I am learning flutter from JustAcademy, They provide very much great environment where people gather and work simultaneously. totally project based training institute.
Awesome Experience. I am a Front-end Web Designer working at Star India for 3 years now. I applied for Full-stack development and my experience has been phenomenal & they really do help with placements exceptionally. Thank you Roshan sir so much.
Android is an open-source mobile operating system developed by Google.
It is mainly used to develop applications for smartphones, tablets, smart TVs, and wearable devices.
Android is based on the Linux Kernel and allows developers to build applications using Java or Kotlin programming languages.
Key Features of Android
Android is currently one of the most widely used mobile operating systems in the world.
Android SDK (Software Development Kit) is a collection of tools used to develop Android applications.
It includes tools such as:
Developers use the Android SDK with Android Studio to build and test Android applications.
Example
Android Studio is the official Integrated Development Environment (IDE) for Android app development.
It is developed by Google and based on IntelliJ IDEA.
Features of Android Studio
Developers use Android Studio to write code, design UI, run apps, and debug applications.
Android architecture consists of several layers.
Main Layers
Linux Kernel
Handles hardware interaction.
Libraries
Provides core functionalities such as graphics and database.
Android Runtime (ART)
Executes Android applications.
Application Framework
Provides APIs for developers.
Applications
Apps built by developers.
Architecture Flow
Applications
Application Framework
Android Runtime + Libraries
Linux Kernel
This layered architecture makes Android flexible and efficient.
An Activity represents a single screen of a mobile application.
For example:
Each screen in an Android app is usually implemented as an Activity.
Example
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
}
Here:
MainActivity is the first screen of the app.
An Intent is a messaging object used to communicate between Android components.
It is mainly used to:
Example
Intent intent = new Intent(MainActivity.this, SecondActivity.class);
startActivity(intent);
This code opens another screen (SecondActivity).
The AndroidManifest.xml file contains important information about the application.
It tells the Android system:
Example
<manifest>
<application>
<activity android:name=".MainActivity" />
</application>
</manifest>
Without the Manifest file, Android cannot run the application.
The Android Emulator is a virtual device used to test Android applications on a computer.
It simulates real Android devices such as:
Developers can run apps without using a physical device.
Example tools used in emulator testing include the emulator inside Android Studio.
A Service is a component that runs in the background without a user interface.
Services are used for tasks like:
Example
Music apps continue playing music even when the app is closed using services.
| Feature | Activity | Service |
|---|---|---|
| UI | Has UI | No UI |
| Purpose | User interaction | Background tasks |
| Example | Login screen | Music playback |
Example
Activity → Displays a screen
Service → Runs tasks in background
A Fragment is a part of a user interface inside an Activity.
It allows developers to divide the UI into smaller reusable components.
Fragments are useful for building dynamic and flexible layouts.
Example
public class MyFragment extends Fragment {
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
return inflater.inflate(R.layout.fragment_layout, container, false);
}
}
Advantages of Fragments
Fragments are commonly used in tab layouts and navigation screens.
RecyclerView is a flexible and efficient component used to display large lists of data.
It is an improved version of ListView.
RecyclerView improves performance by reusing item views instead of creating new ones.
Components of RecyclerView
Example
RecyclerView recyclerView = findViewById(R.id.recyclerView);
recyclerView.setLayoutManager(new LinearLayoutManager(this));
RecyclerView is used in apps like:
The Activity Lifecycle describes the different states an activity goes through during its lifetime.
Main Lifecycle Methods
| Method | Description |
|---|---|
| onCreate() | Activity is created |
| onStart() | Activity becomes visible |
| onResume() | Activity starts interacting |
| onPause() | Activity partially hidden |
| onStop() | Activity not visible |
| onDestroy() | Activity destroyed |
Example
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
Understanding lifecycle helps developers manage resources and avoid crashes.
Gradle is the build automation system used in Android development.
It is responsible for:
Example dependency
implementation 'com.squareup.retrofit2:retrofit:2.9.0'
Gradle automatically downloads required libraries.
An APK (Android Package Kit) is the file format used to distribute and install Android applications.
It contains:
Users download APK files from sources such as the Google Play Store.
SharedPreferences is used to store small amounts of data locally in key-value pairs.
It is commonly used for:
Example
SharedPreferences prefs = getSharedPreferences("MyPrefs", MODE_PRIVATE);
SharedPreferences.Editor editor = prefs.edit();
editor.putString("username", "Rajan");
editor.apply();
SharedPreferences stores data persistently even after the app closes.
SQLite is a lightweight local database used in Android applications.
