Question on Threads

[Natalie Ap]

Hi all,

I tried the following code and strangely i am getting an unexpected output.

When i try to print the total on lines 3 and 4 within the run() method, it is printed correctly each time.

But when i make a call to the getTotal() method as on lines 1 and 2 , the total is printed different each time! Can anyone please tell me what is going wrong here?

Thanks!

  

package packageX.packageY;


import java.util.ArrayList;

public class Thread1 {

	public static int startlimit1 = 0;
	public static int limit1 = 500;
	public static int startlimit2 = 500;
	public static int limit2 = 1000;
	
	
	public void runDemo() {		
			ArrayList<Integer> list1 = new ArrayList<Integer>();
			ArrayList<Integer> list2 = new ArrayList<Integer>();
			for(int i = startlimit1; i < limit1; i++) {
				list1.add(i);
			}
			for(int i = startlimit2; i < limit2; i++) {
				list2.add(i);
			}
			
			MyThread1 th1 = new MyThread1(list1, "1");
			th1.start();	
			MyThread1 th2 = new MyThread1(list2, "2");
			th2.start();	
			
			int total1 = th1.getTotal();
			System.out.println("Total1!! "  + total1);
			int total2 = th2.getTotal();	//line 1
			System.out.println("Total2!! "  + total2);
			int sumtotal = total1 + total2;    //line 2
			System.out.println("Total with Thread:" + sumtotal);
		
	}
	
	
	
	public static void main(String[] args) {
		Thread1 d = new Thread1 ();
		d.runDemo();
	}

}

class MyThread1 extends Thread {
	
	ArrayList<Integer> list;
	int total1;
	int total2;
	String name;
	
	MyThread1(ArrayList<Integer> list, String name) {
		this.list = list;	
		this.name = name;
	}
	
	public void run() {		
		if(name.equalsIgnoreCase("1") ) {
			for(int i = 0; i < list.size(); i++) {			
				Integer num = list.get(i);
				total1 += num;
			}	
			System.out.println("Total1 in run: "  + getTotal()); //line 3
		}
		else {
			for(int i = 0; i < list.size(); i++) {			
				Integer num = list.get(i);
				total2 += num;
			}	
			System.out.println("Total2 in run: "  + getTotal()); //line 4
		}		
	}
	
	public int getTotal() {
		if(name.equalsIgnoreCase("1")) {
			return total1;
		}
		else{
			return total2;		
		}
	}
	
}

  

[Vijitha Kumara]

By the time line1 & line2 gets executed (in main thread),two threads(th1 & th2) aren't started. So you will get 0 printed for those two lines.

[Adeel Ansari]

- First of all, you don't need total1 and total2 in MyThread1 class. Just one total will work
- And you may not require name either. As I can see, you are using to make conditions between total1 and total2

The output may be partial for the one but not the other
Or
The output may not be partial for the one and but for the other
Or
The output may not be partial for both
Or
The output may be partial for both

It depends if the run() method would manage to complete itself and able to assign the values to total1, andr total2, before the main thread invoke getTotal(), then none of them would be partial.

where partial = not computed complete list

[Adeel Ansari]

Line1 and line2..
prints the current value of the totals, in the middle of the execution


Line3 and Line4
prints the total after execution of full list

[Natalie Ap]

Adeel Ansari wrote:- First of all, you don't need total1 and total2 in MyThread1 class. Just one total will work
- And you may not require name either. As I can see, you are using to make conditions between total1 and total2

The output may be partial for the one but not the other
Or
The output may not be partial for the one and but for the other
Or
The output may not be partial for both
Or
The output may be partial for both

It depends if the run() method would manage to complete itself and able to assign the values to total1, andr total2, before the main thread invoke getTotal(), then none of them would be partial.

where partial = not computed complete list

Hi Adeel,

Thanks for your reply..

I thought that using the same variable total for both threads is causing the problem. Introduction of Variables total1 and total2 were simply a part of the experimentation to check if that was really the issue!

Anyway, i understood your ans. But if getTotal() appears AFTER calling the run() method in the code sequence, why is it that the execution of the run method is not complete by then?

Thanks.

[Adeel Ansari]

Thats the fun in threading pal, and therefore we say it sometimes advanced topic. However, all those thread would be independent in execution, so there may be the case where one thread ends up its execution while the other still in execution. I hope you get my point.

Well, I think I should elaborate it more. Actually, here its not about what appears first in sequence. All the threads are given with their work, and they got started on it. Now, its on scheduler that which one got more time to complete its work. Every thread would be given an opportunity to complete its task, but how is depends on the underlying scheduler, i.e. operating system. It may be the case where main thread which spawns other two threads completes its execution before any of those, then the program would wait for both of those thread to complete their tasks and then exits.

[Adeel Ansari]

Okay, precisely and specifically, main thread would not hold there and wait other two threads to complete their job, until and unless its made to wait by calling wait() on it. After spawning both, it would straight go to the next line and execute that, if it gets the turn. It wouldn't wait for them to complete their tasks.

Now I am pretty much hopeful that I clarify the thing.

[Natalie Ap]

Adeel Ansari wrote:Okay, precisely and specifically, main thread would not hold there and wait other two threads to complete their job, until and unless its made to wait by calling wait() on it. After spawning both, it would straight go to the next line and execute that, if it gets the turn. It wouldn't wait for them to complete their tasks.

Now I am pretty much hopeful that I clarify the thing.

Yeah! Pretty much!
Thanks.. Will catch hold of you the next time i have a problem with threads!

[Adeel Ansari]

Sure. Just hang here for sometime and you will learn a lot, from folks a lot more knowledgeable and senior than me.

You are welcome, Pats.