implementation 'com.google.firebase:firebase-analytics:17.2.1'
implementation 'com.google.firebase:firebase-database:19.2.0'
implementation 'com.google.firebase:firebase-messaging:18.0.0'
implementation 'com.squareup.okhttp3:okhttp:3.4.1'

classpath "com.android.tools.build:gradle:4.0.1"

//make a Database Reference object first
DatabaseReference databaseReference;
//following will create a sub parent in your database of name Hello
//Note:- This parent will be only visible if it has a child or value
databaseReference= FirebaseDatabase.getInstance().getReference("Hello");
//this will create a child named child1 inside Hello with the message as msg
databaseReference.child("child1").setValue("msg");
//by using push() multiple messages can be added
databaseReference.push().child("child1").setValue("msg");

//following can be used for retrieving values from the database and this will also work whenever the
//values are changed on the database
ValueEventListener postListener = new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            //data is returned in the form of hashmap 
            //iterating the hasmap will give the desired data
            HashMap hashMap = (HashMap) dataSnapshot.getValue();
            System.out.println(hashMap);
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            // Error...
        }
    };
    databaseReference.addValueEventListener(postListener);