java.util.ConcurrentModificationException in android studio?

Question

I want to create a chat app with android studio and when I want to display users in my app, app crashed and my code is below:

private void readChats()
{
    mUsers = new ArrayList<>();

    reference = FirebaseDatabase.getInstance().getReference("Users");

    reference.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot)
        {
            mUsers.clear();

            for (DataSnapshot snapshot : dataSnapshot.getChildren())
            {
                User user = snapshot.getValue(User.class);

                for (String id : userList){
                    assert user != null;
                    if (user.getId().equals(id)) {
                        if (mUsers.size() != 0) {
                            for (User user1 : mUsers) {
                                if (!user.getId().equals(user1.getId())) {
                                    mUsers.add(user);
                                }
                            }
                        }else {
                            mUsers.add(user);
                        }
                    }
                }
            }
            userAdapter = new UserAdapter(getContext(), mUsers);
            recyclerView.setAdapter(userAdapter);

        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError)
        {

        }
    });
}

and my error is below:

    java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.next(ArrayList.java:831)
    at com.example.mahdi.chatapp.Fragments.ChatsFragment$2.onDataChange(ChatsFragment.java:101)

and error from this line:

for (User user1 : mUsers)

I can't fix this error please help me:

jDroidCoder Group · Accepted Answer · 2019-03-12 10:33:16Z

2

You can't change this list ("mUsers") in loop, because loop use count items for limits. If you make temp variable, or use this code:

for (User user : new ArrayList< User >(mUsers)) {
  if (!user.getId().equals(user1.getId())) {
 }

}

I hope it helpful

answered Mar 12, 2019 at 10:33

jDroidCoder Group

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Comments

GaripTipici · Accepted Answer · 2019-03-12 10:35:38Z

1

If you use plain for loop instead of enhanced for loop, your problem is resolved.

answered Mar 12, 2019 at 10:35

GaripTipici

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Comments

Andy Turner · Accepted Answer · 2019-03-12 10:35:44Z

if (mUsers.size() != 0) {
    for (User user1 : mUsers) {
        if (!user.getId().equals(user1.getId())) {
            mUsers.add(user);
        }
    }
}else {
    mUsers.add(user);
}

Aside from the ConcurrentModificationException (which comes from the mUsers.add(user) inside the loop over mUsers), I don't think this is the logic you intend. This would add user to the list N times, where N is the number of users in the list with differing IDs.

I suspect you might want something like:

if (mUsers.stream().noneMatch(u -> user.getId().equals(u.getId())) {
  mUsers.add(user);
}

which adds user once, if no other user with that ID is present.

You might also consider using a Map<String, User>, where the key is the user's ID. Then you could use:

map.computeIfAbsent(user.getId(), k -> user);

Akash Manna · Accepted Answer · 2020-04-23 14:53:06Z

I was too facing this issue. Better you can use the below code snippet which will make sure that only one user has been added in the ChatFragment.

        reference=FirebaseDatabase.getInstance().getReference("Users");

        reference.addValueEventListener(new ValueEventListener() {

            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                mUsers.clear();

                for(DataSnapshot snapshot:dataSnapshot.getChildren()){


                    User user=snapshot.getValue(User.class);

                    //Display 1 user from chat
                    for (String id:usersList){
                        assert user != null;
                        if(user.getId().equals(id)){
                            if(mUsers.size()!=0){
                                int flag=0;
                               for(User u : mUsers) {
                                   if (user.getId().equals(u.getId())) {
                                       flag = 1;
                                       break;
                                   }
                               }
                               if(flag==0)
                                   mUsers.add(user);
                            }else{

                                mUsers.add(user);
                            }
                        }
                    }
                }

                userAdapter=new UserAdapter(getContext(),mUsers,true);
                recyclerView.setAdapter(userAdapter);

Thank You

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