Top 40+ Android Interview Questions & Answers

Android is a publicly available Operating System (OS) utilised in various mobile devices, such as smartphones and tablets. When an Android application runs, it operates within its dedicated process and runs on its distinct instance of either the Dalvik Virtual Machine (DVM) or the Android RunTime (ART).

Android was founded by Andy Rubin.

Android architecture encompasses a layered structure within the Android stack, which comprises the Operating System (OS), middleware, and applications. Each layer within the Android architecture delivers specific services to the layer immediately above it.

The Android stack is organised into five distinct layers:

1) Linux Kernel: This layer is responsible for device management, memory management, power management, resource access, and device drivers.

2) Libraries: Within this layer, a collection of libraries, including the open-source Web browser engine WebKit, the well-known libc library, audio and video playback and recording libraries, SQLite database for application data storage and sharing, as well as SSL libraries for internet security, are present.

3) Android Runtime: At this level, you find core libraries along with the choice of either the Dalvik Virtual Machine (DVM) or the Android RunTime (ART) for executing Android applications. DVM is optimised for mobile devices, offering swift performance and lower memory consumption. In Android version 5.0 and beyond (API level 21), ART was introduced to replace DVM for executing Android apps.

4) Android framework: This layer encompasses Android APIs, which include functionalities for the User Interface (UI), resources, content providers for data sharing, location services, telephony, and package management. It provides a rich set of interfaces and classes to facilitate the development of Android applications.

5) Android applications: The top most layer includes various applications such as the home screen, games, contacts, settings, browsers, and more. These applications make use of the underlying Android framework, Android runtime, and libraries to provide a wide range of user experiences and functionalities on Android devices.

The activity class holds significant importance within Android components. Every Android app, regardless of its size or scalability, includes at least one activity class. In contrast to most programming languages where the main() method serves as the entry point for program execution, in Android, the operating system triggers the code within an activity.

The most employed programming languages for crafting Android applications include:

1) Java: Java has been the go-to choice for many newcomers and is widely adopted by Android Developers. Popular Integrated Development Environments (IDEs) for Android App Development with Java encompass Eclipse, NetBeans, and IntelliJ.

2) Kotlin: Kotlin, a relatively modern, secure, and object-oriented, cross-platform programming language, has gained popularity for Android Application Development. Kotlin is compatible with IDEs like Android Studio and Eclipse.

3) C#: Developers have the option to create native iOS and Android mobile applications using the C# language, with Visual Studio standing out as a prime tool for Android App Development with C#.

4) Python: Python, known for its dynamism and object-oriented approach, finds acclaim in fields like Machine Learning (ML). For Android App Development in Python, Code editors such as Pydroid 3, Dcoder, and spck code editor are commonly used.

Other languages can also be leveraged for Android Development, including C++ and HTML 5. IDEs tailored for C++ include C4droid, CppDroid, and AIDE, while those for HTML encompass Acode, and spck code editor, among others.

XML, which stands for Extensible Markup Language, serves as a versatile markup language similar in nature to HTML and is employed to depict data structures. XML boasts an inherent characteristic of being easily comprehensible, making it equally accessible to both humans and machines. This dual readability aspect makes it a choice for various applications.

XML exhibits a noteworthy quality in its scalability and simplicity when it comes to development. In the context of Android, XML emerges as a prominent choice for designing layouts due to its lightweight nature. This inherent lightness ensures that XML doesn't introduce unnecessary bulk or complexity to the layout, thereby promoting efficiency in the development process.

The following are the differences between Dalvik and ART:
 

The following are the advantages of Android:

1) Open-source nature: Android is an open-source platform, which means it is freely accessible without the need for licenses, distribution fees, or development costs. This open nature encourages innovation and enables developers to create and distribute applications without financial barriers.

2) Cross-platform compatibility: Android offers support for a wide range of OS, making it a versatile choice for Android Developers. It is compatible with Windows, Mac, and Linux platforms, allowing developers to work on their preferred operating system.

3) Wide technology integration: Android is equipped to harness an array of cutting-edge technologies, such as camera functionality, Bluetooth connectivity, Wi-Fi capabilities, speech recognition, and data transfer technologies like EDGE. This extensive technology support empowers developers to create diverse and feature-rich applications.

