我对空点异常有一些问题

Posted 2023-02-24

【中文标题】我对空点异常有一些问题

【英文标题】：I have some problem with Null Point Exception

【发布时间】：2019-11-02 18:56:42

【问题描述】：

我试图创建一个Uid 代码，但遇到了关于NullPointException 的问题：

    package com.example.musicmates;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.ActionBarDrawerToggle;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.NotificationCompat;
import androidx.drawerlayout.widget.DrawerLayout;
import androidx.recyclerview.widget.RecyclerView;
import androidx.appcompat.widget.Toolbar;

import android.content.Intent;
import android.os.Bundle;
import android.view.MenuItem;
import android.view.View;
import android.widget.TextView;
import android.widget.Toast;


import com.google.android.material.navigation.NavigationView;
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;
import com.google.firebase.database.annotations.NotNull;
import com.squareup.picasso.Picasso;

import java.util.Set;

import de.hdodenhof.circleimageview.CircleImageView;

public class MainActivity extends AppCompatActivity 
private NavigationView navigationView;
private DrawerLayout drawerLayout;
private RecyclerView postList;
private Toolbar mToolbar;
private CircleImageView NavProfileImage;
private TextView NavProfileUserName;
private ActionBarDrawerToggle actionBarDrawerToggle;
private FirebaseAuth mAuth;
private DatabaseReference UsersRef;
String currentUserID;
    @Override
    protected void onCreate(Bundle savedInstanceState) 
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        navigationView = findViewById(R.id.navigaition_view);

        mAuth = FirebaseAuth.getInstance();
        currentUserID = mAuth.getCurrentUser().getUid();

        UsersRef = FirebaseDatabase.getInstance().getReference().child("Users");


        mToolbar = findViewById(R.id.main_page_toolbar);
        setSupportActionBar(mToolbar);
        getSupportActionBar().setTitle("Home");

        drawerLayout = findViewById(R.id.drawable_layout);
        actionBarDrawerToggle = new ActionBarDrawerToggle(MainActivity.this,drawerLayout,R.string.drawer_open,R.string.drawer_close);
        drawerLayout.addDrawerListener(actionBarDrawerToggle);
        actionBarDrawerToggle.syncState();
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        View navView = navigationView.inflateHeaderView(R.layout.navigation_header);
        NavProfileImage = navView.findViewById(R.id.nav_profile_image);
        NavProfileUserName = navView.findViewById(R.id.nav_profile_full_name);


        UsersRef.child(currentUserID).addValueEventListener(new ValueEventListener() 
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) 
                if(dataSnapshot.exists())
                    String fullname = dataSnapshot.child("fullname").getValue().toString();
                    String image = dataSnapshot.child("profileimage").getValue().toString();
                    NavProfileUserName.setText(fullname);
                    Picasso.get().load(image).placeholder(R.drawable.profile).into(NavProfileImage);
                
            

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) 

            
        );

        navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() 
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) 
                UserMenuSelector(menuItem);
                return false;
            
        );

    


    @Override
    protected void onStart() 
        super.onStart();
        FirebaseUser currentUser = mAuth.getCurrentUser();
        if(currentUser == null)
            SendUserToLoginActivity();
        
        else
            CheckUserExistence();
        
    

    private void CheckUserExistence()
    
        final String current_user_id = mAuth.getCurrentUser().getUid();

        UsersRef.addValueEventListener(new ValueEventListener() 
            @Override
            public void onDataChange(DataSnapshot dataSnapshot)
            
                if(!dataSnapshot.hasChild(current_user_id))
                
                    SendUserToSetupActivity();
                
            

            @Override
            public void onCancelled(DatabaseError databaseError) 

            
        );

    

    private void SendUserToSetupActivity() 
        Intent setupIntent = new Intent(MainActivity.this, SetupActivity.class);
        setupIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
        startActivity(setupIntent);
        finish();
    

    private void SendUserToLoginActivity() 
        Intent loginIntent = new Intent(MainActivity.this,LoginActivity.class);
        loginIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK|Intent.FLAG_ACTIVITY_CLEAR_TASK);
        startActivity(loginIntent);
        finish();
    

