Predict the output and why is it what it is ? - Thread related Q

[Anupreet Arora]

-----------------------------------------------------
class Test
{
public static void main(String[] args)
{
System.out.println(Thread.activeCount());
}
}
------------------------------------------------------
Predict the output and why is it what it is ?
Cheers
Anupreet

[Vicken Karaoghlanian]

i believe the output is 1, because main is executed as a thread.
wait for other replies just to make sure.

[S. Sumukh]

I tried the above...code...
and o/p is 2 !!
can some one explain tht....
sumukh

[Anupreet Arora]

Hi,
Yes, the output to my surprise is 2 ! One of the active threads is the main thread. Which is the other one?
If I try to enumerate the active threads, as in the following code, I still get a null for the second thread! So why does the compiler say that the count is 2?
----------------------------------------------
class Test
{
public static void main(String[] args)
{
System.out.println(Thread.activeCount());
Thread tarray[] = new Thread[Thread.activeCount()];
Thread.enumerate(tarray);
for(int i = 0; i<tarray.length; i++)
{
System.out.println(i+": "+tarray[i]);
}
}
}
-----------------------------------------------
The output here is :
2
0: Thread[main,5,main]
1: null
-----------------------------------------------
So if the second thread is null, why is the count there?

[Vicken Karaoghlanian]

i don't know about you guys, but the output i get is always 1.
i compiled it several times... and each time the output is 1.
i am using JDK 1.4

[Anitha Lingam]

I'm using JDK1.4 and the output i get is 1.

[Inja Bory]

When I'm using Blackdown-1.3.1-02a-FCS the output is 1, but with Blackdown-1.4.1-01 the output I get is 2.
Any ideas?

[Anupreet Arora]

Hi Guys!
It never came to my mind that the response could be virtual machine implemenation dependent. Thanks for the inputs.
I ran the code from command prompt using jdk1.4_1, the answer i get is 2. If I run the code in VisualAge For Java, which has jdk1.2 on an IBM virtual machine, the answer is 1.
Maybe the Java Pundits at the ranch might like to comment on the issue.
Regards,
Anupreet

[Ernest Friedman-Hill]

The JVM implementation includes a number of threads of its own; for example, the garbage collection machinery includes two threadsin many JVMs, although these are usually not in the same ThreadGroup as the main thread. If you start jdb on any Java program, type "run" and then type the command "threads" you'll see the initial set of threads your JVM uses. Here's 1.4.1_01 on Linux:

Group system:
  (java.lang.ref.Reference$ReferenceHandler)0xfe Reference Handler cond. waiting
  (java.lang.ref.Finalizer$FinalizerThread)0xfd  Finalizer         cond. waiting
  (java.lang.Thread)0xfc                         Signal Dispatcher running
Group main:
  (java.lang.Thread)0x1                          main              running
  (java.util.logging.LogManager$Cleaner)0x121    Thread-0          unknown
main[1]

But anyway, yes, the answer will be JVM dependent.