What if you pass a no-arg in the Thread constructor and don't extend the Thread class?

Question

What is happening in the background if I do this:

class TestThread {
     public static void main(String[] args) {
         Thread t = new Thread();
         t.start();

         System.out.println(t.getName());
     }
}

I know that to create a new thread you must override the run() method by either extending the Thread class or by implementing the Runnable interface.

If we implement the Runnable interface we have to provide the target run method where the code which has to run concurrently is provided.

Also, If we do not override the run() method and do not extend the Thread or implement the Runnable, the main() thread will execute.

I would like to know as to what exactly will happen in the background when I execute the above code? Does the main have a run() method like other Threads? Will this create a new Thread in addition to the main thread?

Barend · Answer 1 · 2018-04-05T18:38:27.790

0

The new thread is created, starts, executes an empty* method, and terminates.

*) not really empty:

public void run() {
    if (target != null) {
        target.run();
    }
}

edited Apr 05 '18 at 18:38

answered Apr 05 '18 at 18:36

Barend

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Thanks, is it created in addition to the main thread. Also is there a run() method in main as well ? – Salil Joshi Apr 05 '18 at 18:38
Where did you find this definition of the run() method ? – Salil Joshi Apr 05 '18 at 18:47

Andrew Tobilko · Accepted Answer · 2018-04-05T20:02:51.350

0

/**
 * If this thread was constructed using a separate
 * Runnable run object, then that
 * Runnable object's run method is called;
 * otherwise, this method does nothing and returns.
 * 
 * Subclasses of Thread should override this method.
 */
public void run() {
    if (target != null) {
        target.run();
    }
}

Since you haven't set a Runnable target, nothing will happen.

Does the main have a run() method like other Threads?

Low-level API can be used for this purpose. They don't necessarily need to create a Thread instance to run a thread. Here is a good discussion: How main thread created by Java?

edited Apr 05 '18 at 20:02

answered Apr 05 '18 at 18:39

Andrew Tobilko

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where did you find this definition of the run() method ? – Salil Joshi Apr 05 '18 at 18:51
@SalilJoshi, in JavaDocs – Andrew Tobilko Apr 05 '18 at 18:52

What if you pass a no-arg in the Thread constructor and don't extend the Thread class?

2 Answers2