是什么使NullpointException出现在我的代码中

Question

我不知道显示关注者关注列表的问题是什么，它正在工作，但几天后(也许有可能我触摸了一些东西......(它不起作用

是什么使它出错 空指针异常？

这是我的错误

2019-06-17 11:53:31.158 8298-8298/com.example.blogapp E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.blogapp, PID: 8298
    java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference
        at com.example.together.Activities.FollowersActivity$5.onDataChange(FollowersActivity.java:157)
        at com.google.firebase.database.Query$1.onDataChange(com.google.firebase:firebase-database@@17.0.0:184)
        at com.google.firebase.database.core.ValueEventRegistration.fireEvent(com.google.firebase:firebase-database@@17.0.0:75)
        at com.google.firebase.database.core.view.DataEvent.fire(com.google.firebase:firebase-database@@17.0.0:63)
        at com.google.firebase.database.core.view.

没有显示红线...只是空点

这是我点击的btn*

 followers.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(getContext(), FollowersActivity.class);
                intent.putExtra("id", profileid);
                intent.putExtra("title","followers");
                startActivity(intent);
            }
        });


        following.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(getContext(), FollowersActivity.class);
                intent.putExtra("id", profileid);
                intent.putExtra("title","following");
                startActivity(intent);
            }
        });

这是我的关注者活动

package com.example.together.Activities;
import android.content.Intent;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.support.v7.widget.Toolbar;
import android.view.View;
import com.example.together.Adapter.UserAdapter;
import com.example.together.Model.User;
import com.example.together.R;
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;
public class FollowersActivity extends AppCompatActivity {
    String id;
    String title;
    List<String> idList;
    RecyclerView recyclerView;
    UserAdapter userAdapter;
    List<User> userList;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_followers);
        Intent intent = getIntent();
        id = intent.getStringExtra("id");
        title = intent.getStringExtra("title");
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        toolbar.setNavigationOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                finish();
            }
        });

        recyclerView = findViewById(R.id.recycler_view);
        recyclerView.setHasFixedSize(true);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));
        userList = new ArrayList<>();
        userAdapter = new UserAdapter(this, userList, false);
        recyclerView.setAdapter(userAdapter);
        idList = new ArrayList<>();
        switch (title){
            case "likes":
                getLikes();
                break;
            case "following":
                getFollowing();
                break;
            case "followers":
                getFollowers();
                break;
        }
    }
    private void getLikes() {
        DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Likes")
                .child(id);
        reference.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                idList.clear();
                for (DataSnapshot snapshot : dataSnapshot.getChildren()){
                    idList.add(snapshot.getKey());
                }
            }
            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
            }
        });
    }

    private void getFollowing(){
        DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Follow")
                .child(id).child("following");
        reference.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                idList.clear();
                for (DataSnapshot snapshot : dataSnapshot.getChildren()){
                    idList.add(snapshot.getKey());
                }
                showUsers();
            }
            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
            }
        });
    }

    private void getFollowers(){
        DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Follow")
                .child(id).child("followers");
        reference.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                idList.clear();
                for (DataSnapshot snapshot : dataSnapshot.getChildren()){
                    idList.add(snapshot.getKey());
                }
                showUsers();
            }
            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
            }
        });
    }

    private void showUsers(){
        DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Users");
        reference.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                userList.clear();
                for (DataSnapshot snapshot : dataSnapshot.getChildren()){
                    User user = snapshot.getValue(User.class);
                    for (String id : idList){
                        if (user.getId().equals(id)){
                            userList.add(user);
                        }
                    }
                }
                userAdapter.notifyDataSetChanged();
            }
            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
            }
        });
    }

}

Answer 1

用户对象为 null，并且您正在尝试从 null 对象检索数据

reference.addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                    userList.clear();
                    for (DataSnapshot snapshot : dataSnapshot.getChildren()){
                        User user = snapshot.getValue(User.class);
// check user exists or not.. here user gets null
                         if(user!=null){
                        for (String id : idList){
                            if (user.getId().equals(id)){
                                userList.add(user);
                            }
                        }}
                    }

Answer 2

检查第一个活动的配置文件 ID。

intent.putExtra("id", profileid);

并确保在此处打印日志是获得配置文件ID的值