Mobile App Developer Interview Questions

Are you looking for great mobile app developer interview questions so that you can hire the best mobile app engineer for your project?

You can expect the demand for mobile app developers to remain high. After all, the mobile app market is predicted to grow at a CAGR of “14.3% from 2024 to 2030” according to authoritative market research reports. The high demand for mobile apps can make it hard to find and hire great mobile app developers. 

To do this, you will need to undertake effective interviews, which makes having the right mobile app developers’ interview questions all the more important. 

You may have chosen to develop native mobile apps.  Mobile app development targeting mobile operating systems like Android and iOS utilizes programming languages like Java/Kotlin and Swift, respectively. Native apps offer the best user experience, security, and performance. 

Alternatively, you can opt for cross-platform development. In that case, you will use mobile frameworks like React Native or Flutter. Cross-platform mobile applications work on both Android and iOS platforms.     

Use the following mobile development interview questions relevant to your choice, to effectively evaluate technical skills. 

Native Android Mobile App Developers Interview Questions

Businesses most often use either Java or Kotlin for native Android development. Here are some example questions for each. Please only use the ones relevant to your choice of Android development language.

Interview questions for junior Android mobile app developers

The following questions are suitable for junior developers:

    1. What are the main advantages of Kotlin over Java?

Conciseness: Kotlin code is often more concise and expressive due to reduced boilerplate code.
Null Safety: Kotlin’s type system prevents NullPointerExceptions at compile time, ensuring code safety and reliability.
Extension Functions: Kotlin allows adding new functions to existing classes without inheriting them, enhancing code readability and reusability.
Coroutines: Kotlin offers coroutines for managing asynchronous operations in synchronous style
Data Classes: Data classes in Kotlin automatically generate equals(), hashCode(), toString(), and copy() methods, simplifying data manipulation and reducing code verbosity.
Sealed Classes: Sealed classes in Kotlin allow defining a restricted hierarchy of types, ensuring exception safety and facilitating work with algebraic data types.

    2. What is the primary difference between an Activity and a Fragment?

Activity: Represents a standalone screen or window in an Android application. It typically corresponds to a single user interaction, such as viewing a list of items or composing a message. Activities have their own lifecycle and can exist independently, being started, paused, resumed, and destroyed based on user interactions and system events.
Fragment: Serves as a reusable portion of an Activity’s UI or behavior. Fragments are modular components that can be combined within a single Activity to create flexible and dynamic user interfaces. They have their own lifecycle but are tightly bound to the lifecycle of their hosting Activity. Fragments enable the decomposition of complex UIs into smaller, more manageable pieces, facilitating code reuse and maintenance.

    3. Explain the layers in Clean Architecture and what each layer is responsible for

Presentation Layer: Handles user interaction and displays data.
Domain Layer: Contains business logic and interfaces.
Data Layer: Manages app data from local and remote sources

    4. How to display different types of items in a RecyclerView?

To display different types of items in a RecyclerView, we need to override the getItemViewType() method in the adapter. This method should return unique values for each item type. Then, create a separate ViewHolder for each item type with its corresponding layout.

    5. What are the methods available for saving the state of an activity or fragment, and how does each method work?

ViewModel: Use ViewModel to store UI-related data, such as UI state or data that needs to survive configuration changes. ViewModel retains its state throughout the activity or fragment lifecycle.
SavedInstanceState: In the onSaveInstanceState() method of the activity or fragment, save data to a Bundle object. This data will automatically be restored during configuration changes or when the activity or fragment is recreated

Mid-level Android mobile app developers interview questions

Interview a mid-level native Android mobile app developer using the following questions:

    1. How do extension functions work, and what do they look like in Java bytecode?

Extension functions in Kotlin are compiled into static methods in a companion Java class. These static methods take the extended class instance as their first argument. This allows Kotlin to add new functionality to existing classes without modifying their source code.

    2. What are the advantages of ListAdapter over RecyclerView.Adapter?

Automatic updates: ListAdapter automatically calculates and updates only changed items, reducing manual updates and enhancing performance.
DiffUtil: DiffUtil provides efficient list comparison, simplifying update processes by identifying fine-grained item changes.
Immutable data convenience: Designed for immutable data collections, it simplifies state management and list updates.
Performance: Automatic change calculations and DiffUtil usage lead to more efficient updates and better performance. Additionally, ListAdapter supports payloads, allowing for partial updates of specific item properties without rebinding the entire item view.

