Mastering Firebase for Android Development

To develop Firebase applications, you can surely use your development environment; the essential requirements are as follows:

There are two ways that you can connect Firebase to your Android project, from the Firebase console creating the new project and adding the dependencies and Google config file. The other way is just to add Firebase support to an Android project directly from Android Studio:

Since we have connected to Firebase, it is as simple as it sounds, all we have to do is select the Add the Realtime Database to your application button in the Firebase assistance window, you will see a dialog for requesting your authorization to change the gradle file with a few dependencies. Tap on Accept Changes, now we are all set to explore Realtime Database abilities.

Essentially, a reference to the database is required. Every Firebase project has its own particular devoted Realtime Database items of which can be examined by opening the project inside the Firebase console and picking the Database option. Inside the console, panels can be selected to show data trees set away in the database, the rules outlined for fetching the access, database use estimations, and so on. 

Firebase databases are usually Representational State Transfer (REST) endpoint references, which we will use to add the data. We will understand how to fetch the reference with the following code snippet:

// fetch reference database
FirebaseDatabase mDatabase = FirebaseDatabase.getInstance();
DatabaseReference mDbRef = mDatabase.getReference("Donor/Name");

The preceding code will fetch the reference, on the off chance that the particular path does not exist now, it is composed automatically inside the tree when data is written at that location.

Fetch an instance of your database employing getInstance() and reference the location you need to write. You can write most of the primitive data types as they also include Java objects:

// Write a message to the database
FirebaseDatabase mDatabase = FirebaseDatabase.getInstance();
DatabaseReference mDbRef = mDatabase.getReference("Donor/Name");
mDbRef.setValue("Parinitha Krishna");

The following screenshot explains the dashboard changes after running the preceding code:

If you notice that there aren't any changes in the dashboard from the write operation, we shall attach an onFailure callback like the following for identifying what's stopping it:

// Write a message to the database
FirebaseDatabase mDatabase = FirebaseDatabase.getInstance();
DatabaseReference mDbRef = mDatabase.getReference("Donor/Name");
mDbRef.setValue("Parinitha Krishna").addOnFailureListener(new OnFailureListener() {
@Override
public void onFailure(@NonNull Exception e) {
        Log.d(TAG, e.getLocalizedMessage());
    }
});

After writing the data into Firebase now it's time to read what we have written. Firebase Realtime Database syncs all the data in real time across platforms and devices. So we have an onDatachanged() callback to read the data:

// Read from the database
mDbRef.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
// This method is called once with the initial value and again
        // whenever data at this location is updated.
String value = dataSnapshot.getValue(String.class);
        Log.d(TAG, "Value is: " + value);
    }

@Override
public void onCancelled(DatabaseError error) {
// Failed to read value
Log.w(TAG, "Failed to read value.", error.toException());
    }
});

Create a model class with constructors and declare a string to fetch the database reference for a unique key to add the list of objects. The model class is as follows: 

public class Users {

private String Name;
private String Email;
private String Phone;

public Users() {
    }

public String getName() {
return Name;
    }

public void setName(String name) {
Name = name;
    }

public String getEmail() {
return Email;
    }

public void setEmail(String email) {
Email = email;
    }

public String getPhone() {
return Phone;
    }

public void setPhone(String phone) {
Phone = phone;
    }

public Users(String name, String email, String phone) {
Name = name;
Email = email;
Phone = phone;
    }

}

Now in the activity class using the DatabaseReference class we can set the object value to Firebase, as follows:

public class MainActivity extends AppCompatActivity {

private static final String TAG = "MainActivity";
private FirebaseDatabase mDatabase;
private DatabaseReference mDbRef;
private String userId;

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

// Write a message to the database
mDatabase = FirebaseDatabase.getInstance();
mDbRef = mDatabase.getReference("Donor/Name");

//Setting firebase unique key for Hashmap list
String userId = mDbRef.push().getKey();
// creating user object
Users user = new Users("Hillary", "hillary@xyz.com", "90097863873", "Tokyo");

mDbRef.child(userId).setValue(user);

