issue with hashmap getting concurrent modification exception

Question

I am getting the below error while using map and performing some remove.How to avoid this ?

Caused by: java.util.ConcurrentModificationException
    at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
    at java.util.HashMap$EntryIterator.next(HashMap.java:834)
    at java.util.HashMap$EntryIterator.next(HashMap.java:832)

   Map<FormField, Object> ItemMap = domainItem.getValues();         
   for (Map.Entry<FormField, Object> ValMap : ItemMap.entrySet()) {         
       List<Field> groupIdList = Mapper.getGroupId(groupFieldId);           
       for (Field field : groupIdList) {
           ItemMap.put(new FormField(field), domainItem.getDomainItemLinkId());
       }
       ItemMap.remove(ValMap.getKey());
   }

jmj · Accepted Answer · 2012-09-11T11:25:32.707

11

Update:

Use Iterator and ConcurrentHashMap to avoid this scenario

Following won't throw exception

    Map<Integer, String> map = new ConcurrentHashMap<Integer, String>();
    map.put(1, "a");
    map.put(2, "b");
    map.put(3, "c");
    map.put(4, "d");
    for (Iterator<Integer> keys = map.keySet().iterator(); keys.hasNext();) {
        Integer key = keys.next();
        String val = map.get(key);
        map.remove(key);
    }

or use another map while iterating and at the end copy it to source

for example:

    Map<Integer, String> dummy = new HashMap<Integer, String>();
    map.put(1, "a");
    map.put(2, "b");
    map.put(3, "c");
    map.put(4, "d");
    dummy.putAll(map);
    for (Iterator<Integer> keys = dummy.keySet().iterator(); keys.hasNext();) {
        Integer key = keys.next();
        String val = map.get(key);
        map.remove(key);
    }
    System.out.println(map);

edited Sep 11 '12 at 11:25

answered Sep 11 '12 at 10:49

jmj

237,923
42
401
438

how to avoid this i need to add the data also. – pars Sep 11 '12 at 10:50
,please explain how can i use a iterator in my above snippet – pars Sep 11 '12 at 11:04
can also show the another map nd copy snippet example because in my case map is already available to me – pars Sep 11 '12 at 11:23

score 0 · Answer 2 · answered Sep 11 '12 at 10:52

0

A Map is sorted by the keys in the key-value pairs. When you add or remove elements from the Map while you are iterating through them, the program essentially loses track of "where" in the Map it is.

To get around this, try making a separate, temporary transfer Map. There is also a class called Iterator which might suit your needs.

answered Sep 11 '12 at 10:52

asteri

11,402
13
60
84

,please explain how can i use a iterator in my above snippet – pars Sep 11 '12 at 11:04
Jigar Joshi explains it with code samples in an answer above. =) – asteri Sep 11 '12 at 12:50

score 0 · Answer 3 · answered Sep 11 '12 at 10:53

0

One way to avoid this issue is to iterate over a copy.

for (Map.Entry<FormField, Object> ValMap : 
     new HashMap<FormField, Object>(ItemMap).entrySet()) {

answered Sep 11 '12 at 10:53

Peter Lawrey

525,659
79
751
1,130

if i create a copy then i can use for (Field field : groupIdList) { ItemMap.put(new FormField(field), domainItem.getDomainItemLinkId()); } ItemMap.remove(ValMap.getKey()); – pars Sep 11 '12 at 10:57
Its not possible to say from the code you have given as your interaction is very complicated. I would move this code into the class of `domainItem` so you can see all the dependencies. – Peter Lawrey Sep 11 '12 at 11:01
Map ItemMap = domainItem.getValues(); for (Map.Entry ValMap : new HashMap(ItemMap).entrySet()) { List groupIdList = Mapper.getGroupId(groupFieldId); for (Field field : groupIdList) { ItemMap.put(new FormField(field), domainItem.getDomainItemLinkId()); } ItemMap.remove(ValMap.getKey()); } – pars Sep 11 '12 at 11:03
You need to include the code for all the methods called in case there are side effects, which is why suggest you put all the code in one place. The reason I am concerned is that half the loop doesn't appear to need to be there so there must be something going on which I cannot see. ;) – Peter Lawrey Sep 11 '12 at 11:04
can u please explain with some other solution,iterator or so.Seems not to be working out in my case – pars Sep 11 '12 at 11:07
Its impossible to say what you should do without know what you are trying to achieve. It doesn't make any sense as it is. – Peter Lawrey Sep 11 '12 at 11:09
i want to loop over map and add some data to it,removing the previous one.So i m doing as mentioned above – pars Sep 11 '12 at 11:11

score 0 · Answer 4 · edited May 23 '17 at 11:57

can be done without a copy of the map, execute this example and take a look at the code:

public static void main(String[] args)
  {

    System.out.println("creating map ...");

    Map<String, String> dummyMap = new HashMap<>();
    while (dummyMap.size() < 10)
    { dummyMap.put(String.valueOf(new Random().nextInt()), String.valueOf(new Random().nextInt())); }


    System.out.println("start, map size: " + dummyMap.size() + ", keys=" + dummyMap.keySet());


    System.out.print("going to remove: ");
    for (Iterator<String> keys = dummyMap.keySet().iterator(); keys.hasNext(); )
    {
      final String key = keys.next();

      // delete map entries per random
      if(new Random().nextInt(3)>1)
      {
        System.out.print(key+" ");
        keys.remove();
      }
    }
    System.out.print("\n");

    System.out.println("done, map size: " + dummyMap.size() + ", keys=" + dummyMap.keySet());
  }

and take a look at this similar question.

HTH,

issue with hashmap getting concurrent modification exception

4 Answers4

Linked