Exercise 13.6 in - A Programmer's Guide to Java SCJP Certification - 3rd Ed

[Rashmi Jaik]

For reference please see the sample chapter on Threads here: http://www.ii.uib.no/~khalid/pgjc3e/

I'm trying to understand the outcome of Exercise 13.6 posted in the sample chapter on Threads in the A Programmer's Guide to Java SCJP Certification - 3rd Edition

The output I get is that R2 is printed twice and R1b is printed once. I don't understand why R2 is being printed twice (instead of just once).

From logically thinking about the outcome, it appears that R2 is printed once (when the thread starts the first time, which invokes R2's run() asynchronously at some point)

then what causes R2 to be printed again?
Is it because of this line? :

new Thread(new R1(),"|R1a|").run()

because the code in the sample never called new Thread(new R1(),"|R1a|").start() , so the R1 thread will never start running.
so calling the run() method manually in R1 , is just that (it is not a thread execution) , so the current thread it prints is R2 ?

[Ruben Soto]

class R1 implements Runnable {
  public void run() {
    System.out.print(Thread.currentThread().getName());
  }
}
public class R2 implements Runnable {
  public void run() {
    new Thread(new R1(),"|R1a|").run();
    new Thread(new R1(),"|R1b|").start();
    System.out.print(Thread.currentThread().getName());
  }
  public static void main(String[] args) {
    new Thread(new R2(),"|R2|").start();
  }
}

This is the way the flow of the program goes:

1. Thread in main() starts new thread with target R2 and name |R2|.
2. Thread |R2| creates a new Thread object with target R1 and name |R1a|. Then, thread |R2| executes the run() method of that Thread object (howevere this doesn't start the thread of execution |R1a|, so the run() in R1 will be run under the current thread, which is |R2|.) This accounts for one |R2| output.
3. Thread |R2| creates a new Thread object with target R1 and name |R1b|, and starts this new thread. Ultimately this will cause the run() method in R1 to be called for thread |R1b|, which accounts for one |R1b| output.
4. The code System.out.print(Thread.currentThread().getName()); is executed under thread |R2|, which accounts for another |R2| output.
5. In total, we have two |R2| outputs and one |R1b| output. Output for |R1a| is never created, since that thread is never started.

[Rashmi Jaik]

Thank you Ruben, for the clear answer it makes a lot of sense now.

From exercise 13.6 and also 13.8 from the book, and your explanation it is clear that just calling the run() method does not actually start the thread.

The thread will only be started asynchronously after calling start() on the thread, otherwise just calling the run() method is like invoking any other method.

[Ruben Soto]

I can see from your explanation that you understand it perfectly, Rashmi.

[NagarajGoud uppala]

Hi Ruben,
thanks for Excellent explanation

[Ruben Soto]

NagarajGoud uppala wrote:Hi Ruben,
thanks for Excellent explanation

No problem! I'm glad it was useful to you.