Ordering threads to run in the order they were created/started

Question

How can i order threads in the order they were instantiated.e.g. how can i make the below program print the numbers 1...10 in order.

public class ThreadOrdering {
    public static void main(String[] args) {

        class MyRunnable implements Runnable{
            private final int threadnumber;

            MyRunnable(int threadnumber){
                this.threadnumber = threadnumber;
            }

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

        for(int i=1; i<=10; i++){
            new Thread(new MyRunnable(i)).start();
        }
    }
}

Answer 1

Simply put, the scheduling of threads is indeterminate.

http://www.janeg.ca/scjp/threads/scheduling.html Dead domain - do not click

WaybackMachine version of the above page

The longer answer is that if you want to do this, you'll need to manually wait for each thread to complete its work before you allow another to run. Note that in this fashion, all the threads will still interleave but they won't accomplish any work until you give the go-ahead. Have a look at the synchronize reserved word.

Answer 2

"I actually have some parts that i want to execute in parallel, and then once the results are generated, I want to merge the results in certain order." Thanks, this clarifies what you're asking.

You can run them all at once, but the important thing is to get their results in order when the threads finish their computation. Either Thread#join() them in the order in which you want to get their results, or just Thread#join() them all and then iterate through them to get their results.

// Joins the threads back to the main thread in the order we want their results.
public class ThreadOrdering {
    private class MyWorker extends Thread {
        final int input;
        int result;
        MyWorker(final int input) {
            this.input = input;
        }
        @Override
        public void run() {
            this.result = input; // Or some other computation.
        }
        int getResult() { return result; }
    }

    public static void main(String[] args) throws InterruptedException {
        MyWorker[] workers = new MyWorker[10];
        for(int i=1; i<=10; i++) {
            workers[i] = new MyWorker(i);
            workers[i].start();
        }

        // Assume it may take a while to do the real computation in the threads.

        for (MyWorker worker : workers) {
            // This can throw InterruptedException, but we're just passing that.
            worker.join();
            System.out.println(worker.getResult());
        }
    }
}