Why the results are strange when using CopyOnWriteArrayList?

Question

when I use multithreads with CopyOnWriteArrayList, the output result sometimes is not the target. The code is:

public class CopyOnWriteArrayListTest {

    public static void main(String[] args) throws InterruptedException {
        while(true){

        CopyOnWriteArrayList<String> ll = new CopyOnWriteArrayList<String>(new String[10]);

        CountDownLatch latch = new CountDownLatch(2);
        ThreadDemo t1 = new ThreadDemo(0, ll, latch);
        ThreadDemo t2 = new ThreadDemo(1, ll, latch);

        t1.start();
        t2.start();

        latch.await();
        System.out.println(ll);
        }
    }
}

public class ThreadDemo extends Thread implements Runnable {
    private int code;
    private List<String> list;
    private CountDownLatch latch;
    public ThreadDemo(int i, List<String> l, CountDownLatch latch){
        code = i;
        list = l;
        this.latch = latch;
    }

    @Override
    public void run() {
        writeList(list);
    }

    private synchronized void writeList(List<String> l){
        l.add(code, String.valueOf(code));
        latch.countDown();
    }

}

And the outputs are [0, 1, null, null, null, null, null, null, null, null, null, null] or [0, null, 1, null, null, null, null, null, null, null, null, null]. It should be always [0, 1, null, null, null, null, null, null, null, null, null, null].

I watched the source code of the CopyOnWriteArrayList, and didn't find the answer to the strange results.

please help me work it out, thanks!

Answer 1

When adding a element you are effectively shifting the list, not replacing at that index so when adding on at 0 the one at position 1 becomes position 2. So the 2nd element being null will occur whenever the insert to position 1 occurs before 0. ie:

CopyOnWriteArrayList<String> ll = new CopyOnWriteArrayList<String>(new String[10]);
ll.add(1, "1");
ll.add(0, "0");
System.out.println(ll); 

>>> [0, null, 1, null, null, null, null, null, null, null, null, null]

This doesn't happen all the time because generally since you start the thread that inserts the 0 first. If you flip the order in which you are spawning your threads you will see it almost constantly.

I think what you are looking for is the "set" method

list.set(code, String.valueOf(code));

Changing to it will give the output you are expecting. Also worth mentioning the COWList is thread safe so the synchronization on writeList is not necessary.