ListItem is missing a constructor with no arguments

问题

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 .

回答1:

As the error message says, you are missing an empty constructor needed by Firebase.

I suggest you add one:

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

来源：https://stackoverflow.com/questions/42541993/listitem-is-missing-a-constructor-with-no-arguments