Develop Reference

Firebase Database Android - Retrieve Information Without ValueEventListeners

I have users on my Application and I store additional information about them in Firebase Database. I need to retrieve additional information in more than one Activity. I do not want to use ValueEventListeners because they are not called unless there is any change in the database. How can I get information about users from Database without using ValueEventListeners?
In my ProfileFragment I need to get name and departmant values.
I get current user from Firebase and I tried to take other information with a function.
talker = new DatabaseTalk();
FirebaseUser currentUser = mAuth.getCurrentUser();
// Get info of logged in user with talker.
loggedInUser = talker.getUserFromID(currentUser.getUid());
This is my DatabaseTalker class to handle read and write operations to database
public class DatabaseTalk {
private FirebaseDatabase mDatabase;
private DatabaseReference UserRef;
private DatabaseReference SurveyRef;
private List<User> userList;
public DatabaseTalk(){
mDatabase = FirebaseDatabase.getInstance();
UserRef = mDatabase.getReference("users");
UserRef.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for(DataSnapshot child: dataSnapshot.getChildren()){
userList.add(child.getValue(User.class));
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
Log.w("Error", "Failed to read value.", databaseError.toException());
}
});
SurveyRef = mDatabase.getReference("surveys");
}
public void WriteUser(User usr){
UserRef.child(usr.getUserID()).setValue(usr);
}
public void WriteSurvey(Survey survey){SurveyRef.push().setValue(survey);}
public User getUserFromID(String id){
for(User usr: userList){
if(usr.getUserID().equals(id))
return usr;
}
return null;
}
}
I think, I can take additional information about users from userList in DatabaseTalk but userList is null always.
EDIT
I changed getUserFromID method. OnDataChange() does not work when I called getUserFromID method.
public User getUserFromID(String id){
DatabaseReference newRef = mDatabase.getReference("users");
DatabaseReference ds = newRef.child(id);
ds.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
userList.add(dataSnapshot.getValue(User.class));
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
return userList.get(0);
}

I solved the problem. It turned out I do not know how Firebase works actually. Since onDataChange() make async calls, writing Listener definitions on a function is useless because onDataChange mostly does not trigger before function terminates and this cause function to return null value.
I make the definition of ValueEventListeners in onCreate methods. It triggers now after few seconds my ProfileFragment created. I think it is better to use Progress Dialogs to wait.
Thanks to everyone who interested in the question.

Firebase user details are not retrieved in Android

I am storing user details 'firstname' and 'lastname' in UserNode. But when i want to retrieve that details then no data is being retrieved. I tried almost all solutions on the internet but nothing solved my problem. Here is my code for retrieving data of the current user:
FirebaseUser userr = FirebaseAuth.getInstance().getCurrentUser();
if (userr != null) {
String name = userr.getDisplayName();
Log.e("value", name);
}
but it says "println needs a message"
I also tried with this but nothing happened:
DatabaseReference DataRef;
DataRef = FirebaseDatabase.getInstance().getReference().child("UserNode");
DataRef.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
String acctname = (String)dataSnapshot.child("firstname").getValue();
Log.e("name", acctname);
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
]1
Please help me I am stuck with it

You're reading a collection of user with a ValueEventListener. As the [Firebase documentation for reading lists with a value event](Listen for value events) explains:
While using a ChildEventListener is the recommended way to read lists of data, there are situations where attaching a ValueEventListener to a list reference is useful.
Attaching a ValueEventListener to a list of data will return the entire list of data as a single DataSnapshot, which you can then loop over to access individual children.
Even when there is only a single [child node], the snapshot is still a list; it just contains a single item. To access the item, you need to loop over the result.
So in your code:
DatabaseReference DataRef;
DataRef = FirebaseDatabase.getInstance().getReference().child("UserNode");
DataRef.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot childSnapshot: dataSnapshot.getChildren()) {
String acctname = (String)childSnapshot.child("firstname").getValue();
Log.i("name", acctname);
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
throw databaseError.toException(); // don't ignore errors
}
});

Using FirebaseUser:
FirebaseUser implements UserInfo and in UserInfo's getDisplayName() documentation says
Returns the user's display name, if available.
So, it is possible that FirebaseUser.getDisplayName() return null when display name is not set. In that case Log.e() receives null as message and therefore prints println needs a message
Using your own structure:
Instead of using type conversion use getValue(Class<T>) like so:
String acctname = dataSnapshot.child("firstname").getValue(String.class);

Please, read how to retrieve data from firebase. I think you have a problem because you don't have Class Model.
Your steps:
Create model UserModel with firstname and lastname field
Use listener (example from docs):
// Attach a listener to read the data at our posts reference
ref.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
Post post = dataSnapshot.getValue(Post.class);
System.out.println(post);
}
#Override
public void onCancelled(DatabaseError databaseError) {
System.out.println("The read failed: " + databaseError.getCode());
}
});
See other answers: How to retrieve data from one single userID Firebase Android and retrieving data from firebase android

Get element by key with Firebase

I've been trying to retrieve an element from my Firebase database using its key. I have a class User and users are present in database.
I want to retrieve an object user using its key with this method :
public User getConnectedUserByUId(final String uid){
DatabaseReference database = FirebaseDatabase.getInstance().getReference();
DatabaseReference ref = database.child("users");
final List<User> connectedUser= new ArrayList<User>();
ref.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot item: dataSnapshot.getChildren()) {
if (item.getKey()==uid)
{
User user= dataSnapshot.getValue(User.class);
connectedUser.add(user);
}
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
return connectedUser.get(0);
}
but it returns an empty list every time.

