join() method usage

[nikita mishra]

class Thread1 implements Runnable {
	
public void run(){
	for(int i = 0; i<1000;i++){
		System.out.println("Thread1="+i);
	}
}

}

class Thread2 implements Runnable {
	
	public void run(){
		
		for(int i = 0; i<1000;i++){
			System.out.println("Thread2="+i);
		}
	}

	}


public class Threading {
	
	public static void main(String[] args) throws InterruptedException {
	
		Thread1 threadObj1 = new Thread1();
		
		Thread thread1 = new Thread(threadObj1);
		thread1.start();
		
		Thread2 threadObj2 = new Thread2();
		Thread thread2 = new Thread(threadObj2);
		thread2.start();
		thread2.join();
		for(int i=0;i<1000;i++){
			System.out.println("main = "+i);
		}
		
	}
}

This code makes the current thread (main method) to execute after the thread2 finishes its job. How to make thread2 to execute after thread1?

[Harpreet Singh janda]

you have to use thread2.join() inside thread1.

[Ankit Garg]

Harpreet Singh janda wrote:you have to use thread2.join() inside thread1.

Not exactly, if you want to start thread2 after thread1 completes, then you'll have to change the lines 34 and 35 i.e.

thread2.start();  
thread2.join();

to

thread1.join();
thread2.start();

If you want thread2 to wait for thread1 to finish but main thread to complete without interruptions, then thread2 needs a reference to thread1 and call the join method on that reference...

[Abimaran Kugathasan]

Ankit Garg wrote:

Harpreet Singh janda wrote:you have to use thread2.join() inside thread1.

Not exactly, if you want to start thread2 after thread1 completes, then you'll have to change the lines 34 and 35 i.e.

thread2.start();  
thread2.join();

to

thread1.join();
thread2.start();

If you want thread2 to wait for thread1 to finish but main thread to complete without interruptions, then thread2 needs a reference to thread1 and call the join method on that reference...

See the coding :

class Thread1 implements Runnable	{
	public void run()	{
		for(int i = 0;i < 10;i++)	{
			System.out.println("Thread1 = " + i);
		}
	}
}

class Thread2 implements Runnable	{
	public void run()	{
		for(int i = 0;i < 10;i++)	{
			System.out.println("Thread2 = " + i);
		}
	}

}

class Threading	{
	public static void main(String[] args)	throws InterruptedException {
		Thread1 threadObj1 = new Thread1();
		Thread thread1 = new Thread(threadObj1);
		thread1.start();

		Thread2 threadObj2 = new Thread2();
		Thread thread2 = new Thread(threadObj2);
		thread1.join();
		thread2.start();
		//thread1.join();

		//

		thread2.join();

		//

		for(int i = 0;i < 10;i++)	{
			System.out.println("Main = " + i);
		}
	}
}

I make it to Thread1 - first, Thread2 - second, Main - 3. But in some cases, I didn't get the whole Output of Thread1?

[Ankit Garg]

But in some cases, I didn't get the whole Output of Thread1?

I don't know how you created and compiled the program, but for me thread1's output always completes before thread2 starts...

[Abimaran Kugathasan]

Ankit Garg wrote:

But in some cases, I didn't get the whole Output of Thread1?

I don't know how you created and compiled the program, but for me thread1's output always completes before thread2 starts...

That's a mistake of mine.... Sorry....

Ankit wrote :

If you want thread2 to wait for thread1 to finish but main thread to complete without interruptions, then thread2 needs a reference to thread1 and call the join method on that reference...

Hi, You mean this.....

class Thread1 implements Runnable	{
	public void run()	{
		for(int i = 0;i < 10;i++)	{
			System.out.println("Thread1 = " + i);
		}
	}
}

class Thread2 implements Runnable	{
	Thread2(Thread t)	{
		try	{
			t.join();
		} catch(InterruptedException e)	{}
	}
	public void run()	{
		for(int i = 0;i < 10;i++)	{
			System.out.println("Thread2 = " + i);
		}
	}

}

class Threading	{
	public static void main(String[] args)	throws InterruptedException {
		Thread1 threadObj1 = new Thread1();
		Thread thread1 = new Thread(threadObj1);
		thread1.start();

		Thread2 threadObj2 = new Thread2(thread1);
		Thread thread2 = new Thread(threadObj2);
		thread1.join();
		thread2.start();
		//thread1.join();

		//

		//thread2.join();

		//

		for(int i = 0;i < 10;i++)	{
			System.out.println("Main = " + i);
		}
	}
}

[Ankit Garg]

Abimaran, I didn't exactly mean the same thing but its almost the same as your code. Your code is also going to stop the main method till thread1 completes, because you have a join call in the main method. Even if you remove that join call, the main method will wait for thread1 to complete because you called join in the constructor of Thread2 class. I was talking about this

class Thread1 implements Runnable	{
	public void run()	{
		for(int i = 0;i < 10;i++)	{
			System.out.println("Thread1 = " + i);
		}
	}
}

class Thread2 implements Runnable	{
	Thread other;
	Thread2(Thread t)	{
		other = t;
	}
	public void run()	{
		try	{
			other.join();
		} catch(InterruptedException e)	{}
		for(int i = 0;i < 10;i++)	{
			System.out.println("Thread2 = " + i);
		}
	}

}

class Threading	{
	public static void main(String[] args)	throws InterruptedException {
		Thread1 threadObj1 = new Thread1();
		Thread thread1 = new Thread(threadObj1);
		thread1.start();

		Thread2 threadObj2 = new Thread2(thread1);
		Thread thread2 = new Thread(threadObj2);
		thread2.start();

		for(int i = 0;i < 10;i++)	{
			System.out.println("Main = " + i);
		}
	}
}

Now the main method will complete without any waiting and thread2 will only start when thread1 is complete...

[Abimaran Kugathasan]

Thanks Ankit Garg!