NullPointerException異常：嘗試空對象上調用虛擬方法無效android.support.v7「引用

Question

得到這樣的錯誤：

FATAL EXCEPTION: main 
Process: kz.brainpull.vector, PID: 14119 
java.lang.NullPointerException: Attempt to invoke virtual method 
    'void android.support.v7.widget.RecyclerView.setHasFixedSize(boolean)' 
    on a null object reference 
at kz.brainpull.vector.ChatFragment.onViewCreated(ChatFragment.java:72) 

如何解決這個問題呢？

ChatFragment

package kz.brainpull.vector; 

import android.content.Intent; 
import android.os.Bundle; 
import android.support.annotation.Nullable; 
import android.support.design.widget.FloatingActionButton; 
import android.support.v4.app.Fragment; 
import android.support.v7.widget.LinearLayoutManager; 
import android.support.v7.widget.RecyclerView; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.TextView; 

import com.google.firebase.auth.FirebaseAuth; 
import com.google.firebase.auth.FirebaseUser; 
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; 

public class ChatFragment extends Fragment { 

    private TextView mTextEmptyChatList; 

    private ChatAdapter mChatAdapter; 
    private DatabaseReference mChatsRef; 
    private ValueEventListener mChatsListener; 

    @Override 
    public void onCreate(@Nullable Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     mChatsRef = getChatsRefOfCurrentUser(); 
     mChatAdapter = new ChatAdapter(mChatsRef); 
    } 

    @Override 
    public void onStart() { 
     super.onStart(); 
     ValueEventListener chatsListener = new ValueEventListener() { 
      @Override 
      public void onDataChange(DataSnapshot dataSnapshot) { 
       if (dataSnapshot.exists()) { 
        mTextEmptyChatList.setVisibility(View.GONE); 
       } else { 
        mTextEmptyChatList.setVisibility(View.VISIBLE); 
       } 
      } 

      @Override 
      public void onCancelled(DatabaseError databaseError) { 

      } 
     }; 
     mChatsRef.addValueEventListener(chatsListener); 
     // Keep copy of chats listener to remove it when app stops 
     mChatsListener = chatsListener; 
    } 

    @Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, 
          Bundle savedInstanceState) { 
     View rootView = inflater.inflate(R.layout.fragment_chat, container, false); 
     return inflater.inflate(R.layout.activity_main, container, false); 
    } 

    @Override 
    public void onViewCreated(View view, @Nullable Bundle savedInstanceState) { 
     super.onViewCreated(view, savedInstanceState); 
     RecyclerView chatList = (RecyclerView) view.findViewById(R.id.recycler_chat_list); 
     chatList.setHasFixedSize(true); 
     chatList.addItemDecoration(new DividerItemDecoration(getActivity(), 
       DividerItemDecoration.VERTICAL_LIST)); 
     chatList.setLayoutManager(new LinearLayoutManager(getActivity())); 
     chatList.setAdapter(mChatAdapter); 

     mTextEmptyChatList = (TextView) view.findViewById(R.id.text_empty_chat_list); 

     FloatingActionButton fabCreateChat = (FloatingActionButton) 
       view.findViewById(R.id.fab_create_chat); 
     fabCreateChat.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View v) { 
       showContact(); 
      } 
     }); 
    } 

    @Override 
    public void onStop() { 
     super.onStop(); 
     if (mChatsListener != null) { 
      mChatsRef.removeEventListener(mChatsListener); 
     } 
    } 

    @Override 
    public void onDestroy() { 
     super.onDestroy(); 
     mChatAdapter.cleanup(); 
    } 

    private DatabaseReference getChatsRefOfCurrentUser() { 
     FirebaseAuth auth = FirebaseAuth.getInstance(); 
     FirebaseUser user = auth.getCurrentUser(); 
     return FirebaseDatabase.getInstance().getReference() 
       .child("chats") 
       .child(user.getUid()); 
    } 

    private void showContact() { 
     startActivity(new Intent(getActivity(), ContactActivity.class)); 
    } 
} 

Answer 1

試試這個：

移動你的代碼onViewCreated()到onCreateView()：你RecyclerView chatList爲null，因爲它不與view這裏

代碼相關聯：

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

    RecyclerView chatList = (RecyclerView) view.findViewById(R.id.recycler_chat_list); 
    chatList.setHasFixedSize(true); 
    chatList.addItemDecoration(new DividerItemDecoration(getActivity(), 
      DividerItemDecoration.VERTICAL_LIST)); 
    chatList.setLayoutManager(new LinearLayoutManager(getActivity())); 
    chatList.setAdapter(mChatAdapter); 

    mTextEmptyChatList = (TextView) view.findViewById(R.id.text_empty_chat_list); 

    FloatingActionButton fabCreateChat = (FloatingActionButton) 
      view.findViewById(R.id.fab_create_chat); 
    fabCreateChat.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 
      showContact(); 
     } 
    }); 

    return view; //return here 
} 

@Override 
public void onViewCreated(View view, @Nullable Bundle savedInstanceState) { 
    super.onViewCreated(view, savedInstanceState); 
}