How to avoid ConcurrentModificationException while iterating over map.entrySet

Question

I have the following code:

static final Map<String, String> map = Collections.synchronizedMap(new HashMap());

    public static void main(String[] args) throws InterruptedException {
        map.put("1", "1");
        map.put("2", "2");
        new Thread(new Runnable() {
            @Override
            public void run() {
                for (Map.Entry<String, String> stringStringEntry : map.entrySet()) {
                    System.out.println("after iterator");
                    try {
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }

                }
            }
        }).start();

        Thread.sleep(50);
        map.remove("1");
        System.out.println("removed");
    }

It produces java.util.ConcurrentModificationException

How can I avoid it ?

Make the run method synchronized.. or your can use concurrent hashmap . https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ConcurrentHashMap.html — Pranav Maniar, Mar 28 '16 at 14:15
Possible duplicate of [How to debug ConcurrentModificationException?](http://stackoverflow.com/questions/840165/how-to-debug-concurrentmodificationexception) — Raedwald, Mar 28 '16 at 14:18
Looks like essentially the same as you previous question: http://stackoverflow.com/questions/36228084/map-concurrentmodificationexception-in-for-header — Raedwald, Mar 28 '16 at 14:20
use `ConcurrentHashMap`. Iterators in `.synchronizedMap()` are not synchronized. — Alex Salauyou, Mar 28 '16 at 14:26

score 1 · Answer 1 · answered Mar 28 '16 at 14:16

1

You cannot remove an element from a collection while iterating it, unless you use an Iterator. Use Iterator.remove() method

answered Mar 28 '16 at 14:16

Dark Knight

8,218
4
39
58

fail save iterator ? – gstackoverflow Mar 28 '16 at 14:18

score 1 · Answer 2 · answered Mar 28 '16 at 14:16

you could use static final Map<String, String> map = new ConcurrentHashMap<>();

    static final Map<String, String> map = new ConcurrentHashMap<>();

public static void main(String[] args) throws InterruptedException {
    map.put("1", "1");
    map.put("2", "2");
    new Thread(new Runnable() {
        @Override
        public void run() {
            for (Map.Entry<String, String> stringStringEntry : map.entrySet()) {
                System.out.println("after iterator");
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

            }
        }
    }).start();

    Thread.sleep(50);
    map.remove("1");
    System.out.println("removed");
}

score 0 · Answer 3 · answered Mar 28 '16 at 18:08

0

If you want to iterate over a Collections.synchronizedMap you must explicitly synchronize the iteration:

        synchronized (map) {
            for (Map.Entry<String, String> stringStringEntry : map.entrySet()) {
                System.out.println("after iterator");
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

            }
         }

answered Mar 28 '16 at 18:08

Louis Wasserman

191,574
25
345
413

looks like I can use simple HasMap at this case – gstackoverflow Mar 28 '16 at 18:29
No. You must synchronize both the iteration and all other operations on the map, and synchronizedMap is the way to do that. – Louis Wasserman Mar 28 '16 at 18:32

How to avoid ConcurrentModificationException while iterating over map.entrySet

3 Answers3