    }
}

The preceding code will add the object into Firebase as follows: 

To read the object data from Firebase Realtime ValueEventListner() whenever there is an update in the database in onDatachanged callback we can read the data changes:

mDbRef.child(userId).addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {

        Users user = dataSnapshot.getValue(Users.class);

        Log.d(TAG, "User name: " + user.getName() + ", email " + user.getEmail());
    }

@Override
public void onCancelled(DatabaseError error) {
// Failed to read value
Log.w(TAG, "Failed to read value.", error.toException());
    }
});

When the code is executed, it will result in fetching the data tree to your project. It is up to us how we make use of the data. 

Since we are using unique key mechanism the data will be added under the Name reference with a unique identifier: 

In Firebase Realtime Database to listen to the data changes, we have addValueEventListener for listening to the multiple nodes. In case you want to check the single value by adding addListenerForSingleValueEvent(), we can do that as well:

mDbRef = mDatabase.getReference("/Donor/name");


ValueEventListener changeListener = new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {

    }

@Override
public void onCancelled(DatabaseError databaseError) {

    }
};

mDbRef.addValueEventListener(changeListener);

When a listener is not required, it should be detached from the database reference object as follows:

mDbRef.removeEventListener(changeListener);

String title = mDataSnapshot.child("message1").child("title").getValue(String.class);

for (DataSnapshot child : mDataSnapshot.getChildren()) {
    Log.i(TAG, child.getKey());
    Log.i(TAG, child.getValue(String.class));
}

To update data, we can use the setValue() method by passing updated values. You can likewise utilize updateChildren() by passing the way to update data without exasperating other child nodes:

String newEmail = "ashokslsk@gmail.com";
mDbRef.child(userId).child("email").setValue(newEmail);

The following screenshot illustrates the updated value for the email field: 

By using the updateChildren() method of the database reference class, we can write the HashMap data structure into Firebase Realtime Database. Let's create a HashMap and add different key-value pairs, each should be reflected in the Realtime Database:

// Write a message to the database
mDatabase = FirebaseDatabase.getInstance();
mDbRef = mDatabase.getReference("Donor/Name");

//Writing Hashmap
Map<String, Object> mHashmap = new HashMap<>();

mHashmap.put("Name 1/title", "Ashok");
mHashmap.put("Name 1/content", "Parinitha");
mHashmap.put("Name 2/title", "Krishna");
mHashmap.put("Name 2/content", "Sumuthra");

mDbRef.updateChildren(mHashmap);

The following screenshot illustrates the HashMap writing in the Firebase console:

Lists are compelling data structures and they help in numerous use cases. Firebase has excellent support for HashMap and lists. Users can append the data according to the unique key from Firebase, or you can create your logic to create a unique identifier. Using the push() method a user can insert the data, and there are many ways to filter and match the data pushed. Let's see how the push() method helps in creating a list. As usual first grab the reference to the database and then using the push() method get the unique key. Using the push() method we can add a new child:

// Write a message to the database
mDatabase = FirebaseDatabase.getInstance();
mDbRef = mDatabase.getReference("Donor/Name");

//Setting firebase unique key for Hashmap list
String key = mDbRef.push().getKey();

mDbRef.child(key).setValue("First item");

Apart from allowing a database to create a list, it is also necessary to receive a data-changed notification from the list. This can be achieved through adding child event listeners. These listeners will notify the app when there is a new child added. We need to implement a couple of callbacks when we use this listener. Most commonly there is the onChildAdded() method when a child is added, and it sends a new data snapshot with data added. Note that onChildChanged() is called when there is an update to the existing node, and onChildRemoved() is called when a child node is removed. However onChildMoved() is called when any alterations change the list order:

ChildEventListener childListener = new ChildEventListener() {
@Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
    }
@Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
    }
@Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
    }
@Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
    }
@Override
public void onCancelled(DatabaseError databaseError) {
    }
};

mDbRef.addChildEventListener(childListener);

There are many ways to perform the query on the list. Firebase has a class named Query to access the database inside the application on specified criteria: 

