Fundamentals - SCJP Sun Certified Programmer for Java Platform SE6 - page 35

21. System.gc();
22. System.out.println(“End of main...”);
23.
24. }
25. }

The calls to gc are an attempt to force garbage collection so we can see when finalize
is invoked on the Dog objects. The fi rst step is determining when the objects are eligible for
garbage collection. Adding the two Dog objects to the Vector creates additional references
to the objects. On line 11 the reference one is set to null , but Snoopy is not eligible yet for
garbage collection because of line 8. The Vector still has a reference to the Snoopy object,
as shown in Figure 1.10.

FIGURE 1.10 The Vector still has a reference to the Snoopy object.

one

null

“Snoopy”
10

two

vector

0

1

Vector<Dog> object

“Lassie”
12

However, when you set vector to null on line 15, it causes the Snoopy object to
immediately become eligible for garbage collection. The Lassie object still has the
reference two pointing to it, so it does not become eligible until after line 19. Here is a
sample output of the GCDemo3 program. (I use the term “sample output” because the output
can change each time the program is executed depending on when the garbage collector
actually invokes the finalize method.)

Calling gc once...
Calling gc twice...
Snoopy is being garbage collected
Calling gc again...
Lassie is being garbage collected
End of main...