Android Designing and understanding how to retrieve realtime data from the Firebase Database


This example assumes that you have already set up a Firebase Realtime Database. If you are a starter, then please inform yourself here on how to add Firebase to your Android project.

First, add the dependency of the Firebase Database to the app level build.gradle file:

compile ''

Now, let us create a chat app which stores data into the Firebase Database.

Step 1: Create a class named Chat

Just create a class with some basic variables required for the chat:

public class Chat{
    public String name, message;

Step 2: Create some JSON data

For sending/retrieving data to/from the Firebase Database, you need to use JSON. Let us assume that some chats are already stored at the root level in the database. The data of these chats could look like as follows:

        "name":"John Doe",
        "message":"My first Message"
        "name":"John Doe",
        "message":"Second Message"
        "name":"John Doe",
        "message":"Third Message"

Step 3: Adding the listeners

There are three types of listeners. In the following example we are going to use the childEventListener:

DatabaseReference chatDb = FirebaseDatabase.getInstance().getReference() // Referencing the root of the database.
        .child("chats"); // Referencing the "chats" node under the root.

chatDb.addChildEventListener(new ChildEventListener() {
    public void onChildAdded(DataSnapshot dataSnapshot, String s) {
        // This function is called for every child id chat in this case, so using the above
        // example, this function is going to be called 3 times.
        // Retrieving the Chat object from this function is simple.
        Chat chat; // Create a null chat object.

        // Use the getValue function in the dataSnapshot and pass the object's class name to
        // which you want to convert and get data. In this case it is Chat.class.
        chat = dataSnapshot.getValue(Chat.class);

        // Now you can use this chat object and add it into an ArrayList or something like
        // that and show it in the recycler view.

    public void onChildChanged(DataSnapshot dataSnapshot, String s) {
        // This function is called when any of the node value is changed, dataSnapshot will
        // get the data with the key of the child, so you can swap the new value with the
        // old one in the ArrayList or something like that.

        // To get the key, use the .getKey() function.
        // To get the value, use code similar to the above one.

    public void onChildRemoved(DataSnapshot dataSnapshot) {
        // This function is called when any of the child node is removed. dataSnapshot will
        // get the data with the key of the child.

        // To get the key, use the s String parameter .

    public void onChildMoved(DataSnapshot dataSnapshot, String s) {
        // This function is called when any of the child nodes is moved to a different position.

        // To get the key, use the s String parameter.

    public void onCancelled(DatabaseError databaseError) {
        // If anything goes wrong, this function is going to be called.

        // You can get the exception by using databaseError.toException();

Step 4: Add data to the database

Just create a Chat class object and add the values as follows:

Chat chat=new Chat();"John Doe";
chat.message="First message from android";

Now get a reference to the chats node as done in the retrieving session:

DatabaseReference chatDb = FirebaseDatabase.getInstance().getReference().child("chats");

Before you start adding data, keep in mind that you need one more deep reference since a chat node has several more nodes and adding a new chat means adding a new node containing the chat details. We can generate a new and unique name of the node using the push() function on the DatabaseReference object, which will return another DatabaseReference, which in turn points to a newly formed node to insert the chat data.


// The parameter is the chat object that was newly created a few lines above.

The setValue() function will make sure that all of the application's onDataChanged functions are getting called (including the same device), which happens to be the attached listener of the "chats" node.