Query mQuery = mDbRef.orderByKey();

ValueEventListener mQueryValueListener = new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
        Iterable<DataSnapshot> snapshotIterator = dataSnapshot.getChildren();
        Iterator<DataSnapshot> iterator = snapshotIterator.iterator();
while (iterator.hasNext()) {
            DataSnapshot next = (DataSnapshot) iterator.next();
            Log.i(TAG, "Value = " + next.child("name").getValue());
        }
    }
@Override
public void onCancelled(DatabaseError databaseError) {
    }
};

mQuery.addListenerForSingleValueEvent(mQueryValueListener);

To delete data, you can call the removeValue() method onto database reference. You can likewise pass null to the setValue() method, which does the same delete operation:

//Removes the entire child
mDbRef.child(userId).removeValue();
//Passing null to remove the calue
mDbRef.chile(userId).child("name").setValue(null);

// Similarly Hashmap can also be removed
Map<String, Object> mHashmap = new HashMap<>();

mHashmap.put("Name 1/title", null);
mHashmap.put("Name 1/content", null);
mHashmap.put("Name 2/title", null);
mHashmap.put("Name 2/content", null);

The following screenshot shows the Firebase console reaction for the delete operation:

//Offline support
FirebaseDatabase.getInstance().setPersistenceEnabled(true);

By default, Firebase sets the rules for users to authenticate before writing or reading operations. We can go to our project in Firebase console and choose the Database option in the left-hand options panel and go to the Rules tab in the Main panel. The default rules are as follows:

{
 "rules": {
 ".read": "auth != null",
 ".write": "auth != null"
 }
}

The following screenshot shows the default security configurations for any Realtime Database project:

Firebase offers a unique way to examine rules in the simulator. In the right-hand corner of the Firebase console's Main window, you will notice a blue button labelled as SIMULATOR, click on it to have a perceptible familiarity. Now we can see that there are two checkboxes and one input field to enter the URL and a toggle button indicating Authenticated. If you toggle it towards the right side, you will see a drop-down allowing you to choose the security provider, and it will also show UID and auth token payload. When we click on the RUN button, it will show the possible responses. 

The following screenshot shows the Simulator in a default state:

If we press the RUN button, the simulator assumes that the user continues not-authenticated, and it will return a Simulated read denied error. 

The following screenshot illustrates the not-authenticated user's state:

Since now we know what a simulator is, let's check what happens when we push the toggle button and choose a provider:

It will grant the access to the database. Likewise, we can run the test on the write operation and understand what happens behind the screen.

Typically, there are four types of database rules. These rules dictate different responsibilities:

Customizing database rules

Database rules are versatile and powerful and if we want to customize certain operations we can achieve it through rules. For instance, if we require giving access to read all the data and no write access to users, we can achieve this by using the following rules:

{
"rules": {
".read": true,
".write": false
}
}

The preceding rules will allow the user to have a read access to the database. Since there isn't any path specified, the complete database is readable, but not writable. If we want to customize on node basis we can take the node name into the rules and we can give the rules as follows. Test the rules in the simulator before publishing them:

{
"rules": {
".write" : false,
"Donor" : {
".read" : true
}
}
}

The following screenshot illustrates custom rules for read-write access to a particular node:

We can customize the rules to a level where we can specify which node needs to be read, which node can be written, which cannot be written, and a lot more.

Data security

Another outstanding feature of Firebase is security. Ensuring that no data is being given access to the unapproved or not-authenticated users. For this problem there is a variable named auth. It is a predefined variable within the database rules. It contains the auth provider, used auth ID, and token and user's UID. Using this we can restrict the database access and grant the application on a use case basis. Consider the following diagram for apprehending the security. There are blood donors and the details are helpful for having authentic donor information:

Consider the following rules that allow only authenticated users to read the data:

 {
"rules": {
".write" : false,
"Donor" : {
"$uid": {
".read": "auth != null && auth.uid == $uid"
}
}
}
}

The following screenshot shows the simulated authenticated user:

In previous examples, we have seen how to set the rules, how to use the predefined variables of rules, and so on. Now let's understand what custom variables are. Anything that has $ and the name that describes the node will be a variable that we can use in rule expressions. Say for instance, the value allocated to the UID will, obviously, rely upon which node is being read, since every user ID node will have a unique value doled out to it. The $ variable gives an approach to store the value to the key and reference rule expressions.