    @Override
    public boolean onOptionsItemSelected(@NonNull MenuItem item) 
        if(actionBarDrawerToggle.onOptionsItemSelected(item))
            return true;
        
        return super.onOptionsItemSelected(item);
    

    private void UserMenuSelector(MenuItem menuItem) 
        switch (menuItem.getItemId())
            case R.id.nav_post:
                Toast.makeText(this, "Add new post", Toast.LENGTH_SHORT).show();
                break;
            case R.id.nav_profile:
                Toast.makeText(this, "Profile", Toast.LENGTH_SHORT).show();
                break;
            case R.id.nav_home:
                Toast.makeText(this, "Home", Toast.LENGTH_SHORT).show();
                break;
            case R.id.nav_friends:
                Toast.makeText(this, "Friends", Toast.LENGTH_SHORT).show();
                break;
            case R.id.nav_find_friends:
                Toast.makeText(this, "Find Friends", Toast.LENGTH_SHORT).show();
                break;
            case R.id.nav_message:
                Toast.makeText(this, "Message", Toast.LENGTH_SHORT).show();
                break;
            case R.id.nav_setting:
                Toast.makeText(this, "Setting", Toast.LENGTH_SHORT).show();
                break;
            case R.id.nav_logout:
                Toast.makeText(this, "Log out", Toast.LENGTH_SHORT).show();
                mAuth.signOut();
                SendUserToLoginActivity();
                break;
        
    

2019-11-02 11:46:36.860 16810-16810/com.example.musicmates E/AndroidRuntime: 致命异常: main 进程：com.example.musicmates，PID：16810 java.lang.RuntimeException：无法启动活动 ComponentInfocom.example.musicmates/com.example.musicmates.MainActivity： java.lang.NullPointerException：尝试调用虚拟方法 'java.lang.String com.google.firebase.auth.FirebaseUser.getUid()' 空对象引用 在 android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2817) 在 android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892) 在 android.app.ActivityThread.-wrap11（未知来源：0） 在 android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593) 在 android.os.Handler.dispatchMessage(Handler.java:105) 在 android.os.Looper.loop(Looper.java:164) 在 android.app.ActivityThread.main(ActivityThread.java:6541) 在 java.lang.reflect.Method.invoke（本机方法） 在 com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) 在 com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767) 原因：java.lang.NullPointerException：尝试调用虚拟方法'java.lang.String com.google.firebase.auth.FirebaseUser.getUid()' 在空对象上 参考 在 com.example.musicmates.MainActivity.onCreate(MainActivity.java:56) 在 android.app.Activity.performCreate(Activity.java:6975) 在 android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1213) 在 android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2770)

【问题讨论】：

你可以添加一些代码...你的MainActivity类

drawerLayout = findViewById(R.id.drawable_layout); actionBarDrawerToggle = new ActionBarDrawerToggle(MainActivity.this,drawerLayout,R.string.drawer_open,R.string.drawer_close); drawLayout.addDrawerListener(actionBarDrawerToggle); actionBarDrawerToggle.syncState(); getSupportActionBar().setDisplayHomeAsUpEnabled(true);查看 navView = navigationView.inflateHeaderView(R.layout.navigation_header); NavProfileImage = navView.findViewById(R.id.nav_profile_image); NavProfileUserName = navView.findViewById(R.id.nav_profile_full_name);

UsersRef.child(currentUserID).addValueEventListener(new ValueEventListener() @Override public void onDataChange(@NonNull DataSnapshot dataSnapshot) if(dataSnapshot.exists()) String fullname = dataSnapshot.child( "fullname").getValue().toString(); String image = dataSnapshot.child("profileimage").getValue().toString(); NavProfileUserName.setText(fullname); Picasso.get().load(image) .placeholder(R.drawable.profile).into(NavProfileImage);

【参考方案1】：

在您的应用程序 build.gradle 中，将其添加到您的依赖项中

compile 'com.google.firebase:firebase-auth:x.x.x'（最新版本）

在 onCreate 中添加此行下方：navigationView = findViewById(R.id.navigaition_view);

mAuth = FirebaseAuth.getInstance();
if (mAuth.getCurrentUser() != null) 
         currentUserID = mAuth.getCurrentUser().getUid();   
        else
             finish();
             SendUserToLoginActivity();
   

从onstart 中删除检查方法。清理并重建您的项目，让我知道它是否有效。

【讨论】：

嗨。我在哪里可以添加 if (FirebaseAuth.getInstance().getCurrentUser() == null) //Go to login else String uid = FirebaseAuth.getInstance().getCurrentUser().getUid();

Inside on Create .

也标出你遇到错误的地方，我会尽力解决问题的。

我刚刚发布了完整的代码。请看一下。谢谢。

@Kevin 请检查。