Firebase ArrayList.size（）null参照の

Question

が吹いたが、それは私にエラー与える：

java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference 

HERESに私のアダプタ：

をここ

public class RecyclerAdapter extends RecyclerView.Adapter<RecyclerAdapter.PostViewHolder> { 
public ArrayList<Getting_Posts> mdataSet; 

public RecyclerAdapter(ArrayList<Getting_Posts> dataSet) { 
    mdataSet = dataSet; 
} 

@Override 
public RecyclerAdapter.PostViewHolder onCreateViewHolder(ViewGroup parent, int viewType) { 
    View inflatedView = LayoutInflater.from(parent.getContext()).inflate(R.layout.post_card_design, parent, false); 
    return new RecyclerAdapter.PostViewHolder(inflatedView);} 

@Override 
public void onBindViewHolder(PostViewHolder holder, int position) { 
    Getting_Posts post = mdataSet.get(position);} 

@Override 
public int getItemCount() { 
    return mdataSet.size();} 

public void add(int position, Getting_Posts post) { 
    mdataSet.add(position, post); 
    notifyItemInserted(position);} 

public class PostViewHolder extends RecyclerView.ViewHolder { 
    TextView Name; 
    public PostViewHolder(View inflatedView) { 
     super(inflatedView); 
     Name = (TextView) inflatedView.findViewById(R.id.PostUsername);}}} 

とは、私の活動のクラスがあります：

private RecyclerView mPostList; 
ArrayList<Getting_Posts> posts; 
private RecyclerAdapter mAdapter; 

    mPostList = (RecyclerView) v.findViewById(R.id.MainPostList); 
    mPostList.setHasFixedSize(true); 
    mPostList.setItemAnimator(new DefaultItemAnimator()); 
    LinearLayoutManager layoutManager = new LinearLayoutManager(getActivity()); 
    mPostList.setLayoutManager(layoutManager); 
    mAdapter = new RecyclerAdapter(posts); 
    mPostList.setAdapter(mAdapter); 
    mAuth = FirebaseAuth.getInstance(); 
    mCurrentUser = mAuth.getCurrentUser(); 

    //Firebase 
    mDatabase = FirebaseDatabase.getInstance().getReference().child("Posts"); 
    mDatabaseFriends = FirebaseDatabase.getInstance().getReference().child("Following").child(mCurrentUser.getUid()); 

    return v; 
} 


@Override 
public void onStart() { 

    mDatabaseFriends.addValueEventListener(new ValueEventListener() { 
     @Override 
     public void onDataChange(DataSnapshot dataSnapshot) { 
      for(DataSnapshot following : dataSnapshot.getChildren()) { 
       mDatabase.orderByChild("UID").equalTo(following.getKey()).addValueEventListener(new ValueEventListener() { 
        @Override 
        public void onDataChange(DataSnapshot dataSnapshot) { 
         Getting_Posts model = dataSnapshot.getValue(Getting_Posts.class); 

         mAdapter.add(mAdapter.mdataSet.size(), dataSnapshot.getValue(Getting_Posts.class)); 
         mAdapter.notifyDataSetChanged();} 

        @Override 
        public void onCancelled(DatabaseError databaseError) { 

        }});}} 

     @Override 
     public void onCancelled(DatabaseError databaseError) { 

     } 
    }); 
    super.onStart(); 
}} 

そして、ここでは私のゲッターとセッターのクラスです：

public class GettingTester { 

private String Name; 

public GettingTester(String Name) { 
    this.Name = Name; 
} 

public String getName() { 
    return Name; 
} 

public void setName(String name) { 
    Name = name; 
} 

そして最後にここに私のJSONです：

"Following" : { 
 
    "ORBBKfZAAUhqI1h7ojQDzYOjgkp1" : { 
 
     "pypD1SYZkbcYesk09WuMUY1AkTf1": true, 
 
     //I need to show this UID posts only but, 
 
    }, 
 
    "Posts" : { 
 
    "-KduqnVVczZf5uibQiZ-" : { 
 
     "Describe" : "gg", 
 
     "Time" : 14881230655, 
 
     "UID" : "pypD1SYZkbcYesk09WuMUY1AkTf1", 
 
     "Username" : "Jone", 
 
     "Wallpaper" : "https://firebasestorage.googleapis.com/v0/b/test-3f4c5.appspot.com/o/Posts%2Fcropped906285501.jpg?alt=media&token=0c1a3a3d-6e48-4c4e-ba59-f5646bf8965f" 
 
    }, 
 
    "-Ke5gJ00CxbjhOuhzLIx" : { 
 
     "Describe" : "hajj", 
 
     "Time" : 1488318465, 
 
     "UID" : "pypD1SYZkbcYesk09WuMUY1AkTf1", 
 
     "Username" : "Dom", 
 
     "Wallpaper" : "https://firebasestorage.googleapis.com/v0/b/test-3f4c5.appspot.com/o/Posts%2Fcropped1717103943.jpg?alt=media&token=a85b2488-5ac7-49a7-9ad0-dbf4e6f29389" 
 
    }, 
 
    "-KeCuiFmUCpN19zwsTsR" : { 
 
     "Describe" : "a", 
 
     "Profile" : "https://lh4.googleusercontent.com/-suB77riNoX8/AAAAAAAAAAI/AAAAAAAAAAA/ADPlhfKMzINn-Ki538Sqf6SRGaXC81-WuQ/s200-c/photo.jpg", 
 
     "Time" : 1488439652, 
 
     "UID" : "z2SNUlLd6mQM8nnlkU2VUws5Ggl2", 
 
     "Username" : "Dom", 
 
     "Wallpaper" : "https://firebasestorage.googleapis.com/v0/b/test-3f4c5.appspot.com/o/Posts%2Fcropped1913886685.jpg?alt=media&token=c8ad26b2-8f09-453b-b48a-aad9e4d8b5c3" 
 
    }, 
 
}

私はユーザーのみを以下のための記事を表示する必要があります。このコードで

Answer 1

：

mAdapter.add(mAdapter.mdataSet.size(), dataSnapshot.getValue(Getting_Posts.class)); 

コード実行上記初めて、mdataSetがnullあります。何故ですか？上記のコードの前にmdataSetを変更するだけで、コードがのinit RecyclerAdapter（mAdapter）からですので：

mAdapter = new RecyclerAdapter(posts); 

ありmdataSetオブジェクトは、我々が知っていることを、postsオブジェクトの値をコピーしている、実際にはnullです。 postsオブジェクトへ

簡単に言えば値がこのwokrsようになります：

ArrayList<Getting_Posts> posts = new ArrayList<>(); 

希望これは