的Android火力显示java.lang.NullPointerException：尝试调用虚拟方法“com.google.firebase.database.ChildEventListener

Question

我得到的代码的下一行以下错误

recentMessages = mDatabase.orderByKey().equalTo(obj).limitToLast(20); 

的误差如下

显示java.lang.NullPointerException：尝试调用虚拟方法 'com.google.firebase.database.ChildEventListener com.google.firebase.database.Query.addChildEventListener（com.google.firebase.database.ChildEventListener）' 上空对象引用

这是代码

public class LikesFeedFragment extends Fragment { 

    private RecyclerView mBlogList; 
    private DatabaseReference mDatabase ; 
    private DatabaseReference mCommentsDatabase; 
    private Query commentsQuery; 
    Query recentMessages; 
    private Query filterLike; 
    private boolean mProcessLike = false; 
    private FirebaseAuth auth; 
    private DatabaseReference mDatabaseLike; 
    private DatabaseReference ref; 

    @Override 
    public void onActivityCreated(Bundle savedInstanceState) { 
     super.onActivityCreated(savedInstanceState); 
     setHasOptionsMenu(true); 
    } 

    @Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, 
          Bundle savedInstanceState) { 
     View v = inflater.inflate(R.layout.fragment_feed, container, false); 

     setHasOptionsMenu(true); 
     mDatabase = FirebaseDatabase.getInstance().getReference().child("Blog"); 
     mDatabaseLike = FirebaseDatabase.getInstance().getReference().child("Likes"); 
     mCommentsDatabase = FirebaseDatabase.getInstance().getReference().child("Comentarios"); 
     ref = FirebaseDatabase.getInstance().getReference().child("Likes"); 
     ref.keepSynced(true); 
     mDatabase.keepSynced(true); 
     mDatabaseLike.keepSynced(true); 
     mCommentsDatabase.keepSynced(true); 
     auth = FirebaseAuth.getInstance(); 

     ref.addValueEventListener(new ValueEventListener() { 
      @Override 
      public void onDataChange(DataSnapshot snapshot) { 
       for (DataSnapshot objSnapshot: snapshot.getChildren()) { 
        String obj = objSnapshot.getKey().toString(); 

        Log.e("Doo 3", obj); 
        recentMessages = mDatabase.orderByKey().equalTo(obj).limitToLast(20); 
       } 
      } 
      @Override 
      public void onCancelled(DatabaseError firebaseError) { 
       Log.e("Read failed", firebaseError.getMessage()); 
      } 
     }); 

     mBlogList = (RecyclerView) v.findViewById(R.id.blog_list); 

     LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getActivity()); 
     linearLayoutManager.setReverseLayout(true); 
     linearLayoutManager.setStackFromEnd(true); 
     //linearLayoutManager.findFirstVisibleItemPosition(); 
     mBlogList.setHasFixedSize(true); 
     mBlogList.setLayoutManager(linearLayoutManager); 
     mBlogList.setNestedScrollingEnabled(false); 
     return v; 
    } 

    @Override 
    public void onStart() { 
     super.onStart(); 

     FirebaseRecyclerAdapter<FeedData,FeedViewHolder> firebaseRecyclerAdapter = new FirebaseRecyclerAdapter<FeedData, FeedViewHolder>(
      FeedData.class, 
      R.layout.item_feed, 
      FeedViewHolder.class, 
      recentMessages) { 

       @Override 
       protected void populateViewHolder(final FeedViewHolder viewHolder, FeedData model, int position) { 

        final String post_key = getRef(position).getKey(); 
        viewHolder.setTitle(model.getTitle()); 
        viewHolder.setDesc(model.getDesc()); 
        viewHolder.setImage(getContext(), model.getThumb_image()); 
        viewHolder.setLikeBtn(post_key); 
        commentsQuery = mCommentsDatabase.orderByChild("id_comment").equalTo(post_key); 

        commentsQuery.addValueEventListener(new ValueEventListener() { 
         @Override 
         public void onDataChange(DataSnapshot dataSnapshot) { 

          long comentCount = dataSnapshot.getChildrenCount(); 

          if (comentCount == 0) { 
           viewHolder.comments.setText("0 Comentarios"); 
          } 
          else { 
           viewHolder.comments.setText(String.valueOf(comentCount) + " Comentarios"); 
          } 
         } 

         @Override 
         public void onCancelled(DatabaseError databaseError) { 

         } 
        }); 

        mDatabaseLike.child(post_key).addValueEventListener(new ValueEventListener() { 
         @Override 
         public void onDataChange(DataSnapshot dataSnapshot) { 

          long Count = dataSnapshot.getChildrenCount(); 
          // Toast.makeText(getActivity(), (int) Count, Toast.LENGTH_SHORT).show(); 
          Log.e("Doo", String.valueOf(Count)); 
          if (Count == 0) { 
           viewHolder.likes.setText("0 likes"); 
           // viewHolder.likes.setVisibility(View.INVISIBLE); 
          } 
          else { 
           //viewHolder.likes.setVisibility(View.VISIBLE); 
           viewHolder.likes.setText(String.valueOf(Count) + " likes"); 
          } 
         } 

         @Override 
         public void onCancelled(DatabaseError databaseError){ 

         } 
        }); 

        viewHolder.imageBlog.setOnClickListener(new View.OnClickListener() { 
         @Override 
         public void onClick(View v) { 

