Island of isolation

[Shiva Mohan]

    
public class Island{
    Island i;
    public static void main(String[] args) {
        Island i2=new Island();
         Island i3=new Island();
          Island i4=new Island();
          
          i2.i=i3;
          i3.i=i4;
           i4.i=i2;
                             //line 6
           i2=null;
           i3=null;
           i4=null;
           
           //when the code reaches here
    }
    
}

When the code reaches here, which 3 objects are GC?At line 6,i4.i refers to Island object and i2.i refers to island object,i3.i refers to island object.three objects(new Islands()) are already having object references(i2.i,i3.i,i4.i) refers to that.i2,i3,i4 are not objects.Those are object references.Then which three objects are eligible for GC here and how?Please help me.

[Burkhard Hassel]

Hi Shiva,

Then which three objects are eligible for GC here and how?

I think:

        Island i2=new Island();    <-this
         Island i3=new Island();   <-this
          Island i4=new Island();  <- and this

I mean the three new Island-objects, not the references.
And I don't think, that this is an example of island of isolation. i2,3,4 are all nulled so all of their member variables (i) don't have an object which they could reference.
And no live thread can access these i2,3,4. So the three above are eligible for garbage collection.

Or am I wrong?


Yours,
Bu.

[wise owen]

What is 'Islands of Isolation' in garbage collection?

[Burkhard Hassel]

Hi Wise,

thank you for correcting me by providing the link.

I was wrong, this IS an example of an island of isolation.
Your nulling the "outer" islands. The member islands form an island of isolation, so GC can finalize the outer islands even though, there are references to them, but these are isolated.



I extended the code a bit, so it prints something:

class Island {
Island i;
int id;
 
Island(int i) {
id = i;
}
 
public static void main(String[] args) {
Island i2 = new Island(2);
Island i3 = new Island(3);
Island i4 = new Island(4);
 
i2.i = i3;
i3.i = i4;
i4.i = i2;
 
///////////line 6
clean("after line 6");
i2 = null;
i3 = null;
i4 = null;
// when the code reaches here
clean("\"when the code reaches here\"");
System.out.println("ready");
}
 
protected void finalize() {
System.out.println("Island " + id + " finalized");
}
 
static void clean(String message) {
System.out.print(message);
System.out.println("\tbefore calling System.gc");
// provoke garbage collector
Object [] dummy = new Object[100000];
dummy=null;  
try {
Thread.sleep(500);
} catch (InterruptedException e) {
System.err.println("interrupted");
}
// call garbage collector
System.gc(); 
System.out.println("after calling gc");
}
}

Prints out: (VM dependent)
after line 6before calling System.gc
after calling gc
"when the code reaches here"before calling System.gc
Island 4 finalized
Island 3 finalized
Island 2 finalized
after calling gc
ready






Yours,

Bu.