为什么我得到这个例外，我在这里做错了什么

Question

无法退回键入

W/System.err:
at com.firebase.client.DataSnapshot.getValue(DataSnapshot.java:183)

caused by : com.fasterxml.jackson.databind.JsonMappingException: 
Can not instantiate value of type [simple type, class hi.example.com.hi.DataModel] from String value; no single-String constructor/factory method.

即使我在我的DataModel类中使用单个字符串构造函数，我在构造函数中一次得到一个字符串值，为什么会发生这种情况，请有人帮助我..

 here is my json file
   {  
      "messages": {
        "matt__walk": {
          "Username": "matt__walk",
          "imageurl": "https://firebasestorage.googleapis.com/v0",
          "text": "hii"
             }
         }
   }

DataModel类：

@JsonIgnoreProperties(ignoreUnknown=true)
public class DataModel {
    private String Username;
    private String imageurl;
    private String text;

    public DataModel(){
    }
    public String getUsername() {
        return Username;
    }
    public String getText() {
        return text;
    }
    public String getImageUrl() {
        return imageurl;
    }
}

MainActivity类：

public class MainActivity extends AppCompatActivity {
    private static final String TAG = "MainActivity";
    Menu menu;
    private Toolbar toolbar;
    private LinearLayoutManager layoutManager;
    private static RecyclerView recyclerView;
    private Firebase ref;
    private String res;
    private DatabaseReference mDatabase;
    private FirebaseRecyclerAdapter<DataModel, MessageViewHolder>    mFirebaseAdapter;

    public static class MessageViewHolder extends RecyclerView.ViewHolder {
        View mViews;
        CircleImageView img;
        TextView name,data;
        public MessageViewHolder(View views) {
            super(views);
            mViews = views;
        }

        public void setText(String Name) {
            TextView name = (TextView) mViews.findViewById(R.id.Name);
            name.setText(Name);
        }

        public void setData(String Data) {
            TextView data = (TextView) mViews.findViewById(R.id.datas);
            data.setText(Data);
        }

        public void setImage(Context cnt, String imageUrl) {
            CircleImageView img = (CircleImageView) mViews.findViewById(R.id.image);
            Picasso.with(cnt).load(imageUrl).into(img);
        }

    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.mainactivity);
        toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        Firebase.setAndroidContext(this);
        recyclerView = (RecyclerView) findViewById(R.id.recycler);
        recyclerView.setHasFixedSize(true);
        layoutManager = new LinearLayoutManager(this);
        ref = new Firebase("https://hi-34.firebaseio.com/messages/matt__walk");
        mFirebaseAdapter = new FirebaseRecyclerAdapter<DataModel, MessageViewHolder>(
                DataModel.class,
                R.layout.rowplaces,
                MessageViewHolder.class,
                ref
        ) {
            @Override
            protected void populateViewHolder(MessageViewHolder messageViewHolder, DataModel dataModel, int i) {


                        messageViewHolder.setText(dataModel.getUsername());
                        messageViewHolder.setData(dataModel.getText());
                 messageViewHolder.setImage(getApplicationContext(),dataModel.getImageUrl());


            }
        };
        mFirebaseAdapter.registerAdapterDataObserver(new RecyclerView.AdapterDataObserver() {
            @Override
            public void onItemRangeInserted(int positionStart, int itemCount) {
                super.onItemRangeInserted(positionStart, itemCount);
                int friendlyMessageCount = mFirebaseAdapter.getItemCount();
    int lastVisiblePosition =    layoutManager.findLastCompletelyVisibleItemPosition();
                if (lastVisiblePosition == -1 ||
                        (positionStart >= (friendlyMessageCount - 1) &&
                                lastVisiblePosition == (positionStart - 1))) {
                    recyclerView.scrollToPosition(positionStart);
                }
            }
        });
        recyclerView.setLayoutManager(layoutManager);
        recyclerView.setAdapter(mFirebaseAdapter);
    }

Answer 1

Recycler视图（和列表视图）显示项目列表。 FirebaseRecyclerAdapter用于填充此类项目列表。

如果您在节点上添加FirebaseRecyclerAdapter，它会尝试将该节点下的节点添加到Recycler视图中。因此，在您的情况下，这意味着它会尝试将Username，imageurl和text添加到列表中。这似乎不是你想到的。要解决此问题，您需要使适配器在树中听一级：

ref = new Firebase("https://hi-34.firebaseio.com/messages");
mFirebaseAdapter = new FirebaseRecyclerAdapter<DataModel, MessageViewHolder>(
        DataModel.class,
        R.layout.rowplaces,
        MessageViewHolder.class,
        ref
) {

这样，它会为populateViewHolder的每个孩子拨打messages，其中第一个/唯一一个是matt__walk。

第二个问题是，JSON中属性名称的大小写与Java类中相应的getter不匹配。

如果使用：

public class DataModel {
    private String username;
    private String imageUrl;
    private String text;

    public DataModel(){
    }
    public String getUsername() {
        return Username;
    }
    public String getText() {
        return text;
    }
    public String getImageUrl() {
        return imageurl;
    }
}

然后：

matt__walk": {
    "username": "matt__walk",
    "imageUrl": "https://firebasestorage.googleapis.com/v0",
    "text": "hii"
}