Top Android Interview Questions and Answers

Basic Android Development

1) What is Android?

Answer:

Android is an open-source operating system based on Linux for mobile devices like smartphones, tablets, and wearables. It is developed by Google and provides an extensive framework for developing mobile applications.

2) What is an Android application?

Answer:

An Android application is a software application that runs on an Android operating system. It can use Java or Kotlin to build, and it works on various Android-based devices like phones, tablets, and smartwatches.

3) What are the components of an Android application?

Answer:

The primary components of an Android app are:

1. Activities: Handles the user interface.
2. Services: Background tasks.
3. Content Providers: Manages data and shared resources.
4. Broadcast Receivers: Interacts with system-wide broadcast announcements.

4) What is an Activity in Android?

Answer:

An Activity is a single screen that users interact with. It represents a UI component where users can interact with the app. For example, a login screen or a contact list screen.

5) What is an Intent in Android?

Answer:

An Intent is an object that allows one activity to start another or perform some action. It is used to pass data between components in Android.

6) What is the difference between explicit and implicit intents?

Answer:

1. Explicit Intent: Targets a specific component, such as an activity or service (e.g., Intent(this, SecondActivity.class)).
2. Implicit Intent: Does not target a specific component, but specifies an action to be performed (e.g., Intent.ACTION_VIEW to open a URL).

7) What are Views and ViewGroups in Android?

Answer:

1. View: A UI element (e.g., TextView, Button, ImageView).
2. ViewGroup: A container that holds other views or layouts (e.g., LinearLayout, RelativeLayout, ConstraintLayout).

8) What is the AndroidManifest.xml file?

Answer:

The AndroidManifest.xml file is the central configuration file for an Android app. It defines components like activities, services, permissions, and other configuration settings.

9) What are Fragments in Android?

Answer:

A Fragment is a modular section of an activity. Fragments represent a portion of the user interface and can be reused across different activities.

10) What is the difference between dp and px?

Answer:

1. dp (Density-independent Pixels): A unit of measurement used to ensure consistent sizing across different screen densities.
2. px (Pixels): A physical unit that varies across different screen densities.

Intermediate Android Development

11) What is the Android activity lifecycle?

Answer:

The Android activity lifecycle includes states like:

1. onCreate(): When the activity is first created.
2. onStart(): When the activity becomes visible.
3. onResume(): When the activity starts interacting with the user.
4. onPause(): When the activity loses focus but is still visible.
5. onStop(): When the activity is no longer visible.
6. onDestroy(): When the activity is being destroyed.

12) What is the difference between onPause() and onStop()?

Answer:

1. onPause(): The activity is still visible but not in the foreground. Used for saving resources like media playback.
2. onStop(): The activity is no longer visible, and resources should be released.

13) What are AsyncTask, Threads, and Handler in Android?

Answer:

1. AsyncTask: A helper class for performing background operations and updating the UI thread.
2. Threads: A separate execution flow in an application.
3. Handler: Used to manage threads and run tasks on the UI thread.

14) What is the difference between a Service and an Activity?

Answer:

1. Activity: A screen with UI for user interaction.
2. Service: Runs in the background without user interaction (e.g., music player, background syncing).

15) What is onSaveInstanceState() in Android?

Answer:

The onSaveInstanceState() method is used to save the state of an activity before it is destroyed, such as when the device is rotated or the app is killed by the system.

16) What are Content Providers in Android?

Answer:

Content Providers allow apps to share data between applications. They provide a structured interface for querying and manipulating data.

17) What is the difference between ArrayList and LinkedList?

Answer:

1. ArrayList: Implements a dynamic array that supports random access. It is faster for read operations.
2. LinkedList: Implements a doubly linked list. It is faster for insertions and deletions but slower for random access.

18) What is the ViewModel in Android?

Answer:

The ViewModel is a lifecycle-conscious component that stores UI-related data and survives configuration changes, ensuring that the data is not lost during activities and fragment recreation.

19) What is the LiveData component in Android?

Answer:

LiveData is a data holder class that can be observed. It is lifecycle-aware and used for managing UI-related data in a way that ensures UI components only update when necessary.

20) What is Retrofit in Android?

Answer:

Retrofit is a type-safe HTTP client for Android that allows you to interact with RESTful web services. It simplifies the process of consuming APIs.

Advanced Android Development

21) What is the difference between Parcelable and Serializable?

Answer:

1. Parcelable: Faster and more efficient for Android, used to serialize objects to pass between activities.
2. Serializable: A slower interface for serializing objects, not optimized for Android.

22) What is Dependency Injection in Android?

Answer:

Dependency Injection is a design pattern that allows you to inject dependencies (like services, repositories) into a class rather than creating them inside the class, leading to cleaner, more maintainable code.

23) What is Dagger2 and Hilt in Android?

Answer:

1. Dagger2: A popular dependency injection library for Android.
2. Hilt: A newer, simpler way to use Dagger2 for dependency injection in Android, provided by Google.

24) What is an AsyncTask in Android?

Answer:

An AsyncTask is a class that helps to perform background operations and update the UI thread in Android, especially useful for short-lived operations.

25) What is a BroadcastReceiver?

Answer:

A BroadcastReceiver listens for system-wide or app-specific events and acts on them. It helps in receiving intents that are broadcast by the system or other applications (e.g., battery low notifications).

26) What is the WorkManager in Android?

Answer:

WorkManager is a background task management API that provides a robust framework for scheduling deferrable, guaranteed background work that runs even if the app is closed or the device is rebooted.

27) What is Firebase Cloud Messaging (FCM)?

Answer:

Firebase Cloud Messaging (FCM) is a service provided by Firebase to send push notifications to users. It supports messaging to individual devices or groups of devices.

28) What is the purpose of Handler in Android?

Answer:

A Handler allows communication between threads by posting runnable tasks to the message queue of a thread. It is often used to update the UI from a background thread.

29) What are Android Jetpack components?

Answer:

Jetpack is a suite of libraries that help developers follow best practices, write less code, and ensure robust apps. It includes components like LiveData, ViewModel, Navigation, WorkManager, Room Database, etc.

30) What is the difference between HashMap and TreeMap?

Answer:

1. HashMap: Stores key-value pairs with no specific order.
2. TreeMap: A sorted map that orders the keys based on their natural ordering or a comparator.

Performance Optimization and Testing

31) How do you optimize an Android application?

Answer:

To optimize an Android app:

1. Use memory-efficient data structures.
2. Profile app performance using Android Profiler.
3. Avoid memory leaks with tools like LeakCanary.
4. Reduce UI thread usage, offload background tasks to worker threads.
5. Use ProGuard or R8 to minimize the app size.

32) How do you test Android applications?

Answer:

Testing Android apps involves:

1. Unit Testing: Testing individual units of code using libraries like JUnit.
2. UI Testing: Using Espresso or UI Automator to simulate user interaction and test the UI.
3. Instrumented Tests: Testing code that interacts with the Android framework.

33) What are the key features of the Android Debug Bridge (ADB)?

Answer:

ADB is a versatile command-line tool for managing Android devices. It allows you to install apps, debug, log outputs, and execute commands directly on a connected Android device or emulator.

34) What is the difference between an Activity and a Fragment?

Answer:

1. Activity: Represents a full screen with a UI.
2. Fragment: Represents a portion of the UI that can be reused across different activities.