What is Android? Architecture
What is Android?
Android is an open-source operating system made by Google, based on the Linux kernel. It runs on phones, tablets, TVs, watches and cars. An Android app is a program written in Kotlin (or Java) with screens designed in XML, packaged and installed on these devices.
Android architecture (layers)
You will mostly work with the top two layers: your app code and the Android framework (classes like Activity, TextView, NotificationManager). Libraries from Jetpack (AndroidX) — RecyclerView, Room, Navigation — sit on top of the framework and make common tasks easier.
From code to app: APK and AAB
The 4 app components
| Component | What it is | Example |
|---|---|---|
| Activity | A screen with a UI | Login screen, home screen |
| Service | Work without a screen | Music playing in background |
| Broadcast Receiver | Reacts to system events | Battery low, boot completed |
| Content Provider | Shares data with other apps | Contacts, photo gallery |
Modern apps usually have one Activity and many Fragments (parts of screens), plus helpers like WorkManager for background tasks. You will learn all of these.
Android versions and API levels
| Version | API level | Note |
|---|---|---|
| Android 8.0 Oreo | 26 | Notification channels – our minSdk |
| Android 10 | 29 | Scoped storage, dark theme |
| Android 12 | 31 | Material You, splash screen API |
| Android 13 | 33 | Notification permission, photo picker |
| Android 15 | 35 | Edge-to-edge by default |
minSdk = oldest Android version your app supports. targetSdk = the version you tested against (Google Play requires a recent one).
Why Kotlin + XML in this course?
Kotlin + XML Views (this course)
- Used in most existing apps
- Asked in college & job interviews
- Every classic view: drawer, RecyclerView, tabs
- Easy to see UI in the layout editor
Jetpack Compose
- Newer UI toolkit (Kotlin code only)
- Great for new apps
- Learn it after this course
- Same Kotlin, ViewModel, Room, Retrofit
Course roadmap
- 1Setup: Android Studio, first app, Kotlin basics
- 2UI: layouts, all common views, Material 3 theme
- 3Screens: intents, RecyclerView, fragments, bottom nav, drawer, tabs
- 4Data: SharedPreferences, Room, MVVM, coroutines, Retrofit, Glide
- 5Device: permissions, camera, notifications, WorkManager, WebView
- 6Project: complete Student Diary app + Play Store release
Summary
- Android = Linux-based OS; apps are written in Kotlin with XML layouts.
- Layers: apps → framework → runtime/libraries → HAL → kernel.
- Gradle builds an APK (install) or AAB (Play Store).
- Components: Activity, Service, Broadcast Receiver, Content Provider.
Open Settings → About phone on your phone and note the Android version. Find its API level in the table above — that tells you which features your phone supports.
