Java 尝试从Firebase实时数据库检索用户详细信息时使用空指针

在我的Android应用程序中，我有一个数据库存储库来联系Firebase实时数据库，还有一个服务层，提供一些方法。我的SaveUserProfile方法可以工作，但我在GetUserFromUid上不断得到一个空指针

它返回的“user”对象为null，我不知道为什么。我数据库中的节点称为“用户”（全部小写），我希望通过用户ID检索用户作为模型，并在屏幕上显示名称和电子邮件

有人能看出我错在哪里吗

我的数据库上下文：

public class DbContext implements IDbContext {

User user = null;
Context context;
DatabaseReference databaseUsers = FirebaseDatabase.getInstance().getReference("users");

 public DbContext(Context context){
   super();
   this.context = context;
}

@Override
public void AddUserAccount(User user1) {
    databaseUsers.child(user1.userId).setValue(user1);
}

@Override
public User GetUserFromFirebase(String uid) {
   databaseUsers.child(uid)
           .addListenerForSingleValueEvent(new ValueEventListener() {
               @Override
               public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                   user = dataSnapshot.getValue(User.class);
               }

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

               }
           });
    return user;
}

我的数据库服务：

public class DbService implements IDbService {

//instance of DbContext for firebase handling
private DbContext dbContext;

public DbService(Context context){
    super();
    dbContext = new DbContext(context);
}

@Override
public User SaveUserProfile(User u) {

    dbContext.AddUserAccount(u);

    return u;
}

@Override
public User GetUserFromUid(String uid) {
    User user = dbContext.GetUserFromFirebase(uid);
    return user;
}

public class DbService implements IDbService {

//instance of DbContext for firebase handling
public DbContext dbContext;

public DbService(Context context){
    super();
    dbContext = new DbContext(context);
}

@Override
public User SaveUserProfile(User u) {

    dbContext.AddUserAccount(u);

    return u;
}

@Override
public User GetUserFromUid(String uid) {
    final User[] user1 = {new User()};
    dbContext.GetUserFromFirebase(uid, new Callback() {
        @Override
        public void myResponseCallback(User user) {
            user1[0] = user;
        }
    });
    return user1[0];
}

我的用户模型：

public class User {

public String userId;
public String name;
public String email;
public String account;

//constructor required for calls to DataSnapshot.getValue(User.class)
public User(){
}

public User(String userId, String name, String email, String account) {
    this.userId = userId;
    this.name = name;
    this.email = email;
    this.account = account;
}

public String getUserId() {
    return userId;
}

public void setUserId(String userId) {
    this.userId = userId;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public String getEmail() {
    return email;
}

public void setEmail(String email) {
    this.email = email;
}

public String getAccount() {
    return account;
}

public void setAccount(String account) {
    this.account = account;
}

我要在其中显示用户详细信息的活动：

public class DetailsActivity extends AppCompatActivity {
//tag
private static final String TAG = DetailsActivity.class.getSimpleName();

//firebase auth
private FirebaseAuth mAuth;

//variables
private TextView inputName, inputEmail;
private DatabaseReference mFirebaseDatabase;
private String userId;
public String currentUserAccount;
public String teacherAccountNav = "Teacher";
public User userDetails;

@RequiresApi(api = Build.VERSION_CODES.KITKAT)
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_details);

    inputName = findViewById(R.id.nameTextView);
    inputEmail = findViewById(R.id.emailTextView);

   

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

    assert user != null;
    userId = user.getUid();

   
    getUserDetails(userId);
    inputName.setText(userDetails.name);
    inputEmail.setText(userDetails.email);
}

public User getUserDetails(String uid){
    DbService dbService = new DbService(this);
    userDetails = dbService.GetUserFromUid(uid);
    return userDetails;
}