    3. How are functions equals(), hashCode(), toString(), copy() overridden in a data class

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equals(): compares all properties of objects and returns true if they match.
hashCode(): generates a unique hash code for the object based on its properties. If the properties of the object change, its hashcode will also change.
toString(): returns string representation including the class name followed by all properties of the object separated by commas.
copy(): creates a copy of the object with the option to modify its properties. This allows for creating modified copies of objects without changing their original values.

    4. What are the differences between Flow, StateFlow, and Channel, and when should each be used?

– Flow: Flow represents an asynchronous sequential stream of data that can potentially emit an infinite number of elements. It’s used for asynchronous processing of sequences of data items, such as fetching data from a network or database.
StateFlow: StateFlow is a specialized type of flow that represents a mutable state with a single current value. It’s used to represent mutable state in an application, such as current user input or screen state.
Channel: Channel represents a bidirectional asynchronous queue for transferring elements between different coroutines. It can be used for message passing between different parts of an application or for implementing the producer-consumer pattern.

     5. What methods are called during the rendering of a view, and what do they do? What does function invalidate() do?

onMeasure(): Called to measure the dimensions of the view within its parent container. You can change the dimensions here if required.
onLayout(): After measuring the dimensions of the view, called to determine the final position of the view within its parent container.
onDraw(): Called when the view is ready to draw its content on the screen. In this method, the view draws itself using a graphics context.
invalidate(): Called to force a view to redraw itself when its content has changed and needs to be updated on the screen. After calling invalidate(), the system again invokes the onMeasure(), onLayout(), and onDraw() methods to refresh the view.

Questions to evaluate senior Android mobile app developers

It’s a good sign if senior mobile developers can answer the following questions.

    1. What are the points to check if a RecyclerView with complex and diverse views starts to freeze?

Layout Hierarchies: We should avoid complex nested layout hierarchies because it affects view rendering
– Heavy operations in View Holder: All data should be prepared in the background thread and provide a completed model to View Holder. We should avoid complex operations or heavy computations in VH
– Check view recycling: We should ensure that the RecyclerView properly recycles its views and doesn’t create new views
– ListAdapter Usage: It’s better to use ListAdapter the default Recycler View adapter for automatic data set change detection and efficient diffing algorithms
– DiffUtils implementation: If we use ListAdapter with DiffUtils, we should check if it’s correctly implemented and efficiently performing differential updates
– Hardware Acceleration: We can try to enable hardware acceleration (if we didn’t) for smoother UI performance
– Memory leaks: The problem may not directly relate to RecyclerView but could be caused by memory leaks elsewhere in the application

    2. Can you provide examples of how memory leaks can occur in an Android application?

– Static References: Holding references to lifecycle owner or context instances in static fields, such as singletons or static collections
Registered Callbacks and Listeners: Forgetting to unregister callbacks or listeners an keep the objects alive longer than necessary
Views’ references: Holding references to views (always a bad idea) will prevent these views and their contexts from garbage collection even after they will be detached
Bitmaps: After loading any bitmap we should release it. If we don’t it will cause memory leak
References in background threads: If threads are not managed properly and are not terminated when they are no longer needed, they can continue to hold references to objects

    3. What are the differences between Service and WorkManager?

Services serve as a lower-level utility, whereas WorkManager is a library. Both are designed for background processing. Under the hood WorkManager can use Services directly or just employ background threads for execution.

Moreover, WorkManager addresses background work restrictions, ensuring task execution even under resource constraints or in Doze mode. It achieves this by optimizing execution strategies.

Native iOS Mobile App Developers Interview Questions

While earlier iOS programmers developed native apps using Objective-C, Swift is now the preferred option for developing native iOS mobile apps. Naturally, you will assess Swift skills while interviewing developers for a native iOS development project. Use the following interview questions to evaluate iOS mobile app developers of different experience levels:

Junior iOS mobile app developers

    1. Can you explain what Optionals are in Swift and why they are useful?

Optionals are a feature designed to handle the absence of a value. An Optional is a type that represents either a wrapped value or nil, indicating the absence of any value at all. They are particularly useful in Swift, a language emphasizing safety, including type and memory safety.  An Optional variable is declared by appending a `?` to the type of the variable Additionally, it would be great if the candidate would mark one of the following: Optional Binding, Force Unwrapping, or Optional Chaining.

     2. What will the outcome of the following code be if “x” is 10 and “y” is 0? 

“`func divide(_x:Int, by y:Int) ->Int? {

  guard y != 0 else {

     return nil

  }

  return x/y

}

“`

The outcome will be nil. The function includes a guard statement that checks if y is 0. If y is indeed 0, the function will exit early and return nil to signify that division by zero is undefined or not allowed. This helps in safely handling potential runtime errors that would arise from dividing by zero.

