尝试从Firebase检索数据，但得到null

Question

我试图从我的firebase数据库中检索数据，并在文本字段中打印一条语句，但我一直得到空值。我正在使用userid尝试到达我想要的子节点。我的textfield一直在说“欢迎null登录为null”我的Firebase数据库

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_welcome);
        final String uid;
        FirebaseUser user;


        user = FirebaseAuth.getInstance().getCurrentUser();
        uid = user.getUid();

        DatabaseReference reference = 
      FirebaseDatabase.getInstance().getReference("User");


         reference.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                fName = dataSnapshot.child(uid).child("fName").getValue(String.class);
                role = dataSnapshot.child(uid).child("role").getValue(String.class);

            }
            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });

        welcomeText = (TextView) findViewById(R.id.welcomeText);
        welcomeText.setText("Welcome " + fName + "! You are logged-in as " + role);

Answer 1

将以下行移动到onDataChange（）回调方法中。

welcomeText = (TextView) findViewById(R.id.welcomeText);
welcomeText.setText("Welcome " + fName + "! You are logged-in as " + role);