calling start() twice on a thread.

[Suresh Babu Venkat]

I remember reading that if we call start twice on a thread we will get IllegalThreadStateException.

but if i execute the code below which i found on http://www.javachamp.com/public/showQuestionDetail.xhtml?itemIndex=17&examId=178809

public class Test extends Thread {

	int id;

	Test(int id) {
		this.id = id;
		start();
	}

	public static void main(String[] args)  {

		Thread t = new Thread(new Test(2));
		t.start();
		System.out.print("main");
	}

	public void run() {
		System.out.print(id);
	}

}
                                                

its doesn't throw any exception and gives output as main22 .


can somebody help me understanding this..

[Ulf Dittmer]

I remember reading that if we call start twice on a thread we will get IllegalThreadStateException.

That's correct, but that's not what's happening here. To understand why, put "System.out.println(Thread.currentThread())" into the run method.

[Abimaran Kugathasan]

There are actually two threads in your program 1. Test, that is your own thread. 2. From Thread class.
In the Test class's Constructor, you start() that thread. and in your main method, your pass this(you Test object) as target to the Thread class thread, and start it.
If you don't extends Thread class and implements Runnable as bellow, you'll get Compilation error, because Test class don't have start() method.

public class Test implements Runnable {

	int id;

	Test(int id) {
		this.id = id;
		start();// Compilation Error!
	}

	public static void main(String[] args)  {

		Thread t = new Thread(new Test(2));
		t.start();
		System.out.print("main");
	}

	public void run() {
		System.out.print(id);
	}

}

[Sahil Kapoor]

Because you are starting two completely different threads although run method called is of same object.

Thread1 :- referenced by t
Thread2 :- anonymous Object

In constructor you are starting Thread2

In main you are starting Thread 1

Thanks !!!

[Abimaran Kugathasan]

Sahil Kapoor wrote:
Thread2 :- anonymous Object

That's not anonymous Thread, that is the Thread the programmer has created(The Test Thread)! The start() method of that thread is inherited from Parent Thread class, which the programmer extends. Check it with the invocation of run() method of this thread.

[Sahil Kapoor]

If it is not Anonymous then how would you control it.....I mean you dont have any reference to it.....except with the JVM whic calls run on it .

[Abimaran Kugathasan]

Sahil Kapoor wrote:If it is not Anonymous then how would you control it.....I mean you dont have any reference to it.....except with the JVM whic calls run on it .

Try this code...

public class Test extends Thread {

	int id;

	Test(int id) {
		this.id = id;
		start();
	}

	public static void main(String[] args)  {
		Test testThread = new Test(2);
		testThread.start(); // Compilation Error, since you start this thread secondly.
		Thread t = new Thread(testThread);
		t.start();
		System.out.println("main");
	}

	public void run() {
		System.out.println(id);
	}

}

[Sahil Kapoor]

Sir ,now its not more Anonymous , you have given it a reference name namely "testThread"

[Pradeep Kr]

Abimaran Kugathasan wrote:

		testThread.start(); // Compilation Error, since you start this thread secondly.

Just to make it clear, I think above Abimaran means RuntimeException not compilation error.

[Abimaran Kugathasan]

Pradeep- Kumar wrote:

Abimaran Kugathasan wrote:

		testThread.start(); // Compilation Error, since you start this thread secondly.

Just to make it clear, I think above Abimaran means RuntimeException not compilation error.

Yea, Exactly! Compiler doesn't know the logic! Thanks mate. It's Great~!