    3 Describe how you will create a user-defined data type in Swift to define days of a week in a type-safe manner.

We can use an enumeration or enumerated data type. These user-defined data types can contain a few related values, and we can use them in Swift in a type-safe manner. We should use the “enum” keyword, and we can define the days of the week in the following 2 ways:

Option 1:

enum DaysofaWeek

{

case Monday

case Tuesday

case Wednesday

case Thursday

case Friday

case Saturday

case Sunday

}

Option 2, where we can write only one “case” and separate the values using commas:

Enum DaysofaWeek

{

case Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday

}

    4. State the value of the variable “q” in the following Swift and explain the reasons.

Code example:

var m1 = [21, 23, 25, 27, 29]

var m2 = m1

m2.append(31)

var q = m1.count

Answer:

The variable “q” will have a value of 5. For a better understanding, notice how no change has taken place to the value of the variable “m1”. The above code assigns the value of “m1” to the variable “m2”. Subsequently, it appends another element to “m2”. However, the count of elements in “m1” remained 5.

    5. How would you perform a network request to fetch data from a REST API in iOS? Additionally, explain how you would parse JSON data received from a server.

Answer: To perform a network request and fetch data from a REST API in iOS, and then parse the JSON data received from a server, we can use Swift’s URLSession.

  • First, we need to define the URL for the REST API endpoint from which you want to fetch data.
  • Then, we can use URLSession to create a data task for retrieving the content of the URL.
  • We also need to define a Codable (or Decodable) model—a struct that conforms to the Codable/Decodable protocol. This struct’s properties should match the keys in the JSON data we expect to receive.
  • Next, we execute the data task of the previously defined URLSession and handle the result via a completion handler (converting the JSON data into our model, etc.).

Middle  iOS mobile app developers 

     1. Explain ARC (Automatic Reference Counting) in Swift and how it works.

ARC is a memory management technique used by Swift (and Objective-C) to manage and free up memory used by instances of reference types (classes, closures). When no active references to an object remain, ARC automatically frees up the memory that the object was using. This helps prevent memory leaks and ensures that memory usage is optimized without the developer having to manually manage the object’s lifecycle.

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  • Every time you create a new instance of a class and assign it to a variable, a reference to that instance is made. ARC increments a count of references to that instance;
  • When a variable is no longer needed, or it’s set to nil, ARC decrements the count of references to that instance.

    2. Explain retain cycles and how you handle them when using closures and delegate patterns in Swift.

Retain cycles in Swift are a common memory management issue where two or more objects hold strong references to each other, preventing them from being deallocated by ARC (Automatic Reference Counting). This results in memory leaks because the involved objects are never released.

  • Handling retain cycles while using closures: When you capture `self` in a closure within a class instance, it’s essential to capture self as weak or unowned to avoid retain cycles;
  • Handling retain cycles while using delegates: In delegate patterns, ensure that the delegate is referenced weakly. This is especially important when the delegate is a class instance.
  • Key concepts there are  weak references, unowned references, and strong references

    3. Describe how you will implement the Cocoa “adapter” design pattern in Swift.

In an app, we might get an API structure like the following:

struct InputAPI: Decodable  {

   let idInput: String

   let createdAtInput: TimeInterval

   let createdByInput: String

   let developedByInput: String

   let finishedAtInput: TimeInterval?

Assume that we don’t plan to depend on the API model. Therefore, we must adopt the object using a wrapper and decouple the view-model layer and model layer.

The following is the wrapper or protocol in our app:

protocol InputWork   {

    var idInput: String  { get }

    var createdAtDateInput:  Date  { get }

    var authorInput: String  { get }

    var finishedAtDateInput: Date?  { get }

}

The following code example will adapt the InputAPI, which will conform to the InputWork protocol:

extension InputAPI:  Task  {

   var authorInput: String  { return createdByInput }

   var createdAtDateInput:  Date  { return Date(timeIntervalSince1968: createdAtInput) }

   var finishedAtDateInput: Date?  { return finishedAtInput.map { Date(timeIntervalSince1968: $0) } }

In the above code, the extension InputAPI adapts the InputAPI object to the InputWork protocol. 

Senior iOS mobile app developers

    1. Describe how you can implement thread safety in Swift when reading is more frequent than writing.

When dealing with concurrent programming where reading is more frequent than writing, using synchronization mechanisms that allow multiple reads while blocking writes can significantly improve performance. This scenario is well-suited to a “reader-writer” lock pattern, where many readers can simultaneously access a resource without blocking each other unless a writer is writing.