编辑下面的内容

我的回拨：

public interface Callback {
void myResponseCallback(User user);
}

我的编辑背景：

public class DbContext implements IDbContext {

User user = null;
Context context;
DatabaseReference databaseUsers = FirebaseDatabase.getInstance().getReference("users");

 public DbContext(Context context){
   super();
   this.context = context;
}

@Override
public void AddUserAccount(User user1) {
    databaseUsers.child(user1.userId).setValue(user1);
}

@Override
public User GetUserFromFirebase(String uid) {
   databaseUsers.child(uid)
           .addListenerForSingleValueEvent(new ValueEventListener() {
               @Override
               public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                   user = dataSnapshot.getValue(User.class);
               }

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

               }
           });
    return user;
}

公共类DbContext实现IDbContext{

User user = null;
Context context;
DatabaseReference databaseUsers = FirebaseDatabase.getInstance().getReference("users");

public DbContext(Context context) {
    this.context = context;
}

@Override
public void AddUserAccount(User user1) {
    databaseUsers.child(user1.userId).setValue(user1);
}

@Override
public void GetUserFromFirebase(String uid, final Callback callback) {
    databaseUsers.child(uid)
            .addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                    user = dataSnapshot.getValue(User.class);
                    callback.myResponseCallback(user);
                }

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

                }
            });

}

我的编辑数据库服务：

public class DbService implements IDbService {

//instance of DbContext for firebase handling
private DbContext dbContext;

public DbService(Context context){
    super();
    dbContext = new DbContext(context);
}

@Override
public User SaveUserProfile(User u) {

    dbContext.AddUserAccount(u);

    return u;
}

@Override
public User GetUserFromUid(String uid) {
    User user = dbContext.GetUserFromFirebase(uid);
    return user;
}

public class DbService implements IDbService {

//instance of DbContext for firebase handling
public DbContext dbContext;

public DbService(Context context){
    super();
    dbContext = new DbContext(context);
}

@Override
public User SaveUserProfile(User u) {

    dbContext.AddUserAccount(u);

    return u;
}

@Override
public User GetUserFromUid(String uid) {
    final User[] user1 = {new User()};
    dbContext.GetUserFromFirebase(uid, new Callback() {
        @Override
        public void myResponseCallback(User user) {
            user1[0] = user;
        }
    });
    return user1[0];
}

我编辑的活动：

    @RequiresApi(api = Build.VERSION_CODES.KITKAT)
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_details);

    inputName = findViewById(R.id.nameTextView);
    inputEmail = findViewById(R.id.emailTextView);

    FirebaseDatabase mFirebaseInstance = FirebaseDatabase.getInstance();

    //reference to 'users' node
    mFirebaseDatabase = mFirebaseInstance.getReference("users");

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

    assert user != null;
    userId = user.getUid();


    getUserFromFirebase(userId);

}

public void getUserFromFirebase(String uid){
    userDetails = (new DbService(this)).GetUserFromUid(uid);
    inputEmail.setText(userDetails.email);
    inputName.setText(userDetails.name);
}

编辑前调试：

编辑后调试：

---

从调试中可以看到，在回调接口之前，userDetails为null。在接口实现之后，userDetails不是null，但是所有的对象值都为null。我不知道这是为什么，因为它们都填充在数据库中。有什么想法吗？

@a\u local\u谢谢。你能对我的代码更具体一点吗？我真的不知道rstand你的链接，对不起，我是一个初学者。我的例子为你提供了一个完整的教程，告诉你如何自己实现它，如果你在这一过程中遇到困难，欢迎你在这里用你已经尝试过的更新你的问题far@a_local_nobody我想我已经实现了你的解决方案。我已经在上面添加了我的编辑以及调试的一些屏幕截图。如您所见，userDetails对象正在填充，但所有值都为null。这些值在数据库中不是null。有什么想法吗？

返回user1[0]；

这也不起作用，因为您收到的值只通过回调模式返回给您，在

GetUserFromFirebase

内部，您将是唯一一个引用用户对象的地方，您基本上什么也不返回，这就是它看起来的样子