          Intent intentBlog = new Intent(getActivity(), BlogSingleActivity.class); 
          intentBlog.putExtra("blog_id",post_key); 

          startActivity(intentBlog); 
         } 
        }); 

        viewHolder.likeButton.setOnLikeListener(new OnLikeListener() { 
         @Override 
         public void liked(final LikeButton likeButton) { 
          mProcessLike = true; 

          mDatabaseLike.addValueEventListener(new ValueEventListener() { 
          @Override 
          public void onDataChange(DataSnapshot dataSnapshot) { 

           if (mProcessLike) { 

            if (!dataSnapshot.child(post_key).hasChild(auth.getCurrentUser().getUid())) { 


             mDatabaseLike.child(post_key).child(auth.getCurrentUser().getUid()).setValue("RandomValue"); 
             likeButton.setLiked(true); 
             mProcessLike = false; 
            } 
           } 
          } 

          @Override 
          public void onCancelled(DatabaseError databaseError) { 

          } 
         }); 


        } 

        @Override 
        public void unLiked(final LikeButton likeButton) { 
         mProcessLike = true; 

         mDatabaseLike.addValueEventListener(new ValueEventListener() { 

          @Override 
          public void onDataChange(DataSnapshot dataSnapshot) { 
           if (mProcessLike) { 

            if (dataSnapshot.child(post_key).hasChild(auth.getCurrentUser().getUid())) { 

             mDatabaseLike.child(post_key).child(auth.getCurrentUser().getUid()).removeValue(); 
             likeButton.setLiked(false); 
             mProcessLike = false; 
            } 
           } 
          } 

          @Override 
          public void onCancelled(DatabaseError databaseError) { 

          } 
         }); 
        } 

       }); 
      } 
     }; 

     mBlogList.setAdapter(firebaseRecyclerAdapter); 
    } 


    public static class FeedViewHolder extends RecyclerView.ViewHolder{ 

     View mView; 
     LikeButton likeButton; 
     DatabaseReference mDatabaseLike; 
     FirebaseAuth mAuth; 
     ImageView imageBlog; 
     TextView likes; 
     TextView comments; 
     public FeedViewHolder(View itemView) { 
      super(itemView); 
      mView = itemView; 
      likeButton = (LikeButton) mView.findViewById(R.id.heart_button); 
      imageBlog = (ImageView) mView.findViewById(R.id.imageBlog); 
      mDatabaseLike = FirebaseDatabase.getInstance().getReference().child("Likes"); 
      likes = (TextView) mView.findViewById(R.id.count_like); 
      comments = (TextView) mView.findViewById(R.id.comentCount); 
      mAuth = FirebaseAuth.getInstance(); 

      mDatabaseLike.keepSynced(true); 
     } 

     public void setLikeBtn(final String post_key){ 

      mDatabaseLike.addValueEventListener(new ValueEventListener() { 
       @Override 
       public void onDataChange(DataSnapshot dataSnapshot) { 
        if (dataSnapshot.child(post_key).hasChild(mAuth.getCurrentUser().getUid())){ 
         likeButton.setLiked(true); 
        }else{likeButton.setLiked(false);} 
       } 

       @Override 
       public void onCancelled(DatabaseError databaseError) { 

       } 
      }); 
     } 



     public void setTitle(String title){ 

      TextView post_title = (TextView) mView.findViewById(R.id.post_title); 
      post_title.setText(title); 

     } 

     public void setDesc(String desc){ 
      TextView desc_title = (TextView) mView.findViewById(R.id.post_desc); 
      desc_title.setText(desc); 

     } 

     public void setImage(final Context ctx, final String image) { 

      // Picasso.with(ctx).load(image).into(imageBlog); 

      Picasso.with(ctx).load(image).networkPolicy(NetworkPolicy.OFFLINE) 
        .placeholder(R.drawable.fondonav).into(imageBlog, new Callback() { 
       @Override 
       public void onSuccess() { 

       } 

       @Override 
       public void onError() { 
        Picasso.with(ctx).load(image).placeholder(R.drawable.fondonav).into(imageBlog); 

       } 
      }); 
     } 
    } 

    @Override 
    public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) { 
     inflater.inflate(R.menu.menu_fragment, menu); 
     super.onCreateOptionsMenu(menu, inflater); 
    } 

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     int id = item.getItemId(); 

     if (id == R.id.add) { 
      Toast.makeText(getActivity(), "Crear publicacion", Toast.LENGTH_SHORT).show(); 
      startActivity(new Intent(getActivity(), PostActivity.class)); 
      return true; 
     } 



     return super.onOptionsItemSelected(item); 
    } 
} 

Answer 1

添加日志语句onStart()，看它是否在onDataChange()之前运行（见下文）初始化recentMessages运行。我怀疑它确实会导致recentMessages在传递到FirebaseRecyclerAdapter时为空。

我也怀疑为什么你在for-loop中有recentMessages的初始化。如果快照没有子节点，则循环不运行并且recentMessages未设置。

public void onDataChange(DataSnapshot snapshot) { 
    for (DataSnapshot objSnapshot : snapshot.getChildren()) { 
     String obj = objSnapshot.getKey().toString(); 

     Log.e("Doo 3", obj); 
     recentMessages = mDatabase.orderByKey().equalTo(obj).limitToLast(20); 
    } 
}