4) Optimised virtual machine: Android employs a highly optimised virtual machine known as the Dalvik Virtual Machine (DVM). DVM is specifically tailored for mobile devices, ensuring efficient and responsive application performance. This optimised virtual machine contributes to the seamless execution of Android apps, enhancing the overall user experience.

A service is a fundamental application component that enables an app to execute tasks in the background, carrying out extended operations without requiring user intervention. Services have the capability to operate persistently in the background, regardless of whether the app is closed or when the user transitions to another application.

Activities can be concluded or shut down at the user's discretion. In contrast, services are engineered to function autonomously in the background. The following are the differences between activities and services:

Aspect Activities Services

Android has used dessert-themed code names for various versions of its operating system in the past. However, this tradition was discontinued with Android 10. The following are some of the notable code names for earlier Android versions:

1) Android 1.5: Cupcake

2) Android 1.6: Donut

3) Android 2.0/2.1: Éclair

4) Android 2.2: Froyo (short for "Frozen Yogurt")

5) Android 2.3: Gingerbread

6) Android 3.0/3.1/3.2: Honeycomb

7) Android 4.0: Ice Cream Sandwich

8) Android 4.1/4.2/4.3: Jelly Bean

9) Android 4.4: KitKat

10) Android 5.0/5.1: Lollipop

11)  Android 6.0: Marshmallow

12)  Android 7.0/7.1: Nougat

13) Android 8.0/8.1: Oreo

14) Android 9: Pie

15) Android 10: Android version naming moved away from dessert names and adopted a numerical approach.

16) Android 11: Continued with the numerical naming approach.

17) Android 12: Also uses numerical naming.

An Android toast serves to furnish users with informative feedback regarding their ongoing interactions. It serves as a messaging mechanism to communicate the status or outcome of operations initiated by the user. In essence, it conveys messages that apprise users of the results or progress of their actions within the application.

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In Android, a Bundle is a data structure that is commonly used to pass data between different components of an Android application, primarily between activities, fragments, and services. It is a key-value pair collection, like a dictionary or associative array in other programming languages, where data is stored as key-value pairs. The primary use of Bundle is to encapsulate and transfer data in a serialised format.

During sleep mode, the CPU is in a low-power state and remains unresponsive to commands from the Android device, except for the Radio Interface Layer and alarm-related functions.

The following are the two primary methods for placing layouts in Android:

XML layouts:

1) XML (eXtensible Markup Language) Layouts: This is the most common and recommended way to define the layout of an Android user interface.

2) XML layout files are created in the res/layout directory of the Android project.

3) These files define the arrangement and properties of UI elements using XML tags and attributes.

4) XML layouts can be previewed in Android Studio's visual designer.

5) They are referenced and inflated in Java or Kotlin code using methods like.

Programmatic layouts:

1) In some cases, you may need to create layouts dynamically in your Java or Kotlin code.

2) This approach involves instantiating layout and view elements in your code and defining their properties programmatically.

3) It's suitable for scenarios where the layout needs to change dynamically based on data or user interactions.

Android offers a range of storage options to cater to diverse Data Management needs. These storage methods include:

Shared preferences:

1) Shared preferences serve as a simple and efficient means to store key-value pairs, mainly used for saving application settings or user preferences.

2) These values are private to the application and can be easily retrieved and modified.

Internal storage:

1) Internal storage refers to a device's built-in storage space where apps can store private data files.

2) This storage is confined to the app, ensuring data privacy and security. It is often used for saving app-specific data.

External storage:

1) External storage provides a means to store data that can be shared between applications and accessed by the user.

2) It includes the device's external memory like an SD card and allows for the storage of larger files such as images, videos, or documents.

SQLite databases:

1) SQLite databases offer a powerful way to organise and manage structured data in a relational database format.

2) Android apps can create, query, and modify databases, making them suitable for applications with substantial data requirements.

Network connection:

1) Network connection represents a storage option where data is stored on remote servers or cloud-based platforms.

2) Apps can utilise network connections to retrieve and exchange data with online services, making it an essential method for data storage and retrieval from external sources.

