Simple Doubt with Threads

[Gitesh Ramchandani]

Hi for the following code

public class A extends Thread {

public void run{

System.out.print("RUNNING");
}

public static void main(String[] args) {
A a = new A();
a.start(); //1
a.run(); //2
}

}

My doubt is, both 1 and 2 display RUNNING. so is it not true that calling run will actually call the run method in Thread class?

Regards,
Gitesh

[Ulf Dittmer]

so is it not true that calling run will actually call the run method in Thread class?

Calling run doesn't "call run", it simply executes the run method. Calling start will in turn lead to the run method being called.

The difference is that calling start will actually start a new thread, while calling run will execute the code in the same thread as the main method runs in.

[Gitesh Ramchandani]

Originally posted by Ulf Dittmer:


Calling run doesn't "call run", it simply executes the run method. Calling start will in turn lead to the run method being called.

The difference is that calling start will actually start a new thread, while calling run will execute the code in the same thread as the main method runs in.

So if I'm asked in which case (out of 1 and 2) will a new thread of execution start, my answer should be a.start(); //1. Am i correct now?

[Ulf Dittmer]

Yes.

[Gitesh Ramchandani]

Thanks.

[Manfred Klug]

Hi Gitesh,

you can see the difference if you modify the sample a little bit.

public class Test extends Thread {
    public void run() {
        System.out.println(Thread.currentThread().getName());
    }
 
    public static void main(String[] args) {
        System.out.println(Thread.currentThread().getName());
        Test a = new Test();
        a.run(); //2
        a.start(); //1
    }
}

[Gitesh Ramchandani]

Originally posted by Manfred Klug:
Hi Gitesh,

you can see the difference if you modify the sample a little bit.

public class Test extends Thread {
    public void run() {
        System.out.println(Thread.currentThread().getName());
    }
 
    public static void main(String[] args) {
        System.out.println(Thread.currentThread().getName());
        Test a = new Test();
        a.run(); //2
        a.start(); //1
    }
}

Thanks Manfred,

I see that when run() is directly called, no new thread is created. Only by calling start a new thread is created.