Firebase 3 (Getting Location)

To access a value in your database, you create a DatabaseReference for that location. Here are three references to locations in your database:

DatabaseReference zonesRef = FirebaseDatabase.getInstance().getReference("ZONES");
DatabaseReference zone1Ref = zonesRef.child("ZONE_1");
DatabaseReference zone1NameRef = zone1Ref.child("ZNAME");

In this snippet: 

• zonesRef points to /ZONES
• zone1Ref points to /ZONES/ZONE_1
• zone1NameRef points to /ZONES/ZONE_1/ZNAME

https://stackoverflow.com/questions/43293935/how-to-get-child-of-child-value-from-firebase-in-android

getValue();
DataSnapshot.getChildren();

To get value from Database on Button click:

You can use addValueEventListener() to retrieve data when an event is triggered.

In your onClick() method you can write :

DatabaseReference mDatabase = FirebaseDatabase.getInstance().getReference();

// In the database whenever any data is changed then the below code snipped is going to be executed.
mDatabase.child("<dbLocationTag>").addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {

                // the changed value is going to be stored into the dataSnapshot
                // to retrieve value from dataSnapshot we write

                String value = dataSnapshot.getValue(String.class);
                mValueView.setText(value);

                // NOTE: if you've multiple childs of same node then you can use Map to display all of them : EG: 
                /*Map<String, String> map = dataSnapshot.getValue(Map.class);
                String val1 = map.get("val1");
                String val2 = map.get("val2");
                String val3 = map.get("val3");
                Log.v("TAG", "Value 1 "+val1);
                Log.v("TAG", "Value 2 "+val2);
                Log.v("TAG", "Value 3 "+val3);
*/
            }

            @Override
            public void onCancelled(FirebaseError firebaseError) {

                mValueView.setText(firebaseError.toString());

            }
        });

for ValueEventListener which update all data, you need to run through all the snapshot to find matching

                for (DataSnapshot childSnapshot: dataSnapshot.getChildren()) {

https://stackoverflow.com/questions/40184797/retrieve-data-from-firebase-database-on-button-click

https://stackoverflow.com/questions/44811526/datasnapshot-has-the-object-but-getvalue-will-return-null

DataSnapshot.getKey();

https://stackoverflow.com/questions/38232140/how-to-get-the-key-from-the-value-in-firebase