ListItem缺少一个没有参数的构造函数

Question

我遇到了问题。我想了解一些关于Firebase等的内容。今天，我制作了RecyclerView，它应该显示我的Firebase数据库的项目。但是，如果我运行它，我得到这个错误：

E/AndroidRuntime: FATAL EXCEPTION: main 
       Process: com.example.andrejulia.painting, PID: 16777 
       com.google.firebase.database.DatabaseException: Class com.example.andrejulia.painting.ListItem is missing a constructor with no arguments 
        at com.google.android.gms.internal.zzbqi$zza.zze(Unknown Source) 
        at com.google.android.gms.internal.zzbqi$zza.zzaG(Unknown Source) 
        at com.google.android.gms.internal.zzbqi.zze(Unknown Source) 
        at com.google.android.gms.internal.zzbqi.zzb(Unknown Source) 
        at com.google.android.gms.internal.zzbqi.zza(Unknown Source) 
        at com.google.firebase.database.DataSnapshot.getValue(Unknown Source) 
        at com.example.andrejulia.painting.RoomActivity$2.onChildChanged(RoomActivity.java:87) 
        at com.google.android.gms.internal.zzblz.zza(Unknown Source) 
        at com.google.android.gms.internal.zzbnz.zzYj(Unknown Source) 
        at com.google.android.gms.internal.zzboc$1.run(Unknown Source) 
        at android.os.Handler.handleCallback(Handler.java:739) 
        at android.os.Handler.dispatchMessage(Handler.java:95) 
        at android.os.Looper.loop(Looper.java:158) 
        at android.app.ActivityThread.main(ActivityThread.java:7229) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120) 

我已经检查了它的五倍多，但我不能，甚至与谷歌的帮助下解决不了，所以我不好需要回答有什么不对我代码，我得到这个错误？

这里是我的适配器：

public class RecyclerAdapter extends RecyclerView.Adapter<RecyclerAdapter.ViewHolder> { 

private List<ListItem> listItems; 

public RecyclerAdapter(List<ListItem> listItems) { 
    this.listItems = listItems; 
} 

@Override 
public RecyclerAdapter.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) { 
    View v = LayoutInflater.from(parent.getContext()) 
      .inflate(R.layout.room_items, parent, false); 
    return new ViewHolder(v); 
} 

@Override 
public void onBindViewHolder(final RecyclerAdapter.ViewHolder holder, int position) { 
    ListItem listItem = listItems.get(position); 

    holder.text_desc.setText(listItem.desc); 
    holder.text_Title.setText(listItem.head); 

    holder.itemView.setOnCreateContextMenuListener(new View.OnCreateContextMenuListener() { 
     @Override 
     public void onCreateContextMenu(ContextMenu menu, View v, ContextMenu.ContextMenuInfo menuInfo) { 
      menu.add(holder.getAdapterPosition(), 0, 0, "test"); 
      menu.add(holder.getAdapterPosition(), 1, 0, "test2"); 
     } 
    }); 
} 

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

public class ViewHolder extends RecyclerView.ViewHolder{ 

    TextView text_Title; 
    TextView text_desc; 

    public ViewHolder(View itemView) { 
     super(itemView); 

     text_Title = (TextView) itemView.findViewById(R.id.room_title); 
     text_desc = (TextView) itemView.findViewById(R.id.room_describtion); 
    } 
} 

这里是我的ListItem类：

public class ListItem { 

String head; 
String desc; 
String key; 

public ListItem(String head, String desc, String key) { 
    this.head = head; 
    this.desc = desc; 
    this.key = key; 
} 

最后我MainActivity

public class RoomActivity extends AppCompatActivity { 

private RecyclerView recyclerView; 
private RecyclerView.Adapter mAdapter; 
private List<ListItem> result; 

private FirebaseDatabase mDatabase = FirebaseDatabase.getInstance(); 
private DatabaseReference mMainReference; 


@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_room); 

    mMainReference = mDatabase.getReference("Room"); 
    settingRecyclerView(); 


    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fabCreateRoom); 
    fab.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 

     } 
    }); 

    updateList(); 
} 

private void settingRecyclerView(){ 
    recyclerView = (RecyclerView) findViewById(R.id.recyclerView); 
    recyclerView.setLayoutManager(new LinearLayoutManager(this)); 
    result = new ArrayList<>(); 



    mAdapter = new RecyclerAdapter(result); 
    recyclerView.setAdapter(mAdapter); 
} 

@Override 
public boolean onContextItemSelected(MenuItem item) { 

    switch(item.getItemId()){ 
     case 0: 
      break; 
     case 1: 
      break; 
    } 

    return super.onContextItemSelected(item); 
} 

private void updateList(){ 
    mMainReference.addChildEventListener(new ChildEventListener() { 
     @Override 
     public void onChildAdded(DataSnapshot dataSnapshot, String s) { 
      result.add(dataSnapshot.getValue(ListItem.class)); 
      mAdapter.notifyDataSetChanged(); 
     } 

     @Override 
     public void onChildChanged(DataSnapshot dataSnapshot, String s) { 
      ListItem listItem = dataSnapshot.getValue(ListItem.class); 

      int index = getItemIndex(listItem); 
      result.set(index, listItem); 
      mAdapter.notifyItemChanged(index); 

     } 

     @Override 
     public void onChildRemoved(DataSnapshot dataSnapshot) { 
      ListItem listItem = dataSnapshot.getValue(ListItem.class); 

      int index = getItemIndex(listItem); 
      result.remove(index); 
      mAdapter.notifyItemRemoved(index); 
     } 

     @Override 
     public void onChildMoved(DataSnapshot dataSnapshot, String s) { 

     } 

     @Override 
     public void onCancelled(DatabaseError databaseError) { 

     } 
    }); 
} 

private int getItemIndex(ListItem list){ 
    int index = -1; 

    for(int i = 0; i < result.size(); i++){ 
     if(result.get(i).key.equals(list.key)){ 
      index = i; 
      break; 
     } 
    } 
    return index; 
} 

我希望你们能帮助我。这里是我的测试火力地堡实时数据库的JSON代码：

{ 
    "Room" : [ null, { 
    "desc" : "Test2", 
    "head" : "Test1" 
    }, { 
    "desc" : "Test4", 
    "head" : "Test3" 
    } ] 

，我会很高兴，如果有人能帮助我，我想感谢你抽出时间，并尝试解决这个问题。

Answer 1

如错误消息所示，您缺少Firebase所需的空构造函数。

我建议你添加一个：

public ListItem() { 
    this("", "", ""); 
}