How to resolve ConcurrentModificationException in my code?

Question

I have tried since last 2 hours to solve this exception..I got the line of code throwing exception.. but didn't find solution..Please help me error occurs @ mUsers.add(user); //in else part

private void readChats()
{

    mUsers = new ArrayList<>();
    databaseReference = FirebaseDatabase.getInstance().getReference("Users");
    databaseReference.addValueEventListener(new ValueEventListener()
    {

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

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

                        //displaying 1 user from chat
                        for (String id : usersList) {
                            if (user.getUserId().equals(id)) {
                                if (mUsers.size() != 0) {
                                    for (User user1 : mUsers) {
                                        if (!user.getUserId().equals(user1.getUserId())) {
                                            mUsers.add(user);
                                        }
                                    }
                                } else {
                                        mUsers.add(user);

                                }
                            }
                        }

                    }//end of for

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

                }
                catch (Exception e)
                {
                    Toast.makeText(getContext(), e.toString(), Toast.LENGTH_SHORT).show();
                }
            }

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

            }
        });



}//end of readChats
//this is the method

You should not modify the container (ArrayList) while you are iterating over it in a for-each loop. Either convert it to a index-based for-loop or create a new ArrayList where you store the planned modifications and do these modifications afte the loop. — Chris623
– Chris623, Commented May 25, 2019 at 6:25

Nick Bapu · Accepted Answer · 2019-05-25 06:22:04Z

1

Because you are editing same ArrayList you are retrieving data. So, its causing concurrency of modification at same time of data access.

Try to edit copied data:

ArrayList<User> tempList = mUsers;
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
    User user = snapshot.getValue(User.class);

    //displaying 1 user from chat
    for (String id : usersList) {
        if (user.getUserId().equals(id)) {
            if (mUsers.size() != 0) {
                for (User user1 : mUsers) {
                    if (!user.getUserId().equals(user1.getUserId())) {
                        tempList.add(user);
                    }
                }
            } else {
                tempList.add(user);
            }
        }
    }
}//end of for

//Copy temp list to main list.
mUsers.addAll(tempList);

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

answered May 25, 2019 at 6:22

Nick Bapu

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1 Comment

Arun.k Over a year ago

No Sir..Showing same error.. but I solved using Concurrent Hash map.. tnx for ur response

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