It stores structured data using tables, rows, and columns.
Example uses
Example
SQLiteDatabase db = openOrCreateDatabase("MyDB", MODE_PRIVATE, null);
db.execSQL("CREATE TABLE users(name TEXT, age INTEGER)");
SQLite is useful when apps need offline data storage.
A Broadcast Receiver is a component that responds to system-wide broadcast messages.
Examples of broadcasts:
Battery low
Network connectivity changes
Phone boot completed
Example
public class MyReceiver extends BroadcastReceiver {
public void onReceive(Context context, Intent intent) {
Toast.makeText(context, "Broadcast received", Toast.LENGTH_SHORT).show();
}
}
Broadcast receivers allow apps to react to system events.
A Content Provider is used to share data between different applications.
It manages access to structured data.
Examples include:
Apps can access shared data using ContentResolver.
Example
ContentResolver resolver = getContentResolver();
Content providers provide secure data sharing between apps.
LinearLayout
Arranges UI elements in a single direction (vertical or horizontal).
RelativeLayout
Arranges elements relative to each other or parent layout.
Comparison
| Layout | Description |
|---|---|
| LinearLayout | Elements in row or column |
| RelativeLayout | Flexible positioning |
Example:
<LinearLayout
android:orientation="vertical">
</LinearLayout>
ViewModel is a component used to manage and store UI-related data in a lifecycle-conscious way.
It helps the app retain data even when the screen rotates or configuration changes occur.
ViewModel is part of Android Jetpack libraries.
Example
public class MyViewModel extends ViewModel {
private String data = "Hello Android";
}
Benefits
LiveData is a lifecycle-aware data holder class.
It allows UI components such as Activities and Fragments to observe data changes automatically.
When the data changes, the UI updates automatically.
Example
MutableLiveData<String> name = new MutableLiveData<>();
name.setValue("Rajan");
Advantages
LiveData is commonly used with ViewModel.
MVVM (Model–View–ViewModel) is a design pattern used in Android development.
It helps organize code and separate business logic from UI logic.
Components
| Component | Role |
|---|---|
| Model | Handles data |
| View | UI layer (Activity/Fragment) |
| ViewModel | Connects View and Model |
Flow
View → ViewModel → Model
Benefits
Retrofit is a popular library used to make API calls in Android applications.
It simplifies network operations and converts JSON data into Java objects.
Example dependency
implementation 'com.squareup.retrofit2:retrofit:2.9.0'
Example API call
Retrofit retrofit = new Retrofit.Builder()
.baseUrl("https://api.example.com/")
.build();
Retrofit is widely used for REST API integration.
Room is a persistence library that provides an abstraction layer over SQLite.
It makes database operations easier and safer.
Room is part of Android Jetpack.
Components of Room
Example Entity
@Entity
public class User {
@PrimaryKey
public int id;
public String name;
}
Room helps developers avoid complex SQL code.
Dependency Injection (DI) is a design pattern used to provide dependencies to classes instead of creating them inside the class.
It improves:
Popular DI libraries in Android:
Example: Instead of creating objects manually, they are injected automatically.
ConstraintLayout is a flexible layout used to design complex UI screens.
It allows developers to position UI elements relative to each other.
Advantages:
Example
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:layout_constraintTop_toTopOf="parent"/>
ConstraintLayout is commonly used in modern Android UI design.
ANR (Application Not Responding) occurs when the app does not respond to user actions for a long time.
This usually happens when heavy operations run on the main UI thread.
Causes of ANR
Solution
Use background threads like:
This prevents the UI from freezing.
ProGuard is a tool used to optimize and secure Android applications.
It performs tasks such as:
Benefits
ProGuard runs during the build process.
Both are used to pass data between Android components.
Serializable
Parcelable
Comparison
| Feature | Serializable | Parcelable |
|---|---|---|
| Speed | Slower | Faster |
| Usage | Java apps | Android apps |
| Performance | Lower | Higher |
Parcelable is generally preferred in Android development.
Coroutines are used to perform asynchronous tasks without blocking the main thread.
They help developers run background operations like:
Coroutines are mainly used with the Kotlin programming language.
Example
import kotlinx.coroutines.*
GlobalScope.launch {
delay(1000)
println("Hello from Coroutine")
}
Benefits
WorkManager is an Android library used to schedule background tasks that must run even if the app closes.
Examples:
Example
val workRequest = OneTimeWorkRequestBuilder<MyWorker>().build()
WorkManager.getInstance(context).enqueue(workRequest)
WorkManager ensures tasks run reliably in the background.
