为什么当我从 Firebase 获取数据时 return NULL？

Question

我正在尝试使用电子邮件搜索从 firebase 检索用户数据以查找用户 ID 并使用该 ID 获取用户的节点子节点，所以我使用该代码来执行此操作：

    mSignin.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            mEmailStr = SignupActivityFragment.removeSpaces(mEmail.getText().toString());
            mPasswordStr = mPassword.getText().toString();

            mAuth.signInWithEmailAndPassword(mEmailStr, mPasswordStr).addOnCompleteListener(getActivity(), new OnCompleteListener<AuthResult>() {
                @Override
                public void onComplete(Task<AuthResult> task) {
                    if (task.isSuccessful()){
                        getExistingUserInfo();
                        if(User!=null) {
                            sglstr = User.get("sgl");
                            if (sglstr.equals("true")) {
                                Intent intent = new Intent(getActivity(), ControllerSGL.class);
                                intent.putExtra("user", User);
                                startActivity(intent);
                            } else if (sglstr.equals("false")) {
                                Intent intent = new Intent(getActivity(), ControllerStudent.class);
                                intent.putExtra("user", User);
                                startActivity(intent);
                            }

                        }
                        else {
                            Toast.makeText(getActivity(), "NULL!!!!!!", Toast.LENGTH_SHORT).show();
                        }
                        Toast.makeText(getActivity(), "Success", Toast.LENGTH_SHORT).show();

                    }else {
                        Toast.makeText(getActivity(), "Not Success", Toast.LENGTH_SHORT).show();
                    }

                }
            });

        }
    });

和getExistingUserInfo方法

public void getExistingUserInfo(){

    FirebaseDatabase database = FirebaseDatabase.getInstance();
    database.getReference().child("allUsers").orderByChild("emailAddress")
            .equalTo(mEmailStr)
            .addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    for (DataSnapshot childSnapshot : dataSnapshot.getChildren()) {
                        uid = childSnapshot.getKey();
                        Log.v("HimaAbosalem",uid);

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

        }
    });
    database.getReference("allUsers").child(uid).addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            //get user data from dataSnapshot
            User=(HashMap<String,String>)dataSnapshot.getValue();
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

我需要使用 HashMap 发送数据抛出意图，但我在 getExistingUserInfo 方法的这一部分给了我 java.lang.NullPointerException: Can't pass null for argument 'pathString' in child()

  database.getReference("allUsers").child(uid).addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            //get user data from dataSnapshot
            User=(HashMap<String,String>)dataSnapshot.getValue();
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

我认为问题是 uid 值等于 null 但我创建了一个日志但事实并非如此，我试图将该 uid 发送给另一个 activity 并且我收到它发现它不是 null 但是当我试图在那个新的 Activity 中获取数据时，它给出了同样的错误！！！

Answer 1

我通过将 getExistingUserInfo 分成两个单独的函数并调用在 onDataChange 处获取 allUsers 的函数来保证用户 ID returns 来解决这个问题这是编辑后的代码：

public void getExistingUserInfo(){

FirebaseDatabase database = FirebaseDatabase.getInstance();
database.getReference().child("allUsers").orderByChild("emailAddress")
        .equalTo(mEmailStr)
        .addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                for (DataSnapshot childSnapshot : dataSnapshot.getChildren()) {
                    uid = childSnapshot.getKey();
                    Log.v("HimaAbosalem",uid);

                }
                getUser(uid);
            }
    @Override
    public void onCancelled(DatabaseError databaseError) {

    }
});
} 
 public void getUser(String uid){
  database.getReference("allUsers").child(uid).addListenerForSingleValueEvent(new  ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        //get user data from dataSnapshot
        User=(HashMap<String,String>)dataSnapshot.getValue();
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {

    }
});
}

为什么当我从 Firebase 获取数据时 return NULL？

Why When i get the data From Firebase it return NULL?

android

firebase

firebase-authentication

firebase-realtime-database