ArrayList$Itr.next 抛出错误 concurrentModificationException

Posted 2023-05-07

【中文标题】ArrayList$Itr.next 抛出错误 concurrentModificationException

【英文标题】：ArrayList$Itr.next throwing error concurrentModificationException

【发布时间】：2020-09-25 18:18:04

【问题描述】：

我正在创建一个simple message app，每次我在new user to message 上click 并单击send icon 后，我在readChats() onDataChanged(@NonNull DataSnapshot dataSnapshot) function 中收到以下错误concurrentModificationException in ArrayList。我进行了研究，遇到了Iterator，但不确定如何将implement 写入我的代码。

代码

public class ChatsFragment extends Fragment 

    private RecyclerView recyclerView;

    private UserAdapter userAdapter;
    private List<User> mUsers;

    FirebaseUser fuser;
    DatabaseReference reference;

    private List<String> usersList;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) 
      View view = inflater.inflate(R.layout.fragment_chats, container, false);

      recyclerView = view.findViewById(R.id.recycler_view);
      recyclerView.setHasFixedSize(true);
      recyclerView.setLayoutManager(new LinearLayoutManager(getContext()));

      fuser = FirebaseAuth.getInstance().getCurrentUser();

      usersList = new ArrayList<>();

      reference = FirebaseDatabase.getInstance().getReference("Chats");
      reference.addValueEventListener(new ValueEventListener() 
          @Override
          public void onDataChange(@NonNull DataSnapshot dataSnapshot) 
              usersList.clear();

              for (DataSnapshot snapshot : dataSnapshot.getChildren()) 
                  Chat chat = snapshot.getValue(Chat.class);

                  if (chat.getSender().equals(fuser.getUid())) 
                      usersList.add(chat.getReceiver());
                  
                  if (chat.getReceiver().equals(fuser.getUid())) 
                      usersList.add(chat.getSender());
                  
              
              readChats();
          

          @Override
          public void onCancelled(@NonNull DatabaseError databaseError) 

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

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

        reference.addValueEventListener(new ValueEventListener() 
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) 
                mUsers.clear();
                //Display 1 user from chats
                for (DataSnapshot snapshot : dataSnapshot.getChildren()) 
                    User user = snapshot.getValue(User.class);
                    for (String id : usersList) 
                        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, true);
                recyclerView.setAdapter(userAdapter);
            

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) 

            
        );
    

【参考方案1】：

这个循环发生concurrentModificationException

for (User user1 : mUsers) 
   if (!user.getId().equals(user1.getId())) 
       mUsers.add(user);
   

因为你在扔mUsers的物品时修改了mUsers

这样做

boolean containsUser = false;

for (User user1 : mUsers) 
   if (user.getId().equals(user1.getId())) 
       containsUser = true;
   


if (!containsUser) 
   mUsers.add(user);