AndroidManifest.xml

<!--    adding Internet permissions-->
    <uses-permission android:name="android.permission.INTERNET"/>

gradle.build (project)

// Top-level build file where you can add configuration options common to all sub-projects/modules.
buildscript {
    repositories {
        google()
        jcenter()
    }
    dependencies {
        // always use latest version of dependencies
        classpath "com.android.tools.build:gradle:4.1.3"
        classpath 'com.google.gms:google-services:4.3.10'
        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

gradle.build (app)

plugins {
    id 'com.android.application'
    id 'com.google.gms.google-services'
}

android {
    compileSdkVersion 32
    buildToolsVersion "30.0.3"

    defaultConfig {
        applicationId "com.learnoset.firebasetutorials"
        minSdkVersion 19
        targetSdkVersion 32
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
}

dependencies {

    implementation 'androidx.appcompat:appcompat:1.4.0'
    implementation 'com.google.android.material:material:1.4.0'
    implementation 'androidx.constraintlayout:constraintlayout:2.1.2'
    testImplementation 'junit:junit:4.+'

    // Firebase Bom Dependency
    implementation platform('com.google.firebase:firebase-bom:29.0.3')

    // Firebase Realtime Dependency
    implementation 'com.google.firebase:firebase-database'

    androidTestImplementation 'androidx.test.ext:junit:1.1.3'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.4.0'
}

activity_main.xml

<?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:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center"
        android:text="Register"
        android:textSize="20sp"
        android:textStyle="bold" />

    <EditText
        android:id="@+id/fullnameET"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="FullName" />

    <EditText
        android:id="@+id/mobileET"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Mobile" />

    <EditText
        android:id="@+id/emailET"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Email" />

    <androidx.appcompat.widget.AppCompatButton
        android:id="@+id/registerBtn"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        android:text="Register Now" />

</LinearLayout>

MainActivity.java

import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.AppCompatButton;

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;

public class MainActivity extends AppCompatActivity {

    private final DatabaseReference databaseReference = FirebaseDatabase.getInstance().getReference();

    // to get available / registered users in Firebase Database
    private int availableUsers = 0;

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

        // getting TextViews from xml file
        final EditText fullnameET = findViewById(R.id.fullnameET);
        final EditText emailET = findViewById(R.id.emailET);
        final EditText mobileET = findViewById(R.id.mobileET);

        final AppCompatButton registerBtn = findViewById(R.id.registerBtn);

        // getting available users from Firebase Database
        databaseReference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {

                // check if database has users as root
                if (snapshot.hasChild("users")) {

                    // getting available / registerd users
                    availableUsers = (int) snapshot.child("users").getChildrenCount();
                } else {
                    // if there is no available user in the Firebase Database then availableUsers remain 0
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        });

        // register new user
        registerBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                // getting strings from EditTexts
                final String fullnameTxt = fullnameET.getText().toString();
                final String emailTxt = emailET.getText().toString();
                final String mobileTxt = mobileET.getText().toString();

                // sending data to firebase into users root
                // availableUsers + 1 increasing user index to save new user details in it
                databaseReference.child("users").child(String.valueOf(availableUsers + 1)).child("fullname").setValue(fullnameTxt);// adding fullname
                databaseReference.child("users").child(String.valueOf(availableUsers + 1)).child("email").setValue(emailTxt); // adding email
                databaseReference.child("users").child(String.valueOf(availableUsers + 1)).child("mobile").setValue(mobileTxt); //  adding mobile

                Toast.makeText(MainActivity.this, "Data Added", Toast.LENGTH_SHORT).show();

            }
        });
    }
}