finalize() question

[Claire Yang]

The following is a review question from book "Java2 Certification All-in-One Exam Guide":

class Test {
public static void main(String[] args) {
Dirty d=new Dirty();
try {
   d.finalize();
} catch (Throwable e) { }
d=null;
Runtime r=Runtime.getRuntime();
r.gc();
r.runFinalization();
}
}
class Dirty {
protected void finalize() throws Throwable {
System.out.println("Dirty finalization");
}
}

According to API:"The finalize method is never invoked more than once by a Java virtual machine for any given object.", why the above code outputs "Dirty finalization" twice?

[Valentin Crettaz]

You are not supposed to invoke finalize() yourself (cf invocation in the try-catch block). The GC will not keep track of the invocation to the finalize method you are doing by yourself, it only keeps track of the invocations it makes itself (trigerred by runFinalization()).

[Claire Yang]

Valentin, thanks a lot for always replying quickly and patiently.