我的“MainActivity”不断崩溃。我不知道是我的代码、数据库设置还是其他原因

Question

我在使用教程要求的新更改时遇到问题。我完全遵循了编码规则（新版本中的新“快照”条目除外），但我的“MainActivity”一直在崩溃。用户仍在身份验证中注册，但在数据库中并没有像我希望的那样注册他们。

错误控制台指向“DatabaseReference userDb = usersDb.child(user.getUid());”作为崩溃的原因。我认为数据库可能有问题，因为不完全清楚如何设置它。如果有新的编码建议可以解决这个问题，或者我是否遗漏了数据库设置中的某些内容，请告诉我。请帮忙。

这里也是我正在关注的教程的链接：https://www.youtube.com/watch?v=RagA8g9A5Qc&list=PLxabZQCAe5fio9dm1Vd0peIY6HLfo5MCf&index=17

    private String userSex;
private String notUserSex;
public void checkUserSex() {
    final FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
    DatabaseReference userDb = usersDb.child(user.getUid());
    userDb.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot snapshot) {
                if (snapshot.exists()) {
                    if (snapshot.child("sex").getValue() != null) {
                        userSex = snapshot.child("sex").getValue().toString();
                        switch (userSex) {
                            case "Male":
                                notUserSex = "Female";
                                break;
                            case "Female":
                                notUserSex = "Male";
                                break;
                        }
                        getOppositeSexUsers();
                    }
                }
            }
        @Override
        public void onCancelled(@NonNull DatabaseError error) {

        }
    });
}

public void getOppositeSexUsers(){
    usersDb.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(@NonNull DataSnapshot snapshot, @Nullable String previousChildName) {
            if (snapshot.exists() && !snapshot.child("connections").child("no").hasChild(currentUId) && !snapshot.child("connections").child("yes").hasChild(currentUId) && snapshot.child("sex").getValue().toString().equals(notUserSex)){
                String profileImageUrl = "default";
                if(snapshot.child("profileImageUrl").getValue()!=null) {
                    if (!snapshot.child("profileImageUrl").getValue().equals("default")) {
                        profileImageUrl = snapshot.child("profileImageUrl").getValue().toString();
                    }
                }
                cards item = new cards(snapshot.getKey(), snapshot.child("name").getValue().toString(), profileImageUrl);
                rowItems.add(item);
                arrayAdapter.notifyDataSetChanged();
            }
        }

Logcat

2021-03-10 15:48:00.372 9915-9915/com.bendes.tinder2 E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.bendes.tinder2, PID: 9915
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.bendes.tinder2/com.bendes.tinder2.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'com.google.firebase.database.DatabaseReference com.google.firebase.database.DatabaseReference.child(java.lang.String)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:223)
    at android.app.ActivityThread.main(ActivityThread.java:7656)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'com.google.firebase.database.DatabaseReference com.google.firebase.database.DatabaseReference.child(java.lang.String)' on a null object reference
    at com.bendes.tinder2.MainActivity.checkUserSex(MainActivity.java:141)
    at com.bendes.tinder2.MainActivity.onCreate(MainActivity.java:48)
    at android.app.Activity.performCreate(Activity.java:8000)
    at android.app.Activity.performCreate(Activity.java:7984)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
    at android.os.Handler.dispatchMessage(Handler.java:106) 
    at android.os.Looper.loop(Looper.java:223) 
    at android.app.ActivityThread.main(ActivityThread.java:7656) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 

2021-03-10 15:48:00.438 9915-9915/com.bendes.tinder2 I/Process：发送信号。 PID：9915 SIG：9

public class MainActivity extends AppCompatActivity {
private cards cards_data[];
private arrayAdapter arrayAdapter;
private int i;

private Button logout;
private FirebaseAuth mAuth;
private String currentUId;
private DatabaseReference usersDb;

ListView listView;
List<cards> rowItems;


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

    usersDb = FirebaseDatabase.getInstance().getReference().child("Users");
    mAuth = FirebaseAuth.getInstance();
    currentUId = mAuth.getCurrentUser().getUid();

    logout = (Button) findViewById(R.id.signout);
    logout.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            FirebaseAuth.getInstance().signOut();
            startActivity(new Intent(MainActivity.this, ChooseLoginOrRegistrationActivity.class));
        }
    });

Answer 1

我知道我哪里出错了，那是我的编码结构。 logcat 中出现了两个错误：checkUserSex();'声明（第 48 行）和数据库引用（第 141 行）。这只是放置我的“checkUserSex();”的情况。在我的“onCreate”的其他声明下方（因此在 usersDb、mAuth 和 currentUId 下方。

Answer 2

由于user.getUid() 返回 NULL，您将收到 Null 指针异常。因此，每当user.getUid() 为NULL 时，您的应用程序必须提示用户登录。登录成功后，您的应用程序必须遵循您的常规工作流程，例如checkUserSex()

以下解决方案可能会对您有所帮助（示例代码显示 Firebase Google 身份验证）

在您的活动中声明以下附加变量

private static final int RC_SIGN_IN = 1;
private FirebaseAuth mAuth;
private GoogleSignInClient mGoogleSignInClient;

修改你的onCreate()方法如下

@Override
protected void onCreate ( Bundle savedInstanceState ) {
    super.onCreate ( savedInstanceState );
    setContentView ( R.layout.activity_main2 );
    GoogleSignInOptions gso = new GoogleSignInOptions.Builder ( GoogleSignInOptions.DEFAULT_SIGN_IN )
            .requestIdToken ( getString ( R.string.default_web_client_id ) )
            .requestEmail ()
            .build ();
    mGoogleSignInClient = GoogleSignIn.getClient ( this, gso );
    mAuth = FirebaseAuth.getInstance();
    checkFirebaseUser ( mAuth.getCurrentUser () );
}

将以下附加方法添加到您的活动中（用于身份验证）

private void checkFirebaseUser ( FirebaseUser currentUser ) {
    if (currentUser != null) {
        //Continue with your regular flow
        checkUserSex ();
    } else {
        //Sign In flow
        Intent signInIntent = mGoogleSignInClient.getSignInIntent ();
        startActivityForResult ( signInIntent, RC_SIGN_IN );
    }
}

@Override
public void onActivityResult ( int requestCode, int resultCode, 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 );
            assert account != null;
            firebaseAuthWithGoogle ( account.getIdToken () );
        } catch (Exception e) {
            Toast.makeText ( this, "login failed", Toast.LENGTH_SHORT ).show ();
        }
    }
}

private void firebaseAuthWithGoogle ( String idToken ) {
    AuthCredential credential = GoogleAuthProvider.getCredential ( idToken, null );
    mAuth.signInWithCredential ( credential )
            .addOnCompleteListener ( this, task -> {
                if (task.isSuccessful ()) {
                    Toast.makeText ( this, "login success", Toast.LENGTH_SHORT ).show ();
                    FirebaseUser user = mAuth.getCurrentUser ();
                    checkFirebaseUser ( user );
                } else {
                    Toast.makeText ( this, "login failed", Toast.LENGTH_SHORT ).show ();
                }
            } );
}

确保在build.gradle 中添加以下依赖项以进行身份​​验证

implementation 'com.google.android.gms:play-services-auth:19.0.0'
implementation 'com.google.firebase:firebase-auth:20.0.3'

在您的 Firebase 控制台中启用 Google 身份验证