With the ability to create custom variables, Firebase has predefined variables that can be used in database rules in many use cases. The following is a list of predefined variables:

".write" : "data.child('Donor').child('Name').child('userid').child('email').exists()"

In this application, we will keep the user interface simple and informative. All we have is one RecyclerView and two buttons for adding and loading the data. The following xml layout code dictates the UI design: 

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/activity_main"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">

    <TextView
android:textColor="@color/colorPrimaryDark"
android:textStyle="bold"
android:textSize="25dp"
android:gravity="center"
android:text="Packt Blood Bank"
android:layout_width="match_parent"
android:layout_height="wrap_content" />
    <View
android:layout_marginLeft="50dp"
android:layout_marginRight="50dp"
android:layout_width="match_parent"
android:layout_height="5dp"
android:background="@color/colorAccent" />

The following code adds the vertical LinearLayout to support the DynamicData:

    <LinearLayout
android:layout_marginTop="10dp"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
        <LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_weight="1"
android:orientation="vertical">

            <android.support.v7.widget.RecyclerView
android:id="@+id/peopleList"
android:layout_width="match_parent"
android:layout_height="match_parent" />
        </LinearLayout>

The above code sets the dynamic lists that load the data from RecyclerView. lets continue updating the same layout to make the UI complete.  

        <View
android:layout_width="match_parent"
android:layout_height="5dp"
android:background="@color/colorPrimary" />
        <LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_weight="1"
android:orientation="vertical">

The following layout adds a scrollable layout for the input controls. 

            <ScrollView
android:scrollIndicators="right"
android:scrollbarStyle="insideOverlay"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:scrollbars="vertical">
                <LinearLayout
android:layout_width="match_parent"
android:layout_height="150px"
android:layout_marginTop="40px"
android:gravity="center"
android:orientation="vertical">
                    <LinearLayout
android:layout_width="match_parent"
android:layout_height="150px"
android:layout_marginTop="40px"
android:gravity="center"
android:orientation="horizontal">
                        <EditText
android:id="@+id/donorNameInput"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_weight="1"
android:hint="Full name"
android:textColor="#000000"
android:textSize="16dp" />
                        <EditText
android:id="@+id/donorCityInput"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_weight="1"
android:hint="City"
android:textColor="#000000" />
                    </LinearLayout>

The following code adds the donor blood group type and email address:

                    <LinearLayout
android:layout_width="match_parent"
android:layout_height="150px"
android:layout_marginTop="40px"
android:gravity="center"
android:orientation="horizontal">
                        <EditText
android:id="@+id/donorBloodGroupInput"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_weight="1"
android:hint="Blood Group"
android:textColor="#000000"
android:textSize="16dp" />
                    </LinearLayout>

                    <LinearLayout
android:layout_width="match_parent"
android:layout_height="150px"
android:layout_marginTop="40px"
android:gravity="center"
android:orientation="horizontal">

                        <EditText
android:id="@+id/donorEmailInput"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_weight="1"
android:hint="Email address"
android:textColor="#000000"
android:textSize="16dp" />

                    </LinearLayout>

The following code adds the buttons to act on the data received from the input fields:

                    <RelativeLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="#36FFFFFF">
                        <Button
android:id="@+id/loadBtn"
android:layout_width="500px"
android:layout_height="150px"
android:text="Load Donors info"
android:textColor="#000000"
android:textStyle="bold" />
                        <Button
android:id="@+id/addBtn"
android:layout_width="500px"
android:layout_height="150px"
android:layout_marginLeft="30px"
android:layout_toRightOf="@id/loadBtn"
android:text="Add Donor info"
android:textColor="#000000"
android:textStyle="bold" />
                    </RelativeLayout>
                </LinearLayout>
            </ScrollView>
        </LinearLayout>
    </LinearLayout>
</LinearLayout>