Possible solutions include:

  • Using `DispatchQueue` with concurrent queues for reads and a barrier block for writes, achievable with GCD (Grand Central Dispatch);
  • Alternative approaches: `NSLock`/`NSRecursiveLock`, `pthread_rwlock_t` (a more outdated and lower-level solution), or by using Actors (the most modern approach, available in Swift 5.5 and later)

    2. I am using the KVO (Key-Value Observing) mechanism in my Swift code. My app requires me to stop observing the changes to the properties of an object even before the deallocation of the object or the observer. I want to prevent memory leaks in this process. How can I achieve this?

You should use the “invalidate()” method of Swift to prevent memory leaks in this process. This method removes observers explicitly. Swift automatically removes observers when the object being observed is deallocated from the memory.

    3. Can you describe a complex app architecture you’ve implemented in the past? Please explain your choice of architecture (e.g., MVVM/VIPER/Clean Architecture/TCA, etc.) and how it helped address specific challenges in the project.

This question evaluates the candidate’s ability to design scalable and maintainable app architectures. The candidate should describe all architectural components (depending on the architecture being described) and explain how these components interact with each other.

Cross-platform React Native Mobile App Developer Interview Questions

A business might sometimes want to develop mobile applications that work on both Android and iOS instead of just one platform. React Native can be a good choice for such a cross-platform development project. This open-source framework is popular for cross-platform development. Evaluate React Native mobile app developers of different experience levels using the following questions:

A Junior React Native Mobile App Developer

Use the following questions when hiring mobile app developers at a junior level:

    1. How should I make a React Native app go back from one screen to the previous screen?

You should use the React Navigation library of React Native, furthermore, you need to use the “goBack” method. The following code example describes its usage:

function MyDemoScreen({ navigation: { goBack } })  {

   return (

      <view>

         <Button onPress={ () => goBack()} title=”go back from MyDemoScreen”/>

      </view>

 );

“ 

    2. How can I execute a certain React Native code after 512 milliseconds?

You can use the “setInterval” method of React Native to execute the required code after 512 milliseconds. Enter “512” in the milliseconds parameter in the following syntax of this method: 

setInterval(function, milliseconds);

Mid-level React Native Mobile App Developers

Interview mid-level React Native mobile developers using the following questions:

     1. I want to prevent performance degradation due to unnecessary renders. Describe a solution.

You can stop unnecessary renders of React Native components. React Native components will re-render by default if their props or states change. That causes too many renders, which is a performance overhead. You can use the React Native “shouldComponentUpdate”, a method to control whether a component should re-render in the case of changes to the property or state.

    2. How can I get the pixel density of different mobile devices in my React Native app?

You should use the “PixelRatio.get()” method of the “PixelRatio” module of React Native. It provides the pixel densities of devices as numbers.  

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Senior React Native Developers

The following questions help you to interview developers claiming to have extensive knowledge and significant past experiences in React Native.

    1. A certain user scenario in my React Native app leads to crashes while all others work. How will I enable users to continue using the parts that are functioning?

You need to prevent the app from crashing and let users continue using the parts of the app that are working. Use the “Error Boundaries” components of React Native. This component prevents the app from crashing and will show users a fall-back user interface.

    2. I notice undesirable spacing in my React Native app layout. Tell me how I can find the root cause. 

Use the “LayoutDebugger” tool offered by React Native. This tool overlays the borders of components. It highlights the dimensions and boundaries of the components. You can then analyze the boundaries and find out discrepancies that might cause undesirable spacing. 

Flutter Interview Questions for Cross-platform Mobile App Development

Many businesses prefer to develop one app with a single source code base that will work on different devices and screen sizes, and hiring mobile app developers with Flutter skills is a great idea for them. A cross-platform development framework written in Dart, Flutter allows native app development for both Android and iOS including their different OS versions. 

Use the following questions to interview Flutter developers of different levels of experience:

Junior Flutter Developers

Junior Flutter mobile app developers should be able to answer the following questions:

     1. How will you use keys to modify a Flutter widget tree containing only stateless widgets?

We can’t modify a Flutter widget tree having only stateless widgets using Flutter keys. Keys in Flutter maintain the state of modified widgets. Stateless Flutter widgets don’t store any state information, therefore, the purpose of Flutter keys is irrelevant to them.   

     2. How can you check if a Flutter variable is null?

In Flutter, we can use the “??” operator for this, which is also called the “null-aware operator”. It checks if a variable is null. We can use this operator to provide a default value if the variable is null.