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An adapter plays a crucial role in Android App Development by facilitating the creation of child views to display items within a parent view. In other words, it acts as an intermediary component responsible for binding and rendering data, typically drawn from a data source, into a visually coherent format within the parent view. This ensures that the User Interface (UI) effectively presents the information or content in a structured and organised manner, enhancing the user's interaction with the application.

A content provider in the context of Android is a crucial component that enables secure and structured access to data from one application to another. It acts as a data store and offers a standardised interface for interacting with data, allowing apps to share and retrieve information while maintaining data integrity and privacy.

AAPT, which stands for Android Asset Packaging Tool, serves as an essential component within the Android Development environment. Its primary function is to manage the intricate process of packaging various assets and resources that are integral to an Android application. These assets may include images, XML files, layouts, and other essential resources.

In Android, you can call (start) another activity by creating an explicit intent to navigate from the current activity to the target activity. Here's a step-by-step guide on how to do it:

1) Create the target activity: First, ensure that the activity you want to call (the target activity) is defined in your Android project. You should have a Java or Kotlin class for the target activity and a corresponding layout XML file.

2) Create an intent: In your current activity (the one from which you want to start the target activity), create an explicit Intent that specifies the target activity's class. You can do this in the event handler or wherever you want to trigger the navigation.

3) Add extra data: You can use the Intent to pass data from the current activity to the target activity. This is optional, but it's a common practice to send information between activities.

4) Start the target activity: Use the startActivity() method to initiate the transition to the target activity.

5) Receive data in the target activity: In the target activity, you can retrieve any data passed through the Intent by using the getIntent() method.

A portable Wi-Fi hotspot serves as a valuable tool for disseminating an internet connection to multiple wireless devices, allowing them to access the internet seamlessly. This feature enables the transformation of a mobile device, such as a smartphone or tablet, into a wireless access point, granting connectivity to other devices like laptops, tablets, or other smartphones in its vicinity. As a result, users can conveniently and wirelessly share their internet connection, providing a versatile and accessible network solution for multiple devices.

ADB, which stands for Android Debug Bridge, is a versatile command-line tool that is an essential part of the Android Development ecosystem. ADB enables communication and interaction between a developer's computer and an Android device or emulator. It serves several important functions, including:

1) Debugging: ADB allows developers to debug their Android applications on physical devices or emulators. By connecting to the device, developers can view logs, access error messages, and test their applications in real time.

2) Installing and managing apps: ADB can be used to install, uninstall, or manage Android applications directly from a computer. This is especially useful for Developers to test different versions of their apps or install apps for testing purposes.

3) File transfer: ADB facilitates the transfer of files between the computer and the Android device or emulator. This is helpful for pushing files, such as APKs, to the device for installation.

The NDK, or Native Development Kit, is a set of tools and libraries provided by Google for Android App Development. It allows developers to write parts of their Android applications in native code languages like C and C++ instead of using Java or Kotlin, which are the more commonly used languages for Android Development.

ANR stands for "Application Not Responding," and it refers to a situation in the Android operating system where an app becomes unresponsive or fails to process user input in a timely manner. When an ANR occurs, the Android system displays a dialog to the user, indicating that the app is not responding and giving the user the option to either wait for the app to respond or close it.

ANRs typically happen when an app performs lengthy or CPU-intensive tasks on the main UI thread, causing the application's user interface to freeze. The Android system monitors apps and their responsiveness, and if it detects that an app is not processing user input or UI updates within a certain time frame (usually around 5 seconds), it triggers an ANR.

The following are the differences between Service and Thread:
 

The following are the differences between View AND ViewGroup:
 

1) MVP: Appropriate for both straightforward and complex applications. 

2) MVVM: Well-suited for extensive projects but may present unnecessary complexity for smaller-scale endeavours.

3) MVC: Suited for smaller projects but may not be optimal for larger-scale endeavours.

JUnit serves as a robust "Unit Testing" framework specifically designed for Java applications. Notably, it comes pre-installed with Android Studio, making it a readily available tool for Android Developers. JUnit plays a dual role as both an automation framework for Unit Testing and UI Testing, enabling Developers to assess the functionality and correctness of their code systematically.