Jetpack Compose is a modern toolkit used to build Android UI using Kotlin code instead of XML.
It simplifies UI development.
Example
@Composable
fun Greeting() {
Text(text = "Hello Android")
}
Advantages
Jetpack Compose is part of Android Jetpack libraries.
Data Binding is a library that allows developers to bind UI components directly to data sources.
It reduces the need to write code like findViewById().
Example
<TextView
android:text="@{user.name}" />
Benefits
View Binding is a feature that allows developers to easily access views without using findViewById().
It generates a binding class for each XML layout.
Example
ActivityMainBinding binding = ActivityMainBinding.inflate(getLayoutInflater());
setContentView(binding.getRoot());
Benefits
The Navigation Component helps developers manage navigation between screens in Android apps.
It simplifies:
Components
Example
findNavController().navigate(R.id.profileFragment)
App signing is the process of digitally signing an Android app before publishing it.
It ensures:
Apps must be signed before uploading to the Google Play Store.
Types
Dalvik Virtual Machine (DVM) was the original virtual machine used in Android to run applications.
It was designed specifically for mobile devices with limited memory and CPU power.
Later, it was replaced by ART (Android Runtime) for better performance.
Features of Dalvik
ART (Android Runtime) is the runtime environment used in modern Android systems.
It replaced Dalvik Virtual Machine.
Advantages of ART
ART uses Ahead-of-Time (AOT) compilation to compile apps before execution.
Android permissions allow apps to access sensitive device features.
Examples:
Example Permission
<uses-permission android:name="android.permission.CAMERA" />
Permissions protect user privacy and device security.
Android Security refers to the mechanisms used to protect user data and applications.
Android provides several security features such as:
These features help prevent malware and unauthorized access to data.
Multithreading allows an application to run multiple tasks at the same time.
Example tasks:
Multithreading prevents the main UI thread from freezing.
Example
new Thread(new Runnable() {
@Override
public void run() {
// background task
}
}).start();
The Main Thread is also called the UI Thread.
It is responsible for:
If heavy tasks run on the main thread, the app may freeze or cause ANR errors.
A Handler is used to send and process messages between threads.
It helps update the UI from a background thread.
Example
Handler handler = new Handler(Looper.getMainLooper());
handler.post(new Runnable() {
@Override
public void run() {
// update UI
}
});
Handlers are commonly used for thread communication.
| Feature | Thread | Handler |
|---|---|---|
| Purpose | Runs background tasks | Communicates between threads |
| UI access | Cannot update UI directly | Can update UI |
| Usage | Heavy operations | UI updates |
Threads perform background work, while handlers manage communication with the UI thread.
Dependency Injection (DI) is a design pattern that provides required objects to a class instead of creating them inside the class.
Benefits include:
Popular DI libraries include:
These libraries simplify dependency management in Android apps.
Performance optimization improves the speed, efficiency, and responsiveness of an Android app.
Common Optimization Techniques
Good optimization improves user experience and app performance.
dp (Density Independent Pixels)
Used for layout dimensions such as width and height.
sp (Scale Independent Pixels)
Used for text size because it respects user font settings.
Example
android:layout_width="100dp"
android:textSize="16sp"
Using dp and sp ensures UI consistency across different screen sizes.
Logcat is a tool used to view system logs and debug messages in Android development.
Developers use it to:
Example
Log.d("MainActivity", "App started");
Logcat is available in Android Studio's debugging tools.
Android development is valuable because Android powers billions of devices worldwide.
Reasons to Learn Android
Many companies publish their apps on the Google Play Store to reach millions of users.
Selenium WebDriver Interview Questions and Answers for Freshers and Experienced Testers in 2026
UI UX Interview Questions and Answers for Freshers and Beginners Covering Design Thinking, Figma, Wireframing, and UX Research in 2026
Figma Interview Questions and Answers for UI UX Designers, Freshers, and Experienced Designers in 2026
SQL, ORM, and Database Interview Questions and Answers for Python Backend Developers and Freshers in
OOP Concepts, Python Classes Interview Questions and Answers for Beginners and Freshers in 2026
REST API, FastAPI, and Django REST Framework Interview Questions and Answers for Python Freshers and Developers in 2026
Flask API, Python Backend Interview Questions and Answers for Beginners and Freshers in 2026
Django Interview, Python Backend Questions and Answers for Freshers and Beginners in 2026
Python DSA, Coding Interview Questions and Answers for Freshers and Beginners in 2026
Version Control, GitHub Interview Questions and Answers for Python Developers and Freshers in 2026
