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Kotlin Interview Questions and Answers

Kotlin Interview Questions and Answers

Question - 1 : - What is Kotlin? / Describe Kotlin in brief.

Answer - 1 : -

Kotlin is a general-purpose, statically typed, open-source programming language that runs on the JVM. It runs on JVM and can be used anywhere Java is used today. It can be compiled either using Java source code or LLVM compiler. It is generally used to develop Android apps, server-side apps, and much more.

Question - 2 : - What do you know about the history of Kotlin?

Answer - 2 : -

Kotlin was developed by the JetBrains team. This project was started in 2010 to develop a language for Android apps development, and officially its first version was released in February 2016. Kotlin was developed under the Apache 2.0 license.

Question - 3 : - What are the most important features of Kotlin?

Answer - 3 : -

The most popular features of kotlin are:

  1. Kotlin is Concise: Kotlin reduces the writing of the extra codes, making Kotlin more concise.
  2. Compact code: Kotlin is an OOPs-based programming language. Its code lines may be reduced by up to 40% compared to Java, making it an excellent choice for software development.
  3. Kotlin is Simple: Kotlin is a simple language to learn. When working with Kotlin, compiling the code is simple, resulting in improved performance for Android development. It also explains which types of data functions can be used throughout the code.
  4. Open Source: Kotlin is open source for Android, and it uses the JVM to combine the benefits of OOPs and functional programming.
  5. Null safety: Kotlin is null safety language. Kotlin aimed to eliminate the NullPointerException (null reference) from the code.
  6. A high number of extensions: Kotlin supports various extension functions and extension properties without modifying the code. It means that it can help to extend the functionality of classes without touching their code. Kotlin may support a variety of extension functions to help developers make existing code more appealing and wonderful.
  7. Full Java Interoperability: Kotlin provides full interoperability for Java code. Java code can utilize Kotlin code, and Kotlin code can use Java code. So, if you are familiar with OOPs programming and good in Java programming language, you can switch to Kotlin development easily. Also, if there are any Java-based applications, they can be used with Kotlin's environment.
  8. Smart Cast: Kotlin supports the smart cast technique. By using this technique, we can reduce the cost of an application while also improving its speed and performance. It technique uses typecasting or immutable data to manage the efficiency of programming. \
  9. Low Learning Curve: Many businesses prefer Kotlin because of its low adoption cost. Most significantly, it is simple for developers to learn, especially if they have programming experience.
  10. Compilation Time: Kotlin is faster and better than Java in terms of its performance and fast compilation time.
  11. Tools-friendly: Kotlin is Tools-friendly. You can build the Kotlin programs by using the command line as well as any of Java IDE.

Question - 4 : - Why did you switch to Kotlin from Java? Why do some developers like to switch to Kotlin from Java?

Answer - 4 : -

The Kotlin programing language seems to be simpler and cleaner than Java. It removes a lot of redundancies in code as compared to Java. Kotlin also offers some useful features that Java doesn't yet support, making the code more idiomatic. Kotlin has been added to Android Studio's list of supported languages recently. So, there is much to expect from Kotlin in easing out the development efforts and good support in the future.

Question - 5 : - How does Kotlin work on Android?

Answer - 5 : -

Kotlin is very much similar to the Java programming language. Like Java, the Kotlin code is also compiled into the Java bytecode and executed at runtime by the Java Virtual Machine, i.e., JVM. For example, when a Kotlin file named Main.kt is compiled, it will eventually turn into a class, and then the bytecode of the class will be generated. The name of the bytecode file will be MainKt.class, and this file will be executed by the JVM.

Question - 6 : - What is the difference between the variable declaration with var and variable declaration with val?

Answer - 6 : -

The variable declaration with var and the variable declaration with val is used for different purposes. If you want to declare some mutable (changeable) variable, you should use var. If you want to declare the immutable variable, you should use val because val variables can't be changed once you have assigned them.

Question - 7 : - What is the difference between the variable declaration with val and variable declaration with const?

Answer - 7 : -

Both the variables that are declared with val and const are immutable in nature. But the difference between the variable declaration with val and variable declaration with const is that the value of the const variable must be known at the compile-time. In contrast, the value of the val variable can be assigned at runtime also.

Question - 8 : - How can you create a singleton in Kotlin?

Answer - 8 : -

We can create a singleton in Kotlin by using an object.


object SomeSingleton  
The above Kotlin object will be compiled to the following equivalent Java code:

public final class SomeSingleton {  
   public static final SomeSingleton INSTANCE;  
   private SomeSingleton() {  
      INSTANCE = (SomeSingleton)this;  
      System.out.println("init complete");  
   static {  
      new SomeSingleton();  
The above way is preferred to implement singletons on a JVM because it enables thread-safe lazy initialization without relying on a locking algorithm like the complex double-checked locking.

Question - 9 : - What is a primary constructor in Kotlin?

Answer - 9 : -

In Kotlin, the primary constructor is a part of the class header. Unlike Java, it doesn't need you to declare a constructor in the body of the class.

Kotlin facilitates you to declare the constructor in the class header itself:

See the following example:

class Person constructor(name: String, age: Int, salary: Int) {  
Just like functions or methods, it takes a series of parameters with their type. These parameters initialize the variables present in the class.

If you do not have any annotations or modifiers (public, private, protected), you can omit the constructor keyword like the following example.

class Person (name: String, age: Int, salary: Int) {  
By removing the constructor keyword, you can get code that is simplified and easy to understand.

Question - 10 : - What do you understand by Null safety in Kotlin?

Answer - 10 : -

Null safety is one of the major advantages of using Kotlin. Kotlin's type system ensures eliminating the danger of null references from code, also known as The Billion Dollar Mistake. One of the most common pitfalls in many programming languages, including Java, is that accessing a member of a null reference will result in a null reference exception. In Java, this would be the equivalent of a NullPointerException or NPE for short.

In Kotlin, the type system distinguishes between references that can hold null (nullable references) and those that cannot (non-null references). For example, a regular variable of type String can not hold null:

var a: String = "abc"  
a = null // compilation error  
To allow nulls, we can declare a variable as nullable string, written "String?":

var b: String? = "abc"  
b = null // ok  

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