java - 如何解决我的代码中的 ConcurrentModificationException？

Question

自过去两个小时以来，我一直在尝试解决此异常..我得到了抛出异常的代码行..但没有找到解决方案..请帮助我发生错误@ mUsers.add(user);//在其他部分

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

Answer 1

因为您正在编辑相同的ArrayList，所以您正在检索数据。因此，它会导致数据访问的同时进行修改的并发性。

尝试编辑复制的数据:

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);