java.util.ConcurrentModificationException in For loop

Question

I am trying to program an IM software, I want to let user leave the conversation and tell his partner that he has left... I prefer to use for loop instead Iterator, seek all the users and get the user who ask to leave and remove him... like that:

   for(Clientuser Cu: EIQserver.OnlineusersList)
          if(Cu.ID.equals(thsisUser.ID)) // find the user who ask to leave 
          {
          Omsg.setBody("@@!&$$$$@@@####$$$$"); //code means : clien! ur parter leaves...
                 sendMessage(Omsg); // sed message to thje partner with that code
                 EIQserver.OnlineusersList.remove(Cu);// remove the partner
                EIQserver.COUNTER--;// decrease counter.

          }

I get Exception: java.util.ConcurrentModificationException

I was using iterators, and to get rid of this exception, I convert to for, but the same exception still appears!! how may I get rid of this exception?

You cannot remove something from a list which you are looping through. — Trick
– Trick, Commented Oct 31, 2013 at 9:20
@Trick unless you remove it using the same iterator you are using on the loop or you are using a collection that supports it, like CopyOnWriteArrayList. — aalku
– aalku, Commented Oct 31, 2013 at 9:24
You should definitely read this (not an answer): oracle.com/technetwork/java/codeconventions-135099.html#367 — aalku
– aalku, Commented Oct 31, 2013 at 9:28
True. By the way @EsmaeelQash, try to follow Java Coding Standards (Naming Conventions). If you will ever pass your code, it create a pain for your follower :) — Trick
– Trick, Commented Oct 31, 2013 at 9:28

Konstantin Yovkov · Accepted Answer · 2013-10-31 09:29:33Z

7

Use Iterator instead of looping. For example:

Iterator<Clientuser> iterator = EIQserver.OnlineusersList.iterator();
while (iterator.hasNext()) {
    Clientuser next = iterator.next();
    if(next.ID.equals(thsisUser.ID)) {
        Omsg.setBody("@@!&$$$$@@@####$$$$"); 
        sendMessage(Omsg); 
        iterator.remove();// remove the partner
    }
}

edited Oct 31, 2013 at 9:29

answered Oct 31, 2013 at 9:24

Konstantin Yovkov

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2 Comments

EsmaeelQash Over a year ago

thank you, kocko, but does iterator.remove() removes the object from the EIQserver.OnlineusersList??

Konstantin Yovkov Over a year ago

Yes, it removes the last fetched object from list.

Tadas S · Accepted Answer · 2013-10-31 09:20:58Z

3

Faulting line: EIQserver.OnlineusersList.remove(Cu);

You can only remove elements from a collection that is being iterated over via the Iterator object you are using to iterate.

for (Iterator<Clientuser> it = EIQserver.OnlineusersList.iterator(); it.hasNext();)
{
    Clientuser cu = it.next();
    if (!cu.ID.equals(thsisUser.ID))
        continue;
    // other code
    it.remove();
}

answered Oct 31, 2013 at 9:20

Tadas S

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Comments

SeniorJD · Accepted Answer · 2013-10-31 09:27:31Z

1

Use Iterator for do something with list in loop:

Iterator<Clientuser> iter = EIQserver.OnlineuserList.iterator();
for(;iter.hasNext();) {
    Clientuser Cu = iterator.next();
    if(Cu.ID.equals(thsisUser.ID)) {
        Omsg.setBody("@@!&$$$$@@@####$$$$"); 
        sendMessage(Omsg); 
        iterator.remove(next);
    }
}

answered Oct 31, 2013 at 9:27

SeniorJD

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Comments

Trick · Accepted Answer · 2013-10-31 09:25:52Z

0

One possible solution is also to transform the Collection to HashMap, save id's to remove and then remove it from the HashMap.

Collection<Integer> removeIds = new ArrayList<Integer>();
Map<Integer,ClientUser> all = new HashMap<Integer,ClientUser>();

for(Clientuser Cu: EIQserver.OnlineusersList) {
all.put(cu.ID,Cu);
      if(Cu.ID.equals(thsisUser.ID)) // find the user who ask to leave 
      {
      Omsg.setBody("@@!&$$$$@@@####$$$$"); //code means : clien! ur parter leaves...
             sendMessage(Omsg); // sed message to thje partner with that code
            EIQserver.COUNTER--;// decrease counter.
    removeIds.add(Cu.ID);

      }
}

answered Oct 31, 2013 at 9:25

Trick

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Comments

Debabrata · Accepted Answer · 2013-10-31 09:31:27Z

-2

As you are Iterating the EIQserver collection class, you can not remove element from the same class. Use a different collection for Iteration and remove the element from EIQserver class.

 List temp = ListofEIQserverobject;
  for(Clientuser Cu: temp.OnlineusersList){
    ..... your code then 
      EIQserver.OnlineusersList.remove(Cu);
    }

edited Oct 31, 2013 at 9:31

answered Oct 31, 2013 at 9:22

Debabrata

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1 Comment

aalku Over a year ago

To do that temp should be a copy of the list, not a reference to itself.

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