我已经检查了五次以上，但是即使在Google的帮助下也无法解决我很少需要回答什么是错的与我的代码，我得到这个错误？
这是我的适配器： $ pre $ RecyclerAdapter扩展了RecyclerView.Adapter< RecyclerAdapter.ViewHolder> { 私人列表< ListItem> listItems中; public RecyclerAdapter（List< ListItem> listItems）{ this.listItems = listItems; $ b @Override public RecyclerAdapter.ViewHolder onCreateViewHolder（ViewGroup parent，int viewType）{ View v = LayoutInflater.from（parent.getContext（））$ b。b .inflate（R.layout.room_items，parent，false）; 返回新的ViewHolder（v）; $ b @Override public void onBindViewHolder（final RecyclerAdapter.ViewHolder持有者，int位置）{ 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菜单，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）; $ b $ p $这些是我的ListItem类：
public class ListItem { 字符串头; String desc; 字符串键; public ListItem（String head，String desc，String key）{ this.head = head; this.desc = desc; this.key = key; }最后我的MainActivity
public class RoomActivity extends AppCompatActivity { private RecyclerView recyclerView; private RecyclerView.Adapter mAdapter; 私人列表< ListItem>结果; 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）{ $ b} }）; updateList（）; private void settingRecyclerView（）{ recyclerView =（RecyclerView）findViewById（R.id.recyclerView）; recyclerView.setLayoutManager（新的LinearLayoutManager（this））; result = new ArrayList<>（）; mAdapter = new RecyclerAdapter（result）; recyclerView.setAdapter（mAdapter）; @Override public boolean onContextItemSelected（MenuItem item）{ switch（item.getItemId（））{ case 0 ： break; 案例1： break; } return super.onContextItemSelected（item）; $ b 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）; } $ b @覆盖 public void onChildRemoved（DataSnapshot dataSnapshot）{ ListItem listItem = dataSnapshot.getValue（ListItem.class）; $ b $ int index = getItemIndex（listItem）; result.remove（index）; mAdapter.notifyItemRemoved（索引）; $ b @Override public void onChildMoved（DataSnapshot dataSnapshot，String s）{ } @Override public void onCancelled（DatabaseError databaseError）{ $ b} }）; private int getItemIndex（ListItem list）{ int index = -1; （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。这里是我测试的Firebase实时数据库JSON代码：
{房间：[null，{desc：Test2，head：Test1}，{desc：Test4，head：Test3 ]如果有人可以帮我，我会很开心，这个问题的解决方法如下：
您缺少Firebase所需的空构造函数。
我建议您添加一个：
public ListItem（）{ this（，，）; }
I got a problem. I am trying to learn a bit about Firebase etc. Today I made RecyclerView which should show Items of my Firebase Database. But if i run it i get this error:
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)I already checked it more than five times but i couldnt solve it not even with the help of Google so I poorly need to answer What is wrong with my code that i get this error?
Here is my Adapter:
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); } }Here is my ListItem class:
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; }And finally my 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; }I hope you guys can help me. Here is my test JSON Code of Firebase Realtime Database:
{ "Room" : [ null, { "desc" : "Test2", "head" : "Test1" }, { "desc" : "Test4", "head" : "Test3" } ]I would be very happy if someone can help me and I wanna thank you for taking the time and try to solve this problem .
解决方案As the error message says, you are missing an empty constructor needed by Firebase.
I suggest you add one:
public ListItem() { this("", "", ""); }
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