尝试使用FireBase在活动中查看“跟随者”和“跟随者时获取NullPointerException

Question

所以我在这里尝试制作一个类似Instagram的应用程序，但仅用于汽车，并且我试图单击关注者和关注者，以查看我关注的人并了解关注我的人，但是我不确定为什么会有NPE ...

我可以成功跟踪ueser，用户也可以跟踪我，并且它们确实存在于我的firebaseRT中，一切正常。

错误在该代码行中带有星号：****** Objects.requireNonNull（getSupportActionBar（））。setTitle（title）; ******

package com.capstone.araba;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;

import androidx.appcompat.widget.Toolbar;

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.capstone.araba.Adapter.UserAdapter;
import com.capstone.araba.Model.User;

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

public class FollowersActivity extends AppCompatActivity {

    String id;
    String title;

    private 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 = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        ******Objects.requireNonNull(getSupportActionBar()).setTitle(title);******
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        toolbar.setNavigationOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                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 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 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 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());
                }
                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){
                        assert user != null;
                        if (user.getId().equals(id)){
                            userList.add(user);
                        }
                    }
                }
                userAdapter.notifyDataSetChanged();
            }

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

            }
        });
    }
}

2019-10-12 12:53:58.911 23299-23299/com.capstone.araba E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.capstone.araba, PID: 23299
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.capstone.araba/com.capstone.araba.FollowersActivity}: java.lang.NullPointerException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2778)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6494)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
     Caused by: java.lang.NullPointerException
        at java.util.Objects.requireNonNull(Objects.java:203)
        at com.capstone.araba.FollowersActivity.onCreate(FollowersActivity.java:49)
        at android.app.Activity.performCreate(Activity.java:7009)
        at android.app.Activity.performCreate(Activity.java:7000)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1214)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2731)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6494) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807) 

Answer 1

美好的一天，您可以只放置布局而不是工具栏！