Firestore / firebase中的空对象引用null对象引用行with（firestoreDB.collection（＆＃34; Recipes＆＃34;））

Question

我正在使用FireStore / Firebase处理一本烹饪书应用程序而且我一直得到以下引用是空指针异常，我有点迷失在我如何解决这个问题上，有人可能有所帮助吗？

E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.example.dmange.kitchen, PID: 25648
              java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.dmange.kitchen/com.example.dmange.kitchen.ShowDetailsActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'com.google.firebase.firestore.CollectionReference com.google.firebase.firestore.FirebaseFirestore.collection(java.lang.String)' on a null object reference
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
                  at android.app.ActivityThread.-wrap11(ActivityThread.java)
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
                  at android.os.Handler.dispatchMessage(Handler.java:102)
                  at android.os.Looper.loop(Looper.java:148)
                  at android.app.ActivityThread.main(ActivityThread.java:5417)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
               Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'com.google.firebase.firestore.CollectionReference com.google.firebase.firestore.FirebaseFirestore.collection(java.lang.String)' on a null object reference
                  at com.example.dmange.kitchen.ShowDetailsActivity.loadRecipeList(ShowDetailsActivity.java:113)
                  at com.example.dmange.kitchen.ShowDetailsActivity.onCreate(ShowDetailsActivity.java:61)
                  at android.app.Activity.performCreate(Activity.java:6251)
                  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
                  at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
                  at android.os.Handler.dispatchMessage(Handler.java:102) 
                  at android.os.Looper.loop(Looper.java:148) 
                  at android.app.ActivityThread.main(ActivityThread.java:5417) 
                  at java.lang.reflect.Method.invoke(Native Method) 
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

这是我的ShowDetailsActiviy

中的代码

参考：https://firebase.google.com/docs/firestore/query-data/get-data#get_all_documents_in_a_collection

public class ShowDetailsActivity extends AppCompatActivity {
private ProgressDialog progressDialog;
private FirebaseFirestore firestoreDB;
private ListenerRegistration firestoreListener;
private static final String TAG = "MainActivity";


RecyclerView recyclerView;
MyAdapter mAdapter;

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

    recyclerView = (RecyclerView) findViewById(R.id.my_recycler_view);
    //recyclerView.setHasFixedSize(true);
    //recyclerView.setLayoutManager(new LinearLayoutManager(ShowDetailsActivity.this));
    progressDialog = new ProgressDialog(ShowDetailsActivity.this);
    progressDialog.setMessage("Data Loading");
    progressDialog.show();
   // databaseReference = FirebaseDatabase.getInstance().getReference(MainActivity.Database_Path);
    loadRecipeList();

    firestoreDB = FirebaseFirestore.getInstance();
    firestoreListener = firestoreDB.collection("Recipes")
            .addSnapshotListener(new EventListener<QuerySnapshot>() {
                @Override
                public void onEvent(@Nullable QuerySnapshot queryDocumentSnapshots, @Nullable FirebaseFirestoreException e) {
                  if (e != null){
                      Log.e(TAG,"Listen failed!",e );
                      return;
                  }

                  List<Recipe> recipeList = new ArrayList<>();
                  for (DocumentSnapshot doc : queryDocumentSnapshots) {
                      Recipe recipe = doc.toObject(Recipe.class);
                      recipe.setId(doc.getId());
                      recipeList.add(recipe);
                  }

                  mAdapter = new MyAdapter(recipeList, getApplicationContext(), firestoreDB);
                  recyclerView.setAdapter(mAdapter);
                }
            });

}

@Override
protected void onDestroy() {
    super.onDestroy();

    firestoreListener.remove();
}

private void loadRecipeList() {
    firestoreDB.collection("Recipes")
            .get()
            .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                @Override
                public void onComplete(@NonNull Task<QuerySnapshot> task) {
                    if (task.isSuccessful()){
                        List<Recipe> recipeList = new ArrayList<>();

                        for (QueryDocumentSnapshot doc : task.getResult()) {
                                Log.d(TAG, doc.getId() + " => " + doc.getData());
                                //HashMap<String, Object> result = task.getResult().getData();
                                Recipe recipe = doc.toObject(Recipe.class);
                               /* recipe.setId(doc.getId());
                                recipeList.add(recipe);*/
                                Log.d(TAG, recipe.getId() + " => " + recipe.getTitle());
                                recipeList.add(recipe);
                        }

                        mAdapter = new MyAdapter(recipeList,getApplicationContext(), firestoreDB);
                        RecyclerView.LayoutManager mLayoutManager = new LinearLayoutManager(getApplicationContext());
                        recyclerView.setLayoutManager(mLayoutManager);
                        recyclerView.setItemAnimator(new DefaultItemAnimator());
                        recyclerView.setAdapter(mAdapter);
                    } else {
                        Log.d(TAG, "Error getting documents: ", task.getException());
                    }
                }
            });
}

}

这是我的模型食谱

package com.example.dmange.kitchen;

import java.util.HashMap;
import java.util.Map;

/**
 * Created by dmange on 2018-04-08.
 */

public class Recipe {
    private String title;
    private String recipe;
    private String Id;

    public Recipe(){}

    public Recipe(String id,String title, String recipe ){
        this.Id = id;
        this.title = title;
        this.recipe = recipe;
    }

    public void setId(String id) {
        Id = id;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public void setRecipe(String recipe) {
        this.recipe = recipe;
    }

    public String getId() {
        return Id;
    }

    public String getTitle() {
        return title;
    }


    public String getRecipe() {
        return recipe;
    }

    public Map<String, Object> toMap() {

        HashMap<String, Object> result = new HashMap<>();
        result.put("Title", this.title);
        result.put("Recipe", this.recipe);

        return result;
    }
}