我对空点异常有一些问题

Question

我试图创建一个Uid代码，我遇到了这个关于NullPointExctive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。实例musicmates E/AndroidRuntime:致命异常：主进程：com。实例musicmates，PID:16810 java。lang.RuntimeException:无法启动活动组件信息{com.example.musicmates/com.example.musicmates.MainActivity}:java。lang.NullPointerException：尝试调用虚拟方法“java”。lang.String.com.google.火力基地。啊。FirebaseUser。android上的空对象引用上的getUid（）。应用程序。活动线程。在android上执行LaunchActivity（ActivityThread.java:2817）。应用程序。活动线程。android上的handleLaunchActivity（ActivityThread.java:2892）。应用程序。活动线程-android上的wrap11（未知来源：0）。应用程序。android上的ActivityThread$H.handleMessage（ActivityThread.java:1593）。操作系统。汉德勒。android上的dispatchMessage（Handler.java:105）。操作系统。活套。android上的loop（Looper.java:164）。应用程序。活动线程。java上的main（ActivityThread.java:6541）。朗，反思一下。方法在com上调用（本机方法）。Android内部的操作系统。合子$MethodandArgscaler。在com上运行（Zygote.java:240）。Android内部的操作系统。合子体。main（ZygoteInit.java:767）由以下原因引起：java。lang.NullPointerException：尝试调用虚拟方法“java”。lang.String.com.google.火力基地。啊。FirebaseUser。getUid（）'位于com上的空对象引用上。实例音乐教师。主要活动。android上的onCreate（MainActivity.java:56）。应用程序。活动android上的performCreate（Activity.java:6975）。应用程序。仪器。android上的callActivityOnCreate（Instrumentation.java:1213）。应用程序。活动线程。performLaunchActivity（ActivityThread.java:2770）

Answer 1

在您的应用程序build.gradle中，将其添加到您的依赖项

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

将此添加到onCreate中的以下行：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。清理和重建你的项目，让我知道它是否有效。