Thread question

[Kaspar Minosiantz]

Hi !
i've been perplexed about a thread life cycle
there is question from preparation to SCJP 1.4
------------------------------------------------------
which of the following is true regarding main Thread
1. It's the thread from which other child threads wil be spawned
2. It must be the last thread to finish execution. When the main thread
stops , the program terminates.
3. It has highest priority
4. main is not a thread .
-------------------------------------------------------
correct answers are 1 & 2
i'm curious about the second answer [ When the main thread stops , the
program terminates. ]
is it realy true ?
Thanks a lot !

[Vad Fogel]

I'd say the option 2 is correct for a single-threaded application, but not for the one where many threads are started. This code

class MainThread{
    public static void main(String s[]){
        Thread t1=new Thread(new A());
        Thread t2=new Thread(new A());
        t1.start();
        t2.start();
        System.out.println("Main is done!!!");
    }
}
class A implements Runnable{
    public void run(){
        try{
            Thread.sleep(1000);
            System.out.println(Thread.currentThread().getName());
        }catch(InterruptedException ie){}
    }
}

produces the output of
Main is done!!!
Thread-0
Thread-1
It's not guaranteed on every OS, but you see, main() thread in this case decided to complete first.

[KATE MOORE]

[QB]I'd say the option 2 is correct for a single-threaded application, but not for the one where many threads are started.[QB]

Are r sure Vad? Check this..
class MainThread{
public static void main(String s[]){
Thread t1=new Thread(new A());
Thread t2=new Thread(new A());
Thread t = Thread.currentThread();
System.out.println("Current Thread:" +t);
t1.start();
t2.start();
//System.out.println("Main is done!!!");
System.out.println(t.isAlive());
}
}
class A implements Runnable{
public void run(){
try{
Thread.sleep(1000);
System.out.println(Thread.currentThread().getName());
}catch(InterruptedException ie){}
}
}
[ November 13, 2003: Message edited by: KATE MOORE ]

[Vad Fogel]

Are r sure Vad? Check this..

isAlive() instance method returns true if the thread is alive. You're assigning the currently executing thread, which is main(), to a reference t. No wonder that when main() flow execution hits

System.out.println(t.isAlive());

the result is true. The main() is still alive at that point. I hope the following code can clarify some of your doubts:

class MainThread{
    [B]static Thread t;[/B]
    public static void main(String s[]){
        Thread t1=new Thread(new A());
        Thread t2=new Thread(new A());
        t1.start();
        t2.start();
        [B]t=Thread.currentThread();
        System.out.println("Main thread is still " + (t.isAlive() ? "alive" :"dead"));[/B]
    }
}
class A implements Runnable{
    public void run(){
        try{
            Thread.sleep(1000);
            System.out.println(Thread.currentThread().getName());
            [B]System.out.println("Main thread is now " + (MainThread.t.isAlive() ? "alive" :"dead"));[/B]
        }catch(InterruptedException ie){}
    }
}

[Sudhakar Krishnamurthy]

Will the actual exam have such ambigious questions and if so which answer to pick and how do we decide?
Any suggestions

[Vad Fogel]

You are not likely to see any ambiguously worded questions on the test. Pretty much all of them use a very clear language, so if you're familiar with material, you'll at least understand "what is asked".

[Kaspar Minosiantz]

Thanks a lot for answers !