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.

public class CopyOnWriteArrayListExample {

  public static void main(final String[] args) {
     List<String> list = new CopyOnWriteArrayList<>();
    list.add("3");
    list.add("2");
    list.add("1");

    Collections.sort(list);
   }
}

Exception in thread "main" java.lang.UnsupportedOperationException
at java.util.concurrent.CopyOnWriteArrayList$COWIterator.set(CopyOnWriteArrayList.java:1049)
at java.util.Collections.sort(Collections.java:159)
at com.sac.list.CopyOnWriteArrayListExample.main(CopyOnWriteArrayListExample.java:15)

Thanks in advance.

I suspect the stack trace is longer than that - showing the operation that failed... — Jon Skeet, Mar 02 '15 at 08:23
Yes , stack trace is longer . Let me edit and put a full trace. — T-Bag, Mar 02 '15 at 08:26

score 11 · Accepted Answer · answered Mar 02 '15 at 08:34

Collections.sort uses ListIterator.set

    ...
    for (int j=0; j<a.length; j++) {
        i.next();
        i.set((T)a[j]);
    }

but CopyOnWriteArrayList's ListIterator does not support the remove, set or add methods.

Workaround:

    Object[] a = list.toArray();
    Arrays.sort(a);
    for (int i = 0; i < a.length; i++) {
        list.set(i, (String) a[i]);
    }

score 6 · Answer 2 · answered May 11 '17 at 12:47

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<Integer> list = new CopyOnWriteArrayList<>();

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

    ArrayList<Integer> 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.

score 6 · Answer 3 · edited May 07 '19 at 15:11

6

In JDK1.8 can use sort(Comparator<? super E> c) directly.

List<Integer> list = new CopyOnWriteArrayList<Integer>();

list.add(3);
list.add(4);
list.add(1);

list.sort(new Comparator<Integer>() {
    @Override
    public int compare(Integer o1, Integer o2) {
        return o1 - o2;
    }
});

edited May 07 '19 at 15:11

RamenChef

5,557
11
31
43

answered Aug 16 '17 at 12:38

Fanley

61
1
3

5

This needs minSDK 24 – Denny Weinberg Aug 13 '18 at 18:39

score 1 · Answer 4 · answered Mar 02 '15 at 08:28

Because a CopyOnWriteArrayList copies itself every time you change it, its Iterator doesn't allow you to make changes the list. If it did, the Iterator would not be thread safe, and thread safety is the whole point of this class. Collections.sort() won't work as it requires an Iterator that supports the set() method.

Vlad · Answer 5 · 2020-04-01T10:40:17.460

Kotlin helper function

inline fun <T, R : Comparable<R>> CopyOnWriteArrayList<T>.sortListBy(crossinline selector: (T) -> R?) {
    if (size > 1) {
        val list = ArrayList(this)
        list.sortBy(selector)
        clear()
        addAll(list)
    }
}

On Android

inline fun <T, R : Comparable<R>> CopyOnWriteArrayList<T>.sortListBy(crossinline selector: (T) -> R?) {
    if (size > 1) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
            sortBy(selector)
        } else {
            val list = ArrayList(this)
            list.sortBy(selector)
            clear()
            addAll(list)
        }
    }
}

How to sort CopyOnWriteArrayList

5 Answers5

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