java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法'boolean java.lang.String.isEmpty（）' [英] java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.isEmpty()' on a null object reference

问题描述

我的应用中有一个错误，我不知道如何解决。当我尝试运行我的应用程序时，它只是说，不幸的是，应用程序已停止运行，并且在logcat中它说：

I have a bug in my app and I don't know how to resolve it. When I try to run my app, it just says, unfortunately app has stopped and in the logcat it says this:

java.lang。 NullPointerException：尝试在空对象引用上调用虚拟方法'boolean java.lang.String.isEmpty（）'在空对象引用上

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.isEmpty()' on a null object reference' on a null object reference

t假定由于用户未登录而使用户进入登录活动。这是指出错误的片段代码。

t is suppose to take the user to the login activity since the user is not logged in. This is the code of the fragment where the error is being pointed to.

package com.app.whatsupafrica;


import android.content.Intent;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

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

import com.firebase.ui.database.FirebaseRecyclerAdapter;
import com.firebase.ui.database.FirebaseRecyclerOptions;
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;


/**
* A simple {@link Fragment} subclass.
*/
public class ChatsFragment extends Fragment
{
private View PrivateChatsView;
private RecyclerView chatsList;

private DatabaseReference ChatsRef, UsersRef;
private FirebaseAuth mAuth;
private String currentUserID;


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


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    PrivateChatsView = inflater.inflate(R.layout.fragment_chats, container, false);


    mAuth = FirebaseAuth.getInstance();
    currentUserID = mAuth.getUid();
    if (mAuth.getUid() == null)
    {
     SendUserToLoginActivity();
    }
    if (! mAuth.getUid().isEmpty())
    {
        ChatsRef = FirebaseDatabase.getInstance().getReference().child("Contacts").child(mAuth.getUid());
    }
    UsersRef = FirebaseDatabase.getInstance().getReference().child("Users");


    chatsList = (RecyclerView) PrivateChatsView.findViewById(R.id.chats_list);
    chatsList.setLayoutManager(new LinearLayoutManager(getContext()));


    return PrivateChatsView;
}


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


    FirebaseRecyclerOptions<Contacts> options =
            new FirebaseRecyclerOptions.Builder<Contacts>()
            .setQuery(ChatsRef, Contacts.class)
            .build();


    FirebaseRecyclerAdapter<Contacts, ChatsViewHolder> adapter =
            new FirebaseRecyclerAdapter<Contacts, ChatsViewHolder>(options) {
                @Override
                protected void onBindViewHolder(@NonNull final ChatsViewHolder holder, int position, @NonNull Contacts model)
                {
                    final String usersIDs = getRef(position).getKey();
                    final String[] retImage = {"default_image"};

                    UsersRef.child(usersIDs).addValueEventListener(new ValueEventListener() {
                        @Override
                        public void onDataChange(@NonNull DataSnapshot dataSnapshot)
                        {
                            if (dataSnapshot.exists())
                            {
                                if (dataSnapshot.hasChild("image"))
                                {
                                    retImage[0] = dataSnapshot.child("image").getValue().toString();
                                    Picasso.get().load(retImage[0]).into(holder.profileImage);
                                }

                                final String retName = dataSnapshot.child("name").getValue().toString();
                                final String retStatus = dataSnapshot.child("status").getValue().toString();

                                holder.userName.setText(retName);
                                // holder.userStatus.setText("Last Seen: " + "\n" + "Date " + " Time");

                                holder.itemView.setOnClickListener(new View.OnClickListener() {
                                    @Override
                                    public void onClick(View v)
                                    {
                                        Intent chatIntent = new Intent(getContext(), ChatActivity.class);
                                        chatIntent.putExtra("visit_user_id", usersIDs);
                                        chatIntent.putExtra("visit_user_name", retName);
                                        chatIntent.putExtra("visit_image", retImage[0]);
                                        startActivity(chatIntent);
                                    }
                                });
                            }
                        }

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

                        }
                    });
                }

                @NonNull
                @Override
                public ChatsViewHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int i)
                {
                    View view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.users_display_layout, viewGroup, false);
                    return new ChatsViewHolder(view);
                }
            };

    chatsList.setAdapter(adapter);
    adapter.startListening();
}




public static class ChatsViewHolder extends RecyclerView.ViewHolder
{
    CircleImageView profileImage;
    TextView userStatus, userName;


    public ChatsViewHolder(@NonNull View itemView)
    {
        super(itemView);

        profileImage = itemView.findViewById(R.id.users_profile_image);
        userStatus = itemView.findViewById(R.id.user_status);
        userName = itemView.findViewById(R.id.user_profile_name);
    }
}
private void SendUserToLoginActivity()
{
    Intent loginIntent = new Intent(getContext(), LoginActivity.class);
    startActivity(loginIntent);
}
}

推荐答案

添加这段代码：

mAuth.getUid();

而不是

mAuth.getCurrentUser().getUid();

当前用户显然是空的

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