无法解决符号singleSnapshot错误

Question

我尝试清理并重建项目。我也尝试了使高速缓存失效并重新启动的方法，但是没有用。

之前没有给出错误，但是我重写了代码，此错误来了。我尝试重新启动所有解决方案，但这没有用。

package ankittx.com.forsale;

import android.content.Intent ;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.preference.PreferenceManager;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.support.v7.widget.GridLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.KeyEvent;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.view.inputmethod.EditorInfo;
import android.widget.EditText;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast; 

import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.Query;
import com.google.firebase.database.ValueEventListener;

import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;

import ankittx.com.forsale.models.HitsList;
import ankittx.com.forsale.models.HitsObject;
import ankittx.com.forsale.models.Post;
import ankittx.com.forsale.util.ElasticSearchAPI;
import ankittx.com.forsale.util.PostListAdapter;
import okhttp3.Credentials;
import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;

public class SearchFragment extends Fragment {

private static final String TAG = "SearchFragment";
private static final String BASE_URL = 
"http://35.232.207.55//elasticsearch/posts/post/";
private static final int NUM_GRID_COLUMNS = 3;

//widgets
private ImageView mFilters;
private EditText mSearchText;

//vars
private String mElasticSearchPassword;
private String mPrefCity;
private String mPrefStateProv;
private String mPrefCountry;
private ArrayList<Post> mPosts;
private RecyclerView mRecyclerView;
private PostListAdapter mAdapter;

@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup 
  container, @Nullable Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_search, container, 
    false);
    mFilters = (ImageView) view.findViewById(R.id.ic_search);
    mSearchText = (EditText) view.findViewById(R.id.input_search);
    mRecyclerView=(RecyclerView)view.findViewById(R.id.recyclerView);

    init();

    return view;
  }

     private void init(){
    mFilters.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Log.d(TAG, "onClick: navigating to filters activity.");
            Intent intent = new Intent(getActivity(), 
     FiltersActivity.class);
            startActivity(intent);
        }
    });

     mSearchText.setOnEditorActionListener(new 
      TextView.OnEditorActionListener() {
        @Override
        public boolean onEditorAction(TextView v, int actionId, KeyEvent 
     event) {
            if(actionId == EditorInfo.IME_ACTION_SEARCH
                ||actionId == EditorInfo.IME_ACTION_DONE
                ||event.getAction() == KeyEvent.ACTION_DOWN
                    ||event.getKeyCode() == KeyEvent.KEYCODE_ENTER){

            }
            return false;
        }
    });
    }
    private void getElasticSearchPassword(){
    Log.d(TAG, "getElasticSearchPassword: retrieving elastic search 
    password");

    Query query = FirebaseDatabase.getInstance().getReference()
            .child(getString(R.string.node_elasticsearch))
            .child(getString(R.string.field_password));

    query.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            DataSnapshot dataSnapshot1 = 
    dataSnapshot.getChildren().iterator().next();
            mElasticSearchPassword = singleSnapshot.getValue().toString();
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {

        }
    });
    }
   }