     3. How can I get the screen size of a device when implementing responsive design in a Flutter app?

You can use the “MediaQuery” class in Flutter to get the screen size and device orientation of a device. Check out the following Dart code example to see its usage:

final MyAppScreenWidth = MediaQuery.of(context).size.width;

if (MyAppScreenWidth > 600)  {

//Write code here for managing larger screens

} else {

//Write code here to manage mobile screens

}

Mid-level Flutter App Developers

Interview mid-level Flutter mobile app developers using the following questions:

    1. How can I display an image from a website in my Flutter app?

You can use the “image.network()” constructor offered by Flutter. It enables you to display an image from a URL in your Flutter app. In the following code example, the app will display a Pixabay image:

Image.network(‘https://pixabay.com/photos/benches-autumn-park-rest-sit-560435/,’)

     2. How should I prevent performance degradation in my Flutter app if I use the “setState” function? 

Do the following to avoid performance degradation when using the “setState” function:

  • Call the function only once per frame.
  • Don’t call this function when there’s no need.
  • Call this method only when the build method changes results meaningfully after detecting a change of state.

     3. I know the value of a Dart variable already, and it won’t change. Between the two code examples below, which approach to defining the variable offers better performance?

Code example 1:

void main() {

// TestVar1 is a constant with the value known even before the coding began

final TestVar1 = “Flutter is developer-friendly”;

Code example 2:

void main() {

// TestVar2 is a constant with the value known even before the coding began

const TestVar2 = “Flutter is developer-friendly”;

Answer: 

The approach in code example 2 will offer better performance. You are using the “const” keyword in this approach, therefore, the value is evaluated at the compile-time itself. The program will not need to assign the value at the runtime, thus delivering better performance. 

Senior Flutter App Development Experts

Assess senior Flutter mobile development professionals using the following questions:

    1. Describe how I can easily change the default animation behavior of the Flutter “ExpansionTile” widget in my app.

You can change the default animation behavior of the “ExpansionTile” widget easily using the “AnimationStyle” widget, which is one of the relatively new features of Flutter. 

Want Expert Mobile App Developers?

The above interview questions should help you with hiring mobile app developers, even if you are a non-technical person. 

However, since your project’s outcome depends on the quality of your app developers, you should strongly consider DevTeam.Space as their first choice for hiring mobile app developers.

We have 1200 expert mobile developers and 62 mobile development teams, all of whom have been thoroughly vetted and work for our platform on a full-time basis. To date, DevTeam.Space’s developers have helped to build and launch hundreds of apps to the Apple App Store or Google Play.  Contact DevTeam.Space for the best mobile app developers or mobile app development teams. 

Frequently Asked Questions

Everything you need to know about and Answers. Can’t find the answer you’re looking for?

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1. How much does it cost to build a mobile app?

The most basic mobile application with just 1 or 2 core functionalities will cost somewhere between $10,000 and $50,000. A more complex enterprise-level app that requires a complete development team comprising of 5 to 12 designers and developers will cost anywhere between $50,000 and $250,000. A complex mobile app that includes multiple innovative core features starts from $250,000 and up. 

App developers don’t need to know all of the programming languages required for native app development. For a native Android mobile app development project, you need developers with either Java or Kotlin skills. A native iOS mobile app developer should know Swift over Objective-C. Assess their SQL skills when evaluating their SQL and NoSQL database skills. 

Soft skills are important in every company, and that’s true in a mobile app agency or clients developing mobile apps. For example, developers should be able to work effectively in a mobile app development team. They need to understand the perspective of users. Developers should be able to explain their code during code review. Soft skills like teamwork, empathy, and communication skills enable them to do these. 

As a part of assessing the software engineering skills of mobile app developers, you should evaluate their knowledge of version control tools. You should expect a mobile app developer to know at least one of the following popular tools for version control:

Git;
Mercurial;
Subversion;
Perforce.

Salary.com puts the average annual salary for a mobile application developer in the United States at $104,863 per year. However, software development companies and freelancer platforms tend to list their developer rates on an hourly basis. Here is a list of mid-level to senior developers’ salaries by country:

USA - $65 - $300 per hour, or $10,400 to $48,000 per month full-time.

Eastern Europe -  $35 - $150 per hour,  or from $5,600 to $24,000 per month full-time.

Western Europe - $50 - $200 per hour, or from $5,600 to $24,000 per month.

India - $10 - $65 per hour, or from $1,600 to $10,400 per month.

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