Android RecyclerView从Firebase取数报错：无法将String转为UserInformation

Can't convert object of type java.lang.String to type com.example.poppy.UserInformation

尝试用RecyclerView从Firebase Database获取数据时，持续触发上述错误，相关代码如下：

Adapter类 #

public class postAdapter extends RecyclerView.Adapter<MyViewHolder> {

    Context context;
    ArrayList<UserInformation> posts;

    public postAdapter(Context context, ArrayList<UserInformation> posts) {
        this.context = context;
        this.posts = posts;
    }

    @Override
    public MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        return new MyViewHolder(LayoutInflater.from(context).inflate(R.layout.card,parent,false));
    }

    @Override
    public void onBindViewHolder(MyViewHolder holder, int position) {
        holder.text.setText(posts.get(position).getPost());
    }

    @Override
    public int getItemCount() {
        return posts.size();
    }
}

Model类 #

public class UserInformation {

    public String post_text;
    public String id;

    public UserInformation(String post_text,String id) {
        this.post_text = post_text;
        this.id=id;
    }

    //public UserInformation (){}

    //Getter
    public String getPost() {
        return post_text;
    }
    public String getId(){return id;}
}

NewsFeed类 #

public class NewsFeedFragment extends Fragment {

    View root;
    RecyclerView recyclerView;
    postAdapter postAdapter;
    String firebaseAuth;
    DatabaseReference databaseReference;
    ArrayList<UserInformation> postsArray;

    public NewsFeedFragment() {
        // Required empty public constructor
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        root=inflater.inflate(R.layout.fragment_news_feed, container, false);
        init();
        return root;
    }

    private void init() {
        recyclerView=root.findViewById(R.id.recycler_newsfeed);
        RecyclerView.LayoutManager layoutManager=new GridLayoutManager(getContext(),1);
        recyclerView.setLayoutManager(layoutManager);
        postsArray=new ArrayList<UserInformation>();
        postAdapter=new postAdapter(getContext(),postsArray);
        recyclerView.setAdapter(postAdapter);

        firebaseAuth=FirebaseAuth.getInstance().getCurrentUser().getUid();
        databaseReference= FirebaseDatabase.getInstance().getReference().child("Posts").child(firebaseAuth);

        databaseReference.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot snapshot) {
                if(snapshot.exists()){
                    for (DataSnapshot postsnapshot : snapshot.getChildren()){
                        UserInformation upload = postsnapshot.getValue(UserInformation.class);
                        postsArray.add(upload);
                        recyclerView.invalidate();
                    }
                    postAdapter.notifyDataSetChanged();
                }
            }

            @Override
            public void onCancelled(DatabaseError error) {
            }
        });
    }
}

Firebase JSON结构 #

{
  "Posts": {
    "SnUFcmk8Llcy7xwWJ5d1fpp47wx2": {
      "-Nc-G0KCloNLkDBzOauf": "I'm here in my backyard with my Lamborghini ♡",
      "-Nc-GBc67pArbwZj_bxV": "Hey I'm here coding and it's fun yall ☺"
    },
    "Ym0gl0vBzUV1omI7ncOeU9NdZWU2": {
      "-Nc5_A1W2dCn5rMGiZsP": "hey yellow"
    }
  },
  "Users": {
    "PMJvIPKaEbR0vlgmFlIhvmvCHC92": {
      "email": "lisa@yahoo.com",
      "firstname": "Lisa",
      "lastname": "jim"
    },
    "SnUFcmk8Llcy7xwWJ5d1fpp47wx2": {
      "email": "ssky33@ymail.com",
      "firstname": "Sky",
      "lastname": "star"
    },
    "Ym0gl0vBzUV1omI7ncOeU9NdZWU2": {
      "email": "jjj@gmail.com",
      "firstname": "Joan",
      "lastname": "Lee"
    }
  }
}

问题原因及修复方案 #

1. 核心问题：数据结构与Model不匹配 #

你的Firebase Posts节点下，每个帖子的值是字符串，但代码里试图将其直接转换为UserInformation对象，这必然触发类型转换错误。UserInformation期望的是包含post_text和id的对象结构，而当前Firebase存储的是纯文本。

2. 次要问题：Model缺少无参构造函数 #

Firebase在反序列化Java对象时，必须依赖无参构造函数，你把UserInformation的无参构造注释掉了，这也会导致反序列化失败。

具体修复步骤： #

方案一：调整Firebase数据结构（推荐）

将帖子存储为对象格式，匹配UserInformation的结构：

"SnUFcmk8Llcy7xwWJ5d1fpp47wx2": {
  "-Nc-G0KCloNLkDBzOauf": {
    "post_text": "I'm here in my backyard with my Lamborghini ♡",
    "id": "-Nc-G0KCloNLkDBzOauf"
  },
  "-Nc-GBc67pArbwZj_bxV": {
    "post_text": "Hey I'm here coding and it's fun yall ☺",
    "id": "-Nc-GBc67pArbwZj_bxV"
  }
}

同时恢复UserInformation的无参构造：

// 取消注释这行
public UserInformation (){}

方案二：适配现有Firebase结构，修改代码逻辑

如果不想改动Firebase数据，直接读取字符串并手动创建UserInformation对象：
修改onDataChange中的循环逻辑：

for (DataSnapshot postsnapshot : snapshot.getChildren()){
    // 直接获取字符串值
    String postContent = postsnapshot.getValue(String.class);
    // 用帖子的key作为id，手动创建对象
    UserInformation upload = new UserInformation(postContent, postsnapshot.getKey());
    postsArray.add(upload);
}
postAdapter.notifyDataSetChanged();

同样需要恢复UserInformation的无参构造函数。

额外优化 #

删除循环内的recyclerView.invalidate()，只需要在循环结束后调用postAdapter.notifyDataSetChanged()即可，后者已经能通知RecyclerView更新数据。

内容的提问来源于stack exchange，提问作者StarGirl