We also need each item layout for the RecyclerView, which is defined as follows. In this layout, all we have is four TextView:

<?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="wrap_content"
android:orientation="vertical">

    <TextView
android:id="@+id/donorName"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center_vertical"
android:padding="10px"
android:textColor="@color/colorPrimary"
android:textSize="25dp"
android:textStyle="bold" />

    <TextView
android:id="@+id/donorCity"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center_vertical"
android:padding="10px"
android:text="+216 54 821 200"
android:textSize="14dp"
android:textStyle="italic" />

    <TextView
android:id="@+id/donorBloodGroup"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center_vertical"
android:padding="10px"
android:text="+216 54 821 200"
android:textSize="14dp"
android:textStyle="italic" />

    <TextView
android:id="@+id/donorEmail"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center_vertical"
android:padding="10px"
android:text="+216 54 821 200"
android:textSize="14dp"
android:textStyle="italic" />

</LinearLayout>

Now that we have our user interface in place, let's dive deep into the Java part. Let's create a class called Donor. This class is a Plain Old Java Object (POJO) class that we will use throughout the application, and the POJO class dictates the structure of the data. 

The following POJO class expresses the idea and the data format that we will save in Firebase. Using the POJO class we will pass the data to the adapter:

package com.ashok.packt.realtime.database.model;

/**
 * Created by ashok.kumar on 20/10/17.
 */
public class Donor {

private String FullName;
private String Email;
private String City;
private String BloodGroup;
public Donor(){

    }

Now within the same class lets create a constructor for passing the data to the POJO:

public Donor(String fullName, String email, String city, String bloodGroup) {
FullName = fullName;
Email = email;
City = city;
BloodGroup = bloodGroup;
    }

public String getFullName() {
return FullName;
    }

public void setFullName(String fullName) {
FullName = fullName;
    }

public String getEmail() {
return Email;
    }

public void setEmail(String email) {
Email = email;
    }

public String getCity() {
return City;
    }

public void setCity(String city) {
City = city;
    }

public String getBloodGroup() {
return BloodGroup;
    }

public void setBloodGroup(String bloodGroup) {
BloodGroup = bloodGroup;
    }
}

Now let's write our Adapter class. The Adapter class requires POJO, view holder, and row layout. Consider spending some time on understanding the RecyclerView adapter:

package com.ashok.packt.realtime.database.adapter;

import android.content.Context;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import com.ashok.packt.realtime.database.R;
import com.ashok.packt.realtime.database.model.Donor;

import java.util.List;

/**
 * Created by ashok.kumar on 20/05/18.
 */

public class RecyclerViewAdapter extends RecyclerView.Adapter<RecyclerViewAdapter.View_Holder>{

private Context mContext;
private List<Donor> ItemList;

public RecyclerViewAdapter(Context mContext, List<Donor> itemList) {
this.mContext = mContext;
ItemList = itemList;
    }

The constructors require context and the list of donor object for setting the data in RecyclerView callbacks. 

@Override
public View_Holder onCreateViewHolder(ViewGroup parent, int viewType) {
        View itemView = LayoutInflater.from(parent.getContext())
                .inflate(R.layout.donor_list_row, parent, false);
return new View_Holder(itemView);
    }

The above Override method will be responsible for inflating the donor list item row. 

@Override
public void onBindViewHolder(View_Holder holder, int position) {

        Donor Item = ItemList.get(position);
        holder.Name.setText(Item.getFullName());
        holder.City.setText(Item.getCity());
        holder.BloodGroup.setText(Item.getBloodGroup());
        holder.Email.setText(Item.getEmail());

    }

@Override
public int getItemCount() {
return ItemList.size();
    }

public class View_Holder extends RecyclerView.ViewHolder {

        TextView Name;
        TextView City;
        TextView BloodGroup;
        TextView Phone;
        TextView Email;

        View_Holder(View itemView) {
super(itemView);

