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When Google announced Kotlin as an official programming language for Android, its popularity among some regions saw a more than sixfold increase. Kotlin is a great fit for developing Android applications due to its 100% interoperability and high performance.

According to Pusher, 2018 -

Kotlin developers rank null safety (81%), extension functions (64%) and Java interoperability (61%) as their favorite features of the language.

According to TechCrunch, “Kotlin is now Google’s preferred language for Android app development.”

Android development will become increasingly Kotlin-first. Code-written in Kotlin often means much less code - less code to type, test, and maintain.

Java had long been the preferred language for Android app development. Over the course of the last two years, the popularity of Kotlin has only increased. Over 50% of professional Android developers now use Kotlin as the language to develop their apps, according to Stack Overflow developer survey. Kotlin has ranked as the