无法从firebase实时数据库检索数据到recyclerview [重复]

Posted 2023-04-19

【中文标题】无法从firebase实时数据库检索数据到recyclerview [重复]

【英文标题】：Unable to retrieve data from firebase realtime database on to recyclerview [duplicate]

【发布时间】：2020-07-03 06:23:28

【问题描述】：

我正在尝试从 firebase 实时数据库中检索数据到我的 recyclerview 。早些时候，我能够在我的 recyclerview 上检索数据，但不知道发生了什么，它突然开始向我显示错误。 我在这一行得到错误

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

而 logcat 窗口中的这个错误

com.google.firebase.database.DatabaseException：com.shivam.chatapp2.Model.User 类未定义无参数构造函数。如果您使用 ProGuard，请确保未剥离这些构造函数。 在 com.google.firebase.database.core.utilities.encoding.CustomClassMapper$BeanMapper.deserialize

我的完整代码

UserAdapter.java：

    public class UserAdapter extends RecyclerView.Adapter<UserAdapter.ViewHolder> 

    private Context mContext;
    private List<User> mUsers;

    public UserAdapter(Context mContext, List<User> mUsers) 
        this.mContext = mContext;
        this.mUsers = mUsers;
    

    @NonNull
    @Override
    public UserAdapter.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) 
        View view= LayoutInflater.from(mContext).inflate(R.layout.user_item,parent,false);

        return new UserAdapter.ViewHolder(view);
    

    @Override
    public void onBindViewHolder(@NonNull UserAdapter.ViewHolder holder, int position) 


        final User user=mUsers.get(position);
        holder.username.setText(user.getFirst());
        holder.itemView.setOnClickListener(new View.OnClickListener() 
            @Override
            public void onClick(View v) 
                Intent intent=new Intent(mContext, MessageActivity.class);
                intent.putExtra("UserName",user.getFirst());
                intent.putExtra("userid", user.getId());
                mContext.startActivity(intent);
            
        );

    

    @Override
    public int getItemCount() 
        return mUsers.size();
    
    public class ViewHolder extends RecyclerView.ViewHolder

        public TextView username;
        public ImageView profile_image;

        public ViewHolder(@NonNull View itemView) 
            super(itemView);
            username=itemView.findViewById(R.id.username);
            profile_image=itemView.findViewById(R.id.profile_image);
        
    

用户.java：

public class User 

    private String id;
    private String First;
    private String Last;
    private String EmailID;

    public User(String id, String first, String last, String emailID) 
        this.id = id;
        First = first;
        Last = last;
        EmailID = emailID;
    


    public User(String userid, String first_name, String eMail) 

    

    public String getId() 
        return id;
    

    public void setId(String id) 
        this.id = id;
    

    public String getFirst() 
        return First;
    

    public void setFirst(String first) 
        First = first;
    

    public String getLast() 
        return Last;
    

    public void setLast(String last) 
        Last = last;
    

    public String getEmailID() 
        return EmailID;
    

    public void setEmailID(String emailID) 
        EmailID = emailID;
    

UserFragment.java：

    public class UsersFragment extends Fragment 
    private RecyclerView recyclerView;
    private UserAdapter mUserAdapter;
    private List<User> mUsers;
    String TAG = "MyTag";

    public UsersFragment() 
        // Required empty public constructor
    


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) 
        // Inflate the layout for this fragment

        View view = inflater.inflate(R.layout.fragment_users, container, false);
        recyclerView = view.findViewById(R.id.recycler_view);
        recyclerView.setHasFixedSize(true);
        recyclerView.setLayoutManager(new LinearLayoutManager(getContext()));
        mUsers = new ArrayList<>();

        readUser();


        return view;
    

    private void readUser() 

        final FirebaseUser firebaseUser=FirebaseAuth.getInstance().getCurrentUser();
        DatabaseReference 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);
                    mUsers.add(user);


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

            

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) 

            
        );

    

【问题讨论】：

简单。将public User() 添加到 User.java 文件中。 Firebase 需要一个空的构造函数。

【参考方案1】：

根据您的错误日志，它指出：

User 类没有默认构造函数，在 User.class 文件中没有指定参数。

在 User.class 文件中为 User 类添加一个默认构造函数，如：

 public User()