Name = (TextView) itemView.findViewById(R.id.donorName);
City = (TextView) itemView.findViewById(R.id.donorCity);
BloodGroup = (TextView) itemView.findViewById(R.id.donorBloodGroup);
Email = (TextView) itemView.findViewById(R.id.donorEmail);
        }
    }

}

Now, MainActivity holds the complete logic for the application by adding the data to Firebase, fetching the data from Firebase, and loading that in RecyclerView. I have also written methods to update, find, and delete for your future reference:

package com.ashok.packt.realtime.database;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.View;
import android.widget.EditText;

import com.ashok.packt.realtime.database.adapter.RecyclerViewAdapter;
import com.ashok.packt.realtime.database.model.Donor;
import com.google.firebase.database.ChildEventListener;
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.Query;
import com.google.firebase.database.ValueEventListener;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class MainActivity extends AppCompatActivity {

private DatabaseReference myDatabaseReference;
private String personId;
private List<Donor> ItemList;
private RecyclerView mRecyclerview;
private RecyclerViewAdapter mAdapter;

Lets initialise all the above code in the onCreate method as shown below:

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

mRecyclerview = (RecyclerView) findViewById(R.id.peopleList);
        RecyclerView.LayoutManager mLayoutManager = new LinearLayoutManager(this);
mRecyclerview.setLayoutManager(mLayoutManager);

// for data persistence
FirebaseDatabase.getInstance().setPersistenceEnabled(true);
myDatabaseReference=FirebaseDatabase.getInstance().getReference("Donor");
personId= myDatabaseReference.push().getKey();


        (findViewById(R.id.addBtn)).setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {

                String FullName = ((EditText)findViewById(R.id.donorNameInput)).getText().toString();
                String Email = ((EditText)findViewById(R.id.donorEmailInput)).getText().toString();
                String City = ((EditText)findViewById(R.id.donorCityInput)).getText().toString();
                String BloodGroup = ((EditText)findViewById(R.id.donorBloodGroupInput)).getText().toString();


                addPerson(FullName,Email, City, BloodGroup);
            }
        });


        (findViewById(R.id.loadBtn)).setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
                readData();
            }
        });
    }

After adding the views its now time to work on the adding and retrieving the data as shown below:

private void addPerson(String name, String Email, String city, String Bloodgroup){
personId= myDatabaseReference.push().getKey();
Donor person = new Donor(name, Email, city, Bloodgroup);
myDatabaseReference.child(personId).setValue(person);
}

private void updatePerson(String name,int phoneNumber){
myDatabaseReference.child(personId).child("fullName").setValue(name);
myDatabaseReference.child(personId).child("phoneNumber").setValue(phoneNumber);
 }

private void removePerson(String name){
myDatabaseReference.child(personId).removeValue();
 }
private void readData(){
ItemList = new ArrayList<>();
myDatabaseReference.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
 Iterable<DataSnapshot> snapshotIterator = dataSnapshot.getChildren();
 Iterator<DataSnapshot> iterator = snapshotIterator.iterator();
while((iterator.hasNext())){
 Donor donor = iterator.next().getValue(Donor.class);
ItemList.add(donor);
mAdapter.notifyDataSetChanged();
 }
 }

@Override
public void onCancelled(DatabaseError databaseError) {

 }
 });

mAdapter = new RecyclerViewAdapter(this, ItemList);
mRecyclerview.setAdapter(mAdapter);

 }

We also can do the specific person search as shown below:

private void findPerson(String name){
 Query deleteQuery = myDatabaseReference.orderByChild("fullName").equalTo(name);
 deleteQuery.addChildEventListener(new ChildEventListener() {
@Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
 Iterable<DataSnapshot> snapshotIterator = dataSnapshot.getChildren();
 Iterator<DataSnapshot> iterator = snapshotIterator.iterator();
while((iterator.hasNext())){
 Log.d("Item found: ",iterator.next().getValue().toString()+"---");
 }
 }

@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) {
 Log.d("Item not found: ","this item is not in the list");
 }
 });
 }
}

When you compile and run the program in your Android device the output will have the following look and feel: