How to Extract a List of Objects from Firebase Datasnapshot on Android

How to retrieve specific list of data from firebase

If you using addValueEventListener for retrieve all university from Firebase

List<University> universityList = new ArrayList<>();
myRef.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot snapshot) {
        universityList.clear();
        for (DataSnapshot postSnapshot: snapshot.getChildren()) {
            University university = postSnapshot.getValue(University.class);
            universityList.add(university);

            // here you can access to name property like university.name

        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        System.out.println("The read failed: " + firebaseError.getMessage());
    }
});

Or if you using addChildEventListener

List<University> universityList = new ArrayList<>();
myRef.addChildEventListener(new ChildEventListener() {
    @Override
    public void onChildAdded(DataSnapshot dataSnapshot, String s) {
         University university = dataSnapshot.getValue(University.class);
         universityList.add(university);
         Log.i(TAG,"add university name = " + university.name);
         ...
    }
    ...
}

Extracting Individual Values From Firebase DataSnapshot

If you already have a snapshot, you can address children with child():

public void onDataChange(DataSnapshot snapshot) {
    System.out.println(snapshot.getKey()); // Food
    System.out.println(snapshot.child("-key1").getValue()); // true
}

You can also loop over the children:

public void onDataChange(DataSnapshot snapshot) {
    for (DataSnapshot child: snapshot.getChildren()) {
        System.out.println(child.getKey()); // "-key1", "-key2", etc
        System.out.println(child.getValue()); // true, true, etc
    }
}

How to retrieve all the user details from database in firebase

Your question isn't very clear. By user data, if you mean the user object of the other users, then simply put, you can not obtain the user object of the other users regardless of their sign-in method. You can only obtain your own user object by the FirebaseAuth.getInstance().getCurrentUser() method.

Having said that, if the data of the users is stored in the realtime database, you can obtain it in the same way that you obtain data from other nodes. Please note, though, that the privacy settings of your app will determine if you can read other users' data or not.

It would help if you show your database structure in the question.

Edit After Viewing Database Structure :-

Alright, from what I understand, this is quite a basic problem. What you need is a childEventListener to retrieve all the children ( in your case - users ) of the trace-5fa8c node.

The code for that is as follows :-

DatabaseReference databaseReference = FirebaseDatabase.getInstance().getReference().child("trace-5fa8c");

databaseReference.addChildEventListener(new ChildEventListener() {
@Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
            userinfo  user = dataSnapshot.getValue(userinfo.class);

            Log.d(TAG, "showData: name: " + user.getName());
            Log.d(TAG, "showData: Mobile: " + user.getMob());
            Log.d(TAG, "showData: Vehicle: " + user.getVehicle());
            Log.d(TAG, "showData: Address: " + user.getaddress());

            //Do whatever further processing you need
        }

        @Override
        public void onChildChanged(DataSnapshot dataSnapshot, String s) {
        }

        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) {

        }

        @Override
        public void onChildMoved(DataSnapshot dataSnapshot, String s) {

        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    };

Also, even though this is not a part of your question, I'd like to add that with each onChildAdded(datasnapshot), you can get a userInfo pojo. So, in your showData() method, instead of creating a new Arraylist each time, maintain a single Arraylist<UserInfo> and keep adding the child pojo to it each time a child is added. This ArrayList object will be the dataset for your ListView object.

How to get all child's data in firebase database?

First retrieve your users datasnapshot.

//Get datasnapshot at your "users" root node
    DatabaseReference ref = FirebaseDatabase.getInstance().getReference().child("users");
    ref.addListenerForSingleValueEvent(
            new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    //Get map of users in datasnapshot
                    collectPhoneNumbers((Map<String,Object>) dataSnapshot.getValue());
                }

                @Override
                public void onCancelled(DatabaseError databaseError) {
                    //handle databaseError
                }
            });

Then loop through users, accessing their map and collecting the phone field.

private void collectPhoneNumbers(Map<String,Object> users) {

    ArrayList<Long> phoneNumbers = new ArrayList<>();

    //iterate through each user, ignoring their UID
    for (Map.Entry<String, Object> entry : users.entrySet()){

        //Get user map
        Map singleUser = (Map) entry.getValue();
        //Get phone field and append to list
        phoneNumbers.add((Long) singleUser.get("phone"));
    }

    System.out.println(phoneNumbers.toString());
}

This listener will only retrieve the datasnapshot when explicitly called. Consider storing a list of numbers under an "allPhoneNumbers" node in addition to your users node. This will make your datasnapshots lighter and easier to process when collecting all numbers. If you have say hundreds of users, the "users" datasnapshot will be way too big and the "allPhoneNumbers" list will be far more efficient.

The above code was tested on your sample database and does work. However, your phone field may need to be casted to String or int depending on your user's phone instance field's type.

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