StackOverflow 46645568

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <RatingBar
        android:id="@+id/ratingBar"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:numStars="5"
        android:stepSize="0.5"/>

</LinearLayout>

MainActivity.java

package com.example.sandbox;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.widget.RatingBar;

import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

import com.example.sandbox.R;

public class FirebaseActivity extends AppCompatActivity {

    private FirebaseDatabase database;
    private RatingBar ratingBar;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        database = FirebaseDatabase.getInstance();

        ratingBar = (RatingBar) findViewById(R.id.ratingBar);

        testRatingBar();
    }

    private void testRatingBar() {
        final DatabaseReference ref = database.getReference("ratingbar").child("rating");

        ref.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                if (dataSnapshot != null && dataSnapshot.getValue() != null) {
                    float rating = Float.parseFloat(dataSnapshot.getValue().toString());
                    ratingBar.setRating(rating);
                }
            }

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

        ratingBar.setOnRatingBarChangeListener(new RatingBar.OnRatingBarChangeListener() {
            @Override
            public void onRatingChanged(RatingBar ratingBar, float rating, boolean fromUser) {
                if (fromUser) ref.setValue(rating);
            }
        });
    }

}