Android studio空对象引用错误答案

【问题标题】：Android studio null object reference errorAndroid studio空对象引用错误
【发布时间】：2022-08-18 21:28:48
【问题描述】：

这是我面临的错误原因：\'java.lang.NullPointerException\' 尝试在空对象引用上调用虚拟方法 \'java.lang.String() 在 com.example.stdio9.MainActivity.getProfileImage(MainActivity.java:230) 在 com.example.stdio9.MainActivity.onCreate(MainActivity.java:84)

这是我的java代码

package com.example.stdio9;

import static com.google.android.gms.auth.api.signin.GoogleSignIn.getClient;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AlertDialog;
import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentTransaction;

import android.content.Intent;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.FrameLayout;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;

import com.example.stdio9.fragment.ExploreFragment;
import com.example.stdio9.fragment.HomeFragment;
import com.example.stdio9.fragment.LibraryFragment;
import com.example.stdio9.fragment.SubscriptionsFragment;
import com.google.android.gms.auth.api.signin.GoogleSignIn;
import com.google.android.gms.auth.api.signin.GoogleSignInAccount;
import com.google.android.gms.auth.api.signin.GoogleSignInClient;
import com.google.android.gms.auth.api.signin.GoogleSignInOptions;
import com.google.android.gms.common.api.ApiException;
import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;
import com.google.android.material.bottomnavigation.BottomNavigationView;
import com.google.firebase.auth.AuthCredential;
import com.google.firebase.auth.AuthResult;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.firebase.auth.GoogleAuthProvider;
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 java.util.HashMap;
import java.util.Objects;

public class MainActivity extends AppCompatActivity {

    Toolbar toolbar;
    BottomNavigationView bottomNavigationView;
    FrameLayout frameLayout;
    private Fragment selectorFragment;

    ImageView user_profile_image;

    GoogleSignInClient mGoogleSignInClient;
    private static final int RC_SIGN_IN = 100;

    FirebaseAuth auth;
    FirebaseUser user;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        getSupportActionBar().setTitle(\"\");

        bottomNavigationView = findViewById(R.id.bottom_navigation);
        frameLayout = findViewById(R.id.frame_layout);

        auth = FirebaseAuth.getInstance();
        user = auth.getCurrentUser();

        user_profile_image = findViewById(R.id.user_profile_image);
        getProfileImage();

        GoogleSignInOptions gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN).requestIdToken(getString(R.string.default_web_client_id)).requestEmail().build();

        mGoogleSignInClient = GoogleSignIn.getClient(MainActivity.this,gso);

        bottomNavigationView.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                switch (item.getItemId()) {
                    case R.id.home:
                        selectorFragment = new HomeFragment();
                        break;

                    case R.id.explore:
                        selectorFragment = new ExploreFragment();
                        break;

                    case R.id.publish:
                        Toast.makeText(MainActivity.this, \"Upload a video\", Toast.LENGTH_SHORT).show();
                        break;

                    case R.id.subscriptions:
                        selectorFragment = new SubscriptionsFragment();
                        break;

                    case R.id.library:
                        selectorFragment = new LibraryFragment();
                        break;

                }

                if (selectorFragment != null) {
                    getSupportFragmentManager().beginTransaction().replace(R.id.frame_layout, selectorFragment).commit();
                }

                return true;

            }
        });

        getSupportFragmentManager().beginTransaction().replace(R.id.frame_layout, new HomeFragment()).commit();

        user_profile_image.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if(user !=null){
                    Toast.makeText(MainActivity.this, \"User Already Signed In\", Toast.LENGTH_SHORT).show();
                }else{
                    showDialog();
                }

            }
        });


    }

    private void showDialog() {
        AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);
        builder.setCancelable(true);

        ViewGroup viewGroup = findViewById(android.R.id.content);
        View view = LayoutInflater.from(getApplicationContext()).inflate(R.layout.item_signin_dialogue,viewGroup,false);

        builder.setView(view);

        TextView txt_google_signIn = view.findViewById(R.id.txt_google_signIn);
        txt_google_signIn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                signIn();
            }
        });
        builder.create().show();
    }

    private void signIn() {
        Intent intent = mGoogleSignInClient.getSignInIntent();
        startActivityForResult(intent,RC_SIGN_IN);
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if(requestCode == RC_SIGN_IN){
            Task<GoogleSignInAccount> task = GoogleSignIn.getSignedInAccountFromIntent(data);

            try{
                GoogleSignInAccount account = task.getResult(ApiException.class);

                AuthCredential credential = GoogleAuthProvider.getCredential(account.getIdToken(),null);
                auth.signInWithCredential(credential).addOnCompleteListener(new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        if(task.isSuccessful()){
                            FirebaseUser firebaseUser = FirebaseAuth.getInstance().getCurrentUser();

                            HashMap<String,Object> map = new HashMap<>();
                            map.put(\"username\",account.getDisplayName());
                            map.put(\"email\",account.getEmail());
                            map.put(\"profile\",String.valueOf(account.getPhotoUrl()));
                            map.put(\"uid\", firebaseUser.getUid());
                            map.put(\"search\",account.getDisplayName().toLowerCase());

