ListItem缺少一个没有参数的构造函数 [英] ListItem is missing a constructor with no arguments

问题描述

我有一个问题。我想了解一些关于Firebase等等。今天，我做了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：致命的例外：主
过程：com。 example.andrejulia.painting，PID：16777 
 com.google.firebase.database.DatabaseException：类com.example.andrejulia.painting.ListItem缺少一个没有参数的构造函数
在com.google.android .gms.internal.zzbqi $ zza.zze（未知来源）
在com.google.android.gms.internal.zzbqi $ zza.zzaG（未知来源）
在com.google.android.gms .internal.zzbqi.zze（未知来源）
在com.google.android.gms.internal.zzbqi.zzb（未知来源）
在com.google.android.gms.internal.zzbqi.zza （未知来源）
在com.google.firebase.database.DataSnapshot.getValue（未知来源）
在com.example.andrejulia.painting.RoomActivity $ 2.onChildChanged（RoomActivity.java:87）$ b $ com在com.google.a ndroid.gms.internal.zzblz.zza（未知来源）
在com.google.android.gms.internal.zzbnz.zzYj（未知来源）
在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）
在android.os.Looper.loop（Looper.java:158）
 at android.app.ActivityThread.main（ActivityThread.java:7229）
在java.lang.reflect.Method.invoke（Native方法）
 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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