized starting point (such as LRU or FIFO) or what ever. So it is not possible to say that the objects might get garbage collected in a particular order at all.

[Vlado Zajac]

The order may depend on order of creating of objects, since finalizable object may be registered somewhere in JVM internally when they are created. Sun JVM uses the same mechanism for this as for soft/weak/phantom references