错误：尝试在空对象引用上调用虚拟方法“ java.lang.String java.lang.Object.toString（）”

Question

我的朋友代码片段试图在其中使用回收站视图显示朋友列表。

package com.rishav.baatcheet;


import android.app.AlertDialog;
import android.content.DialogInterface;
import android.content.Intent;
import android.os.Bundle;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import com.firebase.ui.database.FirebaseRecyclerAdapter;
import com.google.firebase.auth.FirebaseAuth;
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 com.squareup.picasso.Picasso;

import de.hdodenhof.circleimageview.CircleImageView;

public class FreindsFragment extends Fragment {

private RecyclerView mFreindsList;

private DatabaseReference mUsersDatabase;
private DatabaseReference mFreindsDatabase;
private FirebaseAuth mAuth;

private String mCurrent_user_id;
private View mMainView;

private String list_user_id;

public FreindsFragment() {
    // Required empty public constructor
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    mMainView = inflater.inflate(R.layout.fragment_freinds , container , false);
    mFreindsList = (RecyclerView) mMainView.findViewById(R.id.freinds_list);
    mAuth = FirebaseAuth.getInstance();

    mCurrent_user_id = mAuth.getCurrentUser().getUid();
    mFreindsDatabase = FirebaseDatabase.getInstance().getReference().child("Friends").child(mCurrent_user_id);
    mFreindsDatabase.keepSynced(true);
    mUsersDatabase = FirebaseDatabase.getInstance().getReference().child("Users");
    mUsersDatabase.keepSynced(true);

    mFreindsList.setHasFixedSize(true);
    mFreindsList.setLayoutManager(new LinearLayoutManager(getContext()));

    return mMainView;
}

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

    FirebaseRecyclerAdapter<Friends , FriendsViewHolder> friendsRecyclerAdapter = new FirebaseRecyclerAdapter<Friends, FriendsViewHolder>(
            Friends.class,
            R.layout.users_single_layout,
            FriendsViewHolder.class,
            mFreindsDatabase
    ) {
        @Override
        protected void populateViewHolder(final FriendsViewHolder friendsViewHolder, final Friends friends, int i) {
            friendsViewHolder.setDate(friends.getDate());

            list_user_id = getRef(i).getKey();
            mUsersDatabase.child(list_user_id).addValueEventListener(new ValueEventListener() {
                @Override
                public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                    final String userName = dataSnapshot.child("name").getValue().toString();
                    String userThumb = dataSnapshot.child("thumb_image").getValue().toString();
                    if (dataSnapshot.hasChild("online")) {
                        Boolean userOnline = (boolean) dataSnapshot.child("online").getValue();
                        friendsViewHolder.setUserOnline(userOnline);
                    }

                    friendsViewHolder.setName(userName);
                    friendsViewHolder.setUserImage(userThumb);
                    friendsViewHolder.mView.setOnClickListener(new View.OnClickListener() {
                        @Override
                        public void onClick(View v) {
                            CharSequence []options = new CharSequence[]{"Open Profile" , "Open Chat"};

                            final AlertDialog.Builder builder = new AlertDialog.Builder(getContext());
                            builder.setTitle("Select Options");
                            builder.setItems(options, new DialogInterface.OnClickListener() {
                                @Override
                                public void onClick(DialogInterface dialog, int which) {
                                    if (which == 0){
                                        Intent profileIntent = new Intent(getContext() , ProfileActivity.class);
                                        profileIntent.putExtra("user_id" , list_user_id);
                                        startActivity(profileIntent);
                                    }
                                    if (which == 1){
                                        Intent chatIntent = new Intent(getContext() , ChatActivity.class);
                                        chatIntent.putExtra("user_id" , list_user_id);
                                        chatIntent.putExtra("user_name" , userName);
                                        startActivity(chatIntent);
                                    }
                                }
                            });

                            builder.show();
                        }
                    });


                }

                @Override
                public void onCancelled(@NonNull DatabaseError databaseError) {

                }
            });
        }
    };
    mFreindsList.setAdapter(friendsRecyclerAdapter);
}

public static class FriendsViewHolder extends RecyclerView.ViewHolder {
    View mView;

    public FriendsViewHolder(View itemView){
        super(itemView);
        mView = itemView;
    }

    public void setDate (String date){
        TextView userNameView = (TextView) mView.findViewById(R.id.user_single_status);
        userNameView.setText(date);
    }

    public void setName(String name){
        TextView userNameView = (TextView) mView.findViewById(R.id.user_single_name);
        userNameView.setText(name);
    }

    public void setUserImage (String thumb_image){
        CircleImageView userImageView = (CircleImageView) mView.findViewById(R.id.user_single_image);
        Picasso.get().load(thumb_image).placeholder(R.drawable.default_avtar101).into(userImageView);
    }

    public void setUserOnline (Boolean online_status){
        CircleImageView userOnlineView = (CircleImageView) mView.findViewById(R.id.user_single_online);

        if (online_status){
            userOnlineView.setVisibility(View.VISIBLE);
        }
        else
            userOnlineView.setVisibility(View.INVISIBLE);

    }
}

}

logcat如下

  

2019-10-09 19：36：28.256 9058-9058 / com.rishav.baatcheet E / AndroidRuntime：FATAL EXCEPTION：main       流程：com.rishav.baatcheet，PID：9058       java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“ java.lang.String java.lang.Object.toString（）”           在com.rishav.baatcheet.FreindsFragment $ 1 $ 1.onDataChange（FreindsFragment.java:85）           com.google.firebase.database.core.ValueEventRegistration.fireEvent（com.google.firebase：firebase-database @@ 19.1.0：75）           com.google.firebase.database.core.view.DataEvent.fire（com.google.firebase：firebase-database @@ 19.1.0：63）           com.google.firebase.database.core.view.EventRaiser $ 1.run（com.google.firebase：firebase-database @@ 19.1.0：55）           在android.os.Handler.handleCallback（Handler.java:754）           在android.os.Handler.dispatchMessage（Handler.java:95）           在android.os.Looper.loop（Looper.java:165）           在android.app.ActivityThread.main（ActivityThread.java:6375）           在java.lang.reflect.Method.invoke（本机方法）           在com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:912）           在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:802）

数据库JSON如下：

{"Users":{
  "client UID":{
    "device_token":"cDItwd-LySE",
    "name":"Rishav",
    "image":"https://firebasestorage.googleapis.com/v0/b...",
    "last_seen":1570629990274,
    "online":true,
    "status":"Default Status",
    "thumb_image":"https://firebasestorage.googleapis.com/v0/b..."
  },
  "another client UID":{
    "device_token":"cDItwd-LySE",
    "name":"Rishav",
    "image":"https://firebasestorage.googleapis.com/v0/b...",
    "last_seen":1570629990274,
    "online":true,
    "status":"Default Status",
    "thumb_image":"https://firebasestorage.googleapis.com/v0/b..."
    }
  },
"Friends":{
  "Current client UID":{
    "Friend client UID":{
      "date":"09-Oct-2019 7:10:34 PM"
    }
  },
  "Friend client UID":{
    "Current client UID":{
      "date":"09-Oct-2019 7:10:34 PM"
    }
  }
  }}