How to sort CopyOnWriteArrayList

Question

I want to sort CopyOnWriteArrayList. But when I tried to run the following code

It is throwing unsorted operation exception.

Answer 1

Evgeniy's solution points in the right way, but list.set(i, (String) a[i]) has to gain the lock on list for each element in the list. If there is a concurrent thread which writes into list this will slow down the loop dramatically.

To minimize blocking it's better to reduce the number of statements which alter list:

    CopyOnWriteArrayList list = new CopyOnWriteArrayList<>();

    // ... fill list with values ...

    ArrayList temp = new ArrayList<>();
    temp.addAll(list);                           
    Collections.sort(temp);

    list.clear();            // 1st time list is locked
    list.addAll(temp);       // 2nd time list is locked

The downside is that if a concurrent thread reads list between clear() and addAll(temp) it will see an empty list wheras with Evgeniy's solution it may see a partially sorted list.