我在创建聊天应用程序时获得了ConcurrentModificationException

Question

  // private List<String> stringList; Declaration at top 
    stringList = new ArrayList<>();
    firebaseUser = FirebaseAuth.getInstance().getCurrentUser();

    databaseReference = FirebaseDatabase.getInstance().getReference("BaatCheet/Chats/");
    databaseReference.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            //userModelList.clear();
            for (DataSnapshot dataSnapshot1 : dataSnapshot.getChildren()) {
                MessageModel messageModel = dataSnapshot1.getValue(MessageModel.class);

                if (messageModel.getSender().equals(firebaseUser.getUid())){
                  stringList.add(messageModel.getReceiver());
                }
                if (messageModel.getReceiver().equals(firebaseUser.getUid())){
                    stringList.add(messageModel.getSender());
                }
            }
            readChat();
        }
        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {
        }
    });

  private void readChat() {

    userModelList = new ArrayList<>();
    databaseReference = FirebaseDatabase.getInstance().getReference("BaatCheet/Users/");

    databaseReference.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            userModelList.clear();
            for (DataSnapshot dataSnapshot1 : dataSnapshot.getChildren()){
                UserModel userModel = dataSnapshot1.getValue(UserModel.class);

                for (String id: stringList){
                    if (userModel.getId().equals(id)){
                        if (userModelList.size() !=0){
                            for (UserModel userModel1 : userModelList){
                                if (!userModel.getId().equals(userModel1.getId())){
                                    userModelList.add(userModel);
                                    Log.d("DataAdded",userModel.getId());
                                } // If the existing list don't have same value for sender and reciever
                            } // end of inner userModel
                        } else {
                            userModelList.add(userModel);
                            Log.d("DataAdded",userModel.getId());
                        } // end of else
                    }   // end of userModel id equals string id
                }   // end of String is loop
            }   // end of DataSnapshot loop

            usersAdapter = new UsersAdapter(userModelList);
            recyclerView.setAdapter(usersAdapter);


        } // end of onDataChange

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {
        }
    });
}// end of readChat()

结果将是最近聊天的RecycerView，这些聊天包含发送者或接收者相互发送的消息。

Answer 1

在下面的代码片段中：

for (UserModel userModel1 : userModelList){
   if (!userModel.getId().equals(userModel1.getId())){
        userModelList.add(userModel);
        Log.d("DataAdded",userModel.getId());
    } // If the existing list don't have same value for sender and reciever
} //

您在迭代userModelList时正在修改userModelList。这是不允许的，这是ConcurrentModificationException的原因。

简化逻辑的方法很少，最简单（尽管不是最好的）是将这个foreach循环转换为一个简单的for i循环。

for (int i = 0; i< userModelList.size(); i++) {
    UserModel userModel1 = userModelList.get(i);
    if (!userModel.getId().equals(userModel1.getId())){
       userModelList.add(userModel);
       Log.d("DataAdded",userModel.getId());
    } // If the existing list don't have same value for sender and reciever
} //