Why is one of the answers main when we are starting a new thread?

Question about the task

Thread.currentThread always returns the current thread

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Name = Thread-0
Name = main
Name = Thread-0
Name = main
Name = main

This is a version of the results I am getting when I run the program. I don't understand why it displays Name = main. After all, we are starting a new ThreadNamePrinter object named tnp which creates and starts a new thread object. I assume it has something to do with the Thread.currentThread() assignment to variable t but doesn't that assign the newly created current thread to that variable? Therefore, shouldn't the answer be Name = Thread-0 Name = Thread-0 etc?

1. In the printMsg method, assign the current thread to the variable t.
2. In the printMsg method, add a delay of 1 millisecond after all the actions.

Requirements:

The printMsg method must get the current thread using Thread.currentThread.
The printMsg method should put the thread to sleep for 1 millisecond.
The printMsg method should call the getName method on the current thread.
The main method should call the printMsg method on the ThreadNamePrinter object 5 times.
The run method must call the printMsg method 5 times.
The ThreadNamePrinter class's printMsg method must be called a total 10 times.

package com.codegym.task.task16.task1621; /* Thread.currentThread always returns the current thread */ public class Solution { static int count = 5; public static void main(String[] args) { ThreadNamePrinter tnp = new ThreadNamePrinter(); tnp.start(); for (int i = 0; i < count; i++) { tnp.printMsg(); } } public static class ThreadNamePrinter extends Thread { public void run() { for (int i = 0; i < count; i++) { printMsg(); } } public void printMsg() { Thread t = Thread.currentThread(); // The current thread should be assigned to the variable t String name = t.getName(); System.out.println("Name = " + name); // Add sleep here try{ t.sleep(1000);}catch (InterruptedException e){} } } }

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Guadalupe Gagnon

Level 37 , Tampa, United States

16 December 2020, 15:30

solution

1000 milliseconds does not equal 1 millisecond. Make sure to fix this. The reason that the display is happening is because you have 2 places that the printMsg() is being called, line 15 and line 22. Line 22 is in the run() method of the ThreadNamePrinter class. When you start a Thread (the start() method), a new Thread is created that runs independently from the main Thread starting with the run() method. At that point the code forks out and they both executed concurrently limited by the machinery (the more cores/threads the CPU can run the better). So this happens at line 13, the new Thread goes to 20 while the main thread goes to line 14. There the main thread is the one that is calling the .printMsg() of the tnb object on line 15, while inside the run method the other Thread is calling the .printMsg() on the current object. Both threads are using the same exact object independently of each other. This is extremely important to realize because both of them can make changes to the same object. If you run this code a bunch of times the output changes here and there, this is because the Threads are executing at different intervals, which is handled by the OS with everything else. This is all advanced and you don't need to worry about it too much at this point, but making sure you are coding thread safe so one thread doesn't modify the data another thread is currently using is extremely important. Don't worry about it now, you'll get more instruction on this and then it will take even more practice for it to sink in.

Andrei

Level 41

16 December 2020, 15:50

Hi Guadalupe, thanks, I had already submitted the task, with the correct ms, this was an earlier version. So you are saying that main method can access and apply on itself the method inside the ThreadNamePrinter class? I see, it makes more sense now. Man, multithreading messed me up in the head...

Guadalupe Gagnon

Level 37 , Tampa, United States

16 December 2020, 15:58

Thread is just a class as any other class in Java or that you create. Any thread can access the methods contained in any class, not just restricted to non-Thread classes. The specific special thing about the Thread class is that it can create separate Threads that can execute instructions. Think of it like a separate program within a program. Both threads (up to 1000's of threads) can execute independently of each other using the same exact resources (with exception to private stuff that lower classes, which could be Threads, can't access).