"Island of isolation" of Garbage Collection

Could anyone please explain the concept of Island of isolation of Garbage Collection?

Object A references object B. Object B references object A. Neither object A nor object B is referenced by any other object. That's an island of isolation.

Basically, an island of isolation is a group of objects that reference each other but they are not referenced by any active object in the application. Strictly speaking, even a single unreferenced object is an island of isolation too.

Edit from Comment:

class A {
   B myB; 
} 
class B { 
   A myA; 
} 

/* later */  
A a = new A(); 
B b = new B();  
a.b = b; 
b.a = a;

Here is a good explanation of this term. Excerpt:

> - "If an object obj1 is garbage collected, but another object obj2 > contains a reference to it, then obj2 > is also eligible for garbage > collection" > - "If object obj2 can access object obj1 that is eligible for garbage > collection, then obj2 is also eligible > for garbage collection" > > This is called "Island of Isolation". > An "island of isolation" describes one > or more objects have NO references to > them from active parts of an > application.

The thing to keep in mind is that objects are only collected if they are referenced, either directly or indirectly, from a GC root object (threads, current local variables, static variables etc). If two (or more) objects reference each other, but are not referenced from a root, then they are eligible for garbage collection.

In fact, if you understand the concept of Mark and Sweep of Garbage Collection, you'll understand better Island of Isolation too:

• The algorithm starts from the GC roots: main thread, local variables in the main method, static variables of the main class.
• The algorithm traverses all object references, starting with the GC roots, and marks every object found as alive.
• All of the heap memory that is not occupied by marked objects is reclaimed. It is simply marked as free, essentially swept free of unused objects.
• If two or more objects are referencing each other but they are not referenced by objects linked with any root, they are in the Island of Isolation, and are swept too.