One of the defining features of JUnit is its use of annotations to specify and control the test execution. Some of the key annotations include "@Test," "@Before," "@After," and more.

DDMS, an acronym for Dalvik Debug Monitor Server, is a multifaceted tool that offers an extensive range of debugging capabilities, encompassing:

1) Facilitating port forwarding services.

2) Enabling screen capture functionality.

3) Providing in-depth insights into thread and heap information.

4) Proficiently tracking network traffic for analysis.

5) Supporting location data spoofing for testing and evaluation.

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In Android, sensors are an essential component for gathering data about the device's physical environment. To interact with sensors and retrieve sensor data, developers utilise Java classes and the Android Sensor Framework. The following are some of the key Java classes related to the use of sensors on Android:

SensorManager: The SensorManager class is a fundamental class for sensor management. It provides methods to access and manage device sensors. Developers use this class to obtain sensor instances, register sensor listeners, and retrieve sensor data. It acts as a gateway to the device's sensors.

Sensor: The Sensor class represents an individual sensor on the device, such as an accelerometer, gyroscope, or light sensor. It provides information about the sensor's type, name, vendor, and power usage. Developers use this class to obtain details about a specific sensor.

SensorEvent: The SensorEvent class encapsulates sensor data, including the sensor type, data values, accuracy, and timestamps. It is used in conjunction with sensor listeners to receive and process sensor data in real-time.

SensorEventListener: The SensorEventListener interface is implemented by developers to receive updates from sensor events. It includes two callback methods: onSensorChanged() for receiving sensor data and onAccuracyChanged() for handling changes in sensor accuracy.

Dagger is a dependency injection framework for Java and Android applications. Dependency injection is a design pattern used to improve the modularity and testability of software by decoupling the classes that depend on one another. Dagger simplifies the process of managing and injecting dependencies, making it a valuable tool for developers.

In Android, a "drawable" folder compiles visual resources that can be utilised as backgrounds, banners, icons, splash screens, and more.

Lag in an Android app can be caused by various factors, and identifying the root causes is essential for providing a smooth user experience. Some common reasons for Android app lag include:

1) Inefficient code: Poorly optimised code, including weak algorithms, excessive memory usage, and CPU-intensive operations, can lead to lag. Code optimisation is crucial for improving app performance.

2) UI thread blocking: Performing time-consuming operations on the main UI thread can cause the app's user interface to freeze. Operations like network requests, database queries, or image processing should be executed on background threads to prevent UI thread blocking.

3) Memory leaks: Unreleased resources or memory leaks can gradually degrade app performance. Developers need to be vigilant in managing resources, closing connections, and cleaning up objects to prevent memory leaks.

4) Excessive garbage collection: Frequent garbage collection cycles can cause app pauses and affect user experience. Minimising object creation and avoiding memory churn can reduce the impact of garbage collection.

5) Large bitmaps and images: Loading and displaying large images or bitmaps without proper scaling or caching can lead to lag. Using appropriate image libraries and techniques for handling images can mitigate this issue.

In Android, a singleton class is a design pattern that restricts the instantiation of a class to a single instance and ensures that there is only one global point of access to that instance throughout the application's lifecycle. Singleton classes are commonly used to manage a shared resource, maintain a single point of control, or encapsulate a global state within an Android app.

AIDL stands for "Android Interface Definition Language." It is a specialised interface definition language used in Android App Development to enable interprocess communication (IPC) between Android components running in different processes.

AIDL allows Developers to define the interface of a service that can be accessed and used by components in other Android applications or within the same application but running in different processes. The primary use of AIDL is to facilitate communication between a client and a service when they run in separate processes.

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ANR stands for "Application Not Responding," and it is an Android system mechanism designed to protect the user experience by identifying when an application is not responding to user interactions. To prevent ANRs in Android, consider the following best practices:

1) Perform work in the background: Avoid performing long-running or blocking operations on the main UI thread. Instead, delegate such tasks to background threads or services. Common examples include network requests, file I/O, and database queries. Android provides tools like AsyncTask, Thread, and Intent Service for handling background work.