                            DatabaseReference reference = FirebaseDatabase.getInstance().getReference().child(\"Users\");
                            reference.child(firebaseUser.getUid()).setValue(map);
                        }
                        else{
                            Toast.makeText(MainActivity.this, task.getException().getMessage(), Toast.LENGTH_SHORT).show();
                        }
                    }
                });
            }
            catch(Exception e){
                Toast.makeText(this, e.getMessage(), Toast.LENGTH_SHORT).show();
            }
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.toolbar_menu, menu);
        return super.onCreateOptionsMenu(menu);
    }

    @Override
    public boolean onOptionsItemSelected(@NonNull MenuItem item) {
        switch (item.getItemId()) {
            case R.id.notification:
                Toast.makeText(this, \"Notification\", Toast.LENGTH_SHORT).show();
                break;

            case R.id.search:
                Toast.makeText(this, \"Search\", Toast.LENGTH_SHORT).show();
                break;

            default:
                return super.onOptionsItemSelected(item);

        }
        return false;
    }

    private void getProfileImage(){
        DatabaseReference reference = FirebaseDatabase.getInstance().getReference().child(\"Users\");
        reference.child(user.getUid()).addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {
                if(snapshot.exists()){
                    String p =snapshot.child(\"profile\").getValue().toString();

                    Picasso.get().load(p).placeholder(R.drawable.ic_baseline_account_circle).into(user_profile_image);
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {
                Toast.makeText(MainActivity.this,error.getMessage(),Toast.LENGTH_SHORT).show();

            }
        });
    }
}

我的 XML 代码：


<?xml version=\"1.0\" encoding=\"utf-8\"?>
<RelativeLayout
    xmlns:android=\"http://schemas.android.com/apk/res/android\"
    xmlns:app=\"http://schemas.android.com/apk/res-auto\"
    xmlns:tools=\"http://schemas.android.com/tools\"
    android:layout_width=\"match_parent\"
    android:layout_height=\"match_parent\"
    tools:context=\".MainActivity\">

    <com.google.android.material.appbar.AppBarLayout
        android:layout_width=\"match_parent\"
        android:layout_height=\"wrap_content\"
        android:id=\"@+id/appBar\">

        <androidx.appcompat.widget.Toolbar
            android:layout_width=\"match_parent\"
            android:layout_height=\"wrap_content\"
            android:id=\"@+id/toolbar\"
            android:background=\"@color/white\"
            app:menu=\"@menu/toolbar_menu\">

            <ImageView
                android:layout_width=\"70dp\"
                android:layout_height=\"50dp\"
                android:src=\"@drawable/icon1\"
                android:adjustViewBounds=\"true\"
                android:id=\"@+id/icon\"/>

             <RelativeLayout
            android:layout_width=\"match_parent\"
            android:layout_height=\"wrap_content\">

            <de.hdodenhof.circleimageview.CircleImageView
                android:layout_width=\"24dp\"
                android:layout_height=\"24dp\"
                android:src=\"@drawable/ic_baseline_account_circle\"
                android:layout_marginStart=\"190dp\"
                android:id=\"@+id/user_profile_image\"/>

        </RelativeLayout>

        </androidx.appcompat.widget.Toolbar>



    </com.google.android.material.appbar.AppBarLayout>

    <FrameLayout
        android:layout_width=\"match_parent\"
        android:layout_height=\"match_parent\"
        android:layout_below=\"@+id/appBar\"
        android:layout_above=\"@+id/bottom_navigation\"
        android:id=\"@+id/frame_layout\"/>


    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:layout_width=\"match_parent\"
        android:layout_height=\"wrap_content\"
        android:background=\"@color/white\"
        app:menu=\"@menu/bottom_menu\"
        android:layout_alignParentBottom=\"true\"
        app:itemTextColor=\"@color/black\"
        app:itemIconTint=\"@color/black\"
        app:itemRippleColor=\"@color/cardview_dark_background\"
        android:id=\"@+id/bottom_navigation\"
        app:labelVisibilityMode=\"labeled\"/>

</RelativeLayout>

Error in this particular lines
 reference.child(user.getUid()).addValueEventListener(new ValueEventListener() {

and `getProfileImage();

我是 android 新手，有人可以帮忙...非常感谢

如果您遇到问题，最好在发布问题时创建MCVE。你发布的差不多400（四百）行代码来解决这个问题。很多人需要解析和尝试在线调试。请编辑您的问题并隔离问题，这样您就可以增加获得帮助的机会。请花点时间阅读how to ask a question。

标签： java android firebase android-fragments firebase-realtime-database

【解决方案1】：

如果它指的是该行：

reference.child(user.getUid()).addValueEventListener(new ValueEventListener()

然后它告诉您“参考”或“用户”为空，并且您正在注意检查它。因此，您需要执行以下操作：

if(reference == null || user == null) return;

就在那一行之前，或者，输入一些逻辑来处理其中任何一个为 NULL 的情况

【讨论】：