very simple example of usage firebase android

different files.java

build.gradle.kts (Project):

plugins {
    alias(libs.plugins.android.application) apply false
    alias(libs.plugins.google.gms.google.services) apply false
}

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build.gradle.kts (Module):

plugins {
    alias(libs.plugins.android.application)
    alias(libs.plugins.google.gms.google.services) // !!!
}

android {
    namespace = "site.sunmeat.helloworld"
    compileSdk = 36

    defaultConfig {
        applicationId = "site.sunmeat.helloworld"
        minSdk = 33
        targetSdk = 36
        versionCode = 1
        versionName = "1.0"
    }

    buildTypes {
        release {
            isMinifyEnabled = false
            proguardFiles(getDefaultProguardFile("proguard-android-optimize.txt"), "proguard-rules.pro")
        }
    }

    compileOptions {
        sourceCompatibility = JavaVersion.VERSION_17
        targetCompatibility = JavaVersion.VERSION_17
    }
}

dependencies {
    implementation(libs.appcompat)
    implementation(libs.activity)
    implementation(libs.constraintlayout)
    implementation(libs.material)
    implementation(libs.firebase.database) // !!!
    implementation(libs.firebase.firestore)
    implementation(libs.firebase.core)
}

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libs.versions.toml:

[versions]
agp = "8.13.1"
firebaseCore = "21.1.1"
firebaseFirestore = "26.0.2"
appcompat = "1.7.1"
material = "1.13.0"
activity = "1.12.0"
constraintlayout = "2.2.1"
googleGmsGoogleServices = "4.4.4"
firebaseDatabase = "22.0.1"

[libraries]
firebase-core = { module = "com.google.firebase:firebase-core", version.ref = "firebaseCore" }
firebase-firestore = { module = "com.google.firebase:firebase-firestore", version.ref = "firebaseFirestore" }
appcompat = { group = "androidx.appcompat", name = "appcompat", version.ref = "appcompat" }
material = { group = "com.google.android.material", name = "material", version.ref = "material" }
activity = { group = "androidx.activity", name = "activity", version.ref = "activity" }
constraintlayout = { group = "androidx.constraintlayout", name = "constraintlayout", version.ref = "constraintlayout" }
firebase-database = { group = "com.google.firebase", name = "firebase-database", version.ref = "firebaseDatabase" }

[plugins]
android-application = { id = "com.android.application", version.ref = "agp" }
google-gms-google-services = { id = "com.google.gms.google-services", version.ref = "googleGmsGoogleServices" }

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AndroidManifest.xml:

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

    <application
        android:allowBackup="true"
        android:dataExtractionRules="@xml/data_extraction_rules"
        android:fullBackupContent="@xml/backup_rules"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.AppCompat.DayNight.DarkActionBar">

        <activity
            android:name=".MainActivity"
            android:exported="true">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
</manifest>

==========================================================================================================

MAinActivity.java:

package site.sunmeat.helloworld;

import android.os.Bundle;
import android.util.Log;
import android.widget.*;
import androidx.appcompat.app.AppCompatActivity;
import com.google.firebase.firestore.FirebaseFirestore;
import com.google.firebase.firestore.QueryDocumentSnapshot;
import java.util.*;

public class MainActivity extends AppCompatActivity {
    private static final String TAG = "MainActivity";
    // колекція в Firestore, куди будемо зберігати числа
    private static final String COLLECTION_NAME = "random_numbers";

    private FirebaseFirestore db;
    private ArrayList<String> numbersList;
    private ArrayAdapter<String> adapter;

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

        // ініціалізація Firestore
        db = FirebaseFirestore.getInstance();

        Button btnSend = findViewById(R.id.btnSend);
        Button btnLoad = findViewById(R.id.btnLoad);
        ListView listView = findViewById(R.id.listView);

        numbersList = new ArrayList<>();
        adapter = new ArrayAdapter<>(this,
                android.R.layout.simple_list_item_1, numbersList);
        listView.setAdapter(adapter);

        // надіслати випадкове число
        btnSend.setOnClickListener(v -> sendRandomNumber());

        // завантажити всі числа з Firestore
        btnLoad.setOnClickListener(v -> loadAllNumbers());
    }

    // генеруємо випадкове число від 1 до 1000 і відправляємо в Firestore
    private void sendRandomNumber() {
        var random = new Random();
        int randomNumber = random.nextInt(1000) + 1;

        // cтворюємо об’єкт, який збережемо
        Map<String, Object> data = new HashMap<>();
        data.put("number", randomNumber);
        data.put("timestamp", System.currentTimeMillis());

        // додаємо документ у колекцію
        db.collection(COLLECTION_NAME)
                .add(data)
                .addOnSuccessListener(documentReference -> {
                    Toast.makeText(MainActivity.this,
                            "Число надіслано: " + randomNumber, Toast.LENGTH_SHORT).show();
                    Log.d(TAG, "Документ додано з ID: " + documentReference.getId());
                })
                .addOnFailureListener(e -> {
                    Toast.makeText(MainActivity.this,
                            "Помилка: " + e.getMessage(), Toast.LENGTH_SHORT).show();
                    Log.w(TAG, "Помилка додавання", e);
                });
    }

    // зчитуємо всі документи з колекції і виводимо числа
    private void loadAllNumbers() {
        db.collection(COLLECTION_NAME)
                .orderBy("timestamp") // сортуємо за часом (не обов’язково)
                .get()
                .addOnCompleteListener(task -> {
                    if (task.isSuccessful()) {
                        numbersList.clear();
                        for (QueryDocumentSnapshot document : task.getResult()) {
                            Long num = document.getLong("number");
                            if (num != null) {
                                numbersList.add(num.toString());
                            }
                        }
                        adapter.notifyDataSetChanged();

                        Toast.makeText(MainActivity.this,
                                "Завантажено " + numbersList.size() + " чисел", Toast.LENGTH_SHORT).show();
                    } else {
                        Log.w(TAG, "Помилка завантаження", task.getException());
                        Toast.makeText(MainActivity.this,
                                "Не вдалося завантажити дані", Toast.LENGTH_SHORT).show();
                    }
                });
    }
}

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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"
    android:orientation="vertical"
    android:padding="16dp">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="20dp"
        android:text="Тест Firestore"
        android:textSize="24sp" />

    <Button
        android:id="@+id/btnSend"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="10dp"
        android:text="Надіслати випадкове число" />

    <Button
        android:id="@+id/btnLoad"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="20dp"
        android:text="Завантажити всі числа" />

    <ListView
        android:id="@+id/listView"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</LinearLayout>

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Project perspective, app / google-services.json: (цей файл маэ бути створено автоматично, якщо не буде - то можна завантажити в розділі Project Overview на сайті https://console.firebase.google.com/

{
  "project_info": {
    "project_number": "1055250888714",
    "project_id": "site-sunmeat-student",
    "storage_bucket": "site-sunmeat-student.firebasestorage.app"
  },
  "client": [
    {
      "client_info": {
        "mobilesdk_app_id": "1:1055250888714:android:577b2fc50b2ddbf7db47f3",
        "android_client_info": {
          "package_name": "site.sunmeat.helloworld"
        }
      },
      "oauth_client": [],
      "api_key": [
        {
          "current_key": "AIzaSyCsB7y4Jq4fSUkjDYxdOsxdy9its6jo1hk"
        }
      ],
      "services": {
        "appinvite_service": {
          "other_platform_oauth_client": []
        }
      }
    }
  ],
  "configuration_version": "1"
}

==========================================================================================================

rules:

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /{document=**} {
      allow read, write: if true;
    }
  }
}