2) Use AsyncTask or Loaders: When updating the UI in response to background tasks, consider using AsyncTask or Loaders to simplify thread management and ensure that UI updates occur on the main thread. These classes handle thread synchronisation for you.

3) Optimise database operations: When working with databases, ensure that database queries and operations are efficient. Use indexing, pagination, and background threads for data manipulation. Avoid running complex queries on the main thread.

The Android NDK (Native Development Kit) is a valuable tool for Android App Developers because it provides the capability to write parts of an Android application in native code, typically in C or C++. The NDK is useful for various reasons:

1) Performance optimisation: One of the primary reasons for using the NDK is to optimise performance-critical parts of an Android application. Native code can execute faster than Java code, making it suitable for tasks that require maximum performance, such as graphics processing, audio processing, and complex calculations.

2) Access to platform-specific features: The NDK allows developers to access and utilise platform-specific features and APIs that are not available through the Android SDK or are better suited for native code. This is particularly useful for tasks like low-level hardware interaction, real-time audio processing, or low-level graphics rendering.

3) Legacy code integration: If an app relies on existing C or C++ libraries, or if you have legacy code that you want to reuse in your Android app, the NDK enables seamless integration. You can wrap and use existing native libraries within your Android application.

1) Inflate Exception

2) Surface.OutOfResourceException

3) SurfaceHolder.BadSurfaceTypeException

4) WindowManager.BadTokenException

When to use Dagger:

1) Large projects: Dagger is most beneficial in large, complex projects where managing dependencies can become challenging. It helps organise and simplify the injection of dependencies.

2) Modular architecture: If you're designing your application using a modular architecture, Dagger can assist in managing dependencies between modules and making them more self-contained.

3) Testability: Dagger facilitates testing by allowing you to inject mock or test dependencies, making it easier to write unit tests for your code.

4) Android Development: For Android App Development, Dagger is a popular choice for implementing dependency injection, particularly when building apps with a high degree of separation of concerns and modularity.

JobScheduler is an Android API introduced in Android 5.0 (API level 21) as part of the JobScheduler API. It provides a flexible and efficient way for developers to schedule and manage background tasks, jobs, or services in Android applications. JobScheduler is designed to improve the efficiency of background processing and to optimise battery life by allowing the system to batch and schedule jobs based on criteria such as network availability and device charging status.

Key features and benefits of JobScheduler include the following:

1) Job scheduling: JobScheduler allows developers to schedule jobs or tasks that need to be executed in the background. These jobs can be periodic or one-time tasks.

2) Battery efficiency: One of the primary goals of JobScheduler is to optimise battery life. It considers the device's current state, including battery level, network connectivity, and charging status, to schedule jobs at times that are most power-efficient.

3) Network awareness: Developers can specify network conditions for job execution. For example, you can schedule a job to run only when a network connection or the device is connected to a specific network type (e.g., Wi-Fi).

4) Idle maintenance: JobScheduler allows developers to set conditions for running jobs only when the device is idle, conserving resources and avoiding interference with the user's activities.

5) Flexibility: The system can optimise the execution of jobs by batching them together or delaying them when resources are limited. This ensures efficient utilisation of system resources.

6) Job constraints: Developers can set job constraints to specify conditions that must be met before the job runs. For example, you can require that a job only executes when the device is charging and connected to Wi-Fi.

7) Backward compatibility: While JobScheduler was introduced in Android 5.0, it is backward-compatible with earlier versions of Android. On devices running Android 4.4 (KitKat) and later, the JobScheduler API is available through the AndroidX library.

8) Job rescheduling: If a job fails to be completed successfully, JobScheduler can automatically reschedule it for execution later.

The Android architecture refers to the structural design and organisation of the Android operating system, which serves as the foundation for developing and running Android applications. It encompasses various layers and components that work together to provide a robust and flexible environment for mobile devices.

In Android App Development, Room is one of the Android Architecture Components introduced by Google. It is a robust and efficient persistence library that provides an abstraction layer over SQLite, making it easier for developers to work with local databases in Android applications. Room simplifies the process of database management, reduces boilerplate code, and encourages the use of best practices for data storage.

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