The issue is here:
if (item.getKey()==uid)
since you are comparing 2 String in java you have to use the method
string.equals(Object other) not the == operator.
Moreover, since you know the key of the data in Firebase you can use it to get the reference without cycling all children.
Something like:
DatabaseReference database = FirebaseDatabase.getInstance().getReference();
DatabaseReference ref = database.child("users").child(uid);

Here you try to check a very specific ID only on changed data. Instead, try using a Firebase Query with filterByKey and not using your own function to achieve that. Here's sample code that I would use to try to replace your function:
DatabaseReference database = FirebaseDatabase.getInstance().getReference();
DatabaseReference ref = database.child("users");
Query connectedUser = ref.equalTo(uid);
connectedUser.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot postSnapshot: dataSnapshot.getChildren()) {
// TODO: handle the post here
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
// Getting Post failed, log a message
Log.w(TAG, "loadPost:onCancelled", databaseError.toException());
// ...
}
});
As specified in the Firebase documentation here: https://firebase.google.com/docs/database/android/lists-of-data#filtering_data

in the line : User user= dataSnapshot.getValue(User.class);
you have to put : User user= item.getValue(User.class);
and you have to check the id after you get the user:
if (user.getKey()==uid){
connectedUser.add(user);
}

There are 2 mistakes and a minor issue:
you are using == to compare two String objects. In java, this is true only if they are the same reference. Use equals instead.
addValueEventListener only adds a listener that gets invoked once after you add it and then every time something changes in the value you are listening to: this is an asynchronous behaviour. You are trying to get data synchronously instead. Please read something about this.
you are fetching useless data: you only need an object but you are fetching tons of them. Please consider to use the closest reference you can to the data you are fetching.
So, in conclusion, here's some code. I'd like to point out right now that forcing synchronous acquisition of naturaly asynchronous data is a bad practice. Nevertheless, here's a solution:
public User getConnectedUserByUId(final String uid){
DatabaseReference database = FirebaseDatabase.getInstance().getReference();
DatabaseReference ref = database.child("users").child(uid);
Semaphore sem = new Semaphore(0);
User[] array = new User[1];
ref.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot item: dataSnapshot.getChildren()) {
if (item.getKey()==uid)
{
User user= dataSnapshot.getValue(User.class);
array[0] = user;
}
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
try
{
sem.tryAcquire(10, TimeUnit.SECONDS);
}
catch (Exception ignored)
{
}
return array[0];
}
EDIT: I've just seen that this post is very old. I'm not sure how I ended up here.

In Firebase Table structure, how the retrieve the items from the subparent?

Below i have attached the firebase table... here i have to list the child of date (chat2,chat1 from 1312206 followed by chat3,chat2,chat1 from 12122016).
Please anyone help how to query the data.Thanks in advance.

You need to point to the correct node and then just attach a listener where you will get the callback.
ValueEventListener eventListener = new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
Chat chat = dataSnapshot.getValue(Chat.class);
// do stuff
}
#Override
public void onCancelled(DatabaseError databaseError) {
// do stuff when there is an error
}
}
DatabaseReference database = FirebaseDatabase.getInstance().getReference();
DatabaseReference chatNode = database.child("chat_room").child("12122016").child("chat1")
chatNode.addValueEventListener(eventListener);
With this listener you'll be notified whenever there is an update on that node.
You can find more information in Firebase documentation.

Getting the relevant key from ValueEventListener

I'm building an app that is basically a table for soccer that updates every time I change it in my DB.
I'm using Firebase in this way:
I'm trying to use an addValueEventListener, but I don't know which one of the values is the one that changes (dif,score,points...) therefore I don't which field in my table I should update.
public class table extends AppCompatActivity {
Button insert;
TextView name, games, win, lost, tie, score, dif, points;
Team A;
DatabaseReference mRootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference FootballRef = mRootRef.child("Football");
DatabaseReference HouseARef = FootballRef.child("HouseA");
protected void onStart() {
super.onStart();
ValueEventListener postListener = new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
// Get Post object and use the values to update the UI
String key = dataSnapshot.getKey() ;
Toast.makeText(table.this,key, Toast.LENGTH_SHORT).show();
// ...
}
#Override
public void onCancelled(DatabaseError databaseError) {
// Getting Post failed, log a message
// ...
}
};
HouseARef.addValueEventListener(postListener);
}
}
See here, when let's say I'm updating the name of housaA, I want to get back the key that got updated (in my case, name), but when I'm using
dataSnapshot.getKey() the Toast I'm getting back is HouseA and not name.

You won't be able to do this using a valuelistener.
Instead put a childeventlistener on the houseA.
Now inside onChildChanged, you should get the child node under houseA which has changed as a datasnapshot and from this snapshot, you can get the key which has changed