顯示java.lang.NullPointerException而從火力地堡

Question

下面檢索數據是我的數據在火力地堡圖片：

所以我試圖找回DoctorsName和Email在Recyclerview時，當他們是兩個醫生recyclerView的項目列表。但我不知道如何使用get()方法在我的Recycleriew中檢索它們。 這是錯誤我得到

java.lang.NullPointerException: Attempt to invoke interface method 'java.lang.Object java.util.List.get(int)' on a null object reference 

下面是我的MainActiviy.java。

import android.app.Dialog; 
import android.content.Context; 
import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 
import android.support.v7.widget.LinearLayoutManager; 
import android.support.v7.widget.RecyclerView; 
import android.view.View; 
import android.widget.Button; 

import com.google.android.gms.common.GoogleApiAvailability; 
import com.google.android.gms.tasks.Task; 
import com.google.firebase.database.ChildEventListener; 
import com.google.firebase.database.DataSnapshot; 
import com.google.firebase.database.DatabaseError; 
import com.google.firebase.database.DatabaseReference; 
import com.google.firebase.database.FirebaseDatabase; 
import com.google.firebase.database.ValueEventListener; 

import java.util.ArrayList; 
import java.util.List; 
import java.util.Map; 


public class MainActivity extends AppCompatActivity { 

    private Button but; 
    private DatabaseReference mRef; 
    private MyAdapter adapter; 
    public RecyclerView recyclerView; 

    public List<ModelClass> movies_; 
    public Context c; 
    private LinearLayoutManager linearLayoutManager; 

    public DatabaseReference mMovieRef ; 


    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     movies_ = new ArrayList<ModelClass>(); 

     recyclerView =(RecyclerView) findViewById(R.id.recyclerview); 

     linearLayoutManager = new LinearLayoutManager(this); 
     recyclerView.setLayoutManager(linearLayoutManager); 
     recyclerView.setHasFixedSize(true); 
     mRef = FirebaseDatabase.getInstance().getReferenceFromUrl("https://askdoctor-546bf.firebaseio.com/Doctors"); 
     // mMovieRef = mRef.child("Doctors"); 


     mRef.addChildEventListener(new ChildEventListener() { 
      @Override 
      public void onChildAdded(DataSnapshot dataSnapshot, String s) { 
       LoadData(dataSnapshot); 
      } 

      @Override 
      public void onChildChanged(DataSnapshot dataSnapshot, String s) { 
       LoadData(dataSnapshot); 
      } 

      @Override 
      public void onChildRemoved(DataSnapshot dataSnapshot) { 

      } 

      @Override 
      public void onChildMoved(DataSnapshot dataSnapshot, String s) { 

      } 

      @Override 
      public void onCancelled(DatabaseError databaseError) { 

      } 
     }); 




    } 



    public void LoadData (DataSnapshot dataSnapshot){ 
     for(DataSnapshot singlesnapshot :dataSnapshot.getChildren()){ 
      String taskTitle =singlesnapshot.getValue(String.class); 


      movies_.add(new ModelClass(taskTitle)); 

      adapter = new MyAdapter(MainActivity.this, movies_); 
      recyclerView.setAdapter(adapter); 



     } 

    } 

} 

這是我的Adapter類。

import android.content.Context; 
import android.support.v7.widget.RecyclerView; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.ViewGroup; 

import com.google.firebase.FirebaseApp; 
import com.google.firebase.storage.FirebaseStorage; 
import com.google.firebase.storage.StorageReference; 
import com.squareup.picasso.Picasso; 

import java.util.ArrayList; 
import java.util.List; 

/** 
* Created by HUZY_KAMZ on 9/16/2016. 
*/ 
public class MyAdapter extends RecyclerView.Adapter<MyViewHolder> { 


    protected Context c; 
    List<ModelClass> movies; 
    FirebaseStorage storage = FirebaseStorage.getInstance(); 

    StorageReference url =storage.getReferenceFromUrl("gs://askdoctor-546bf.appspot.com/"); 

    private List<ModelClass> itemList = null; 


    public MyAdapter(Context c, List<ModelClass> movies) { 
     this.c = c; 
     this.movies = movies; 
    } 

    @Override 
    public MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) { 

     View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_activity,parent,false); 
     MyViewHolder holder = new MyViewHolder(v); 
     return holder; 
    } 

    @Override 
    public void onBindViewHolder(MyViewHolder holder, int position) { 
     final ModelClass feedItems = itemList.get(position); 

     holder.name.setText(movies.get(position).getName()); 
     holder.email_txt.setText(movies.get(position).getEmail()); 
     // PicassoClient.downloadImage(c,movies.get(position).getUrl(),holder.img); 
     //Download image using picasso library 
     Picasso.with(c).load(url+ feedItems.getName()) 
       .error(R.mipmap.ic_launcher). 
       resize(120, 50) 
       .placeholder(R.mipmap.ic_launcher).into(holder.img); 




    } 

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

Answer 1

在你onBindViewHolder

最終ModelClass feedItems = itemList.get（位置）;

itemList中爲空，你試圖從一個空列表的東西，所以這就是爲什麼你得到一個顯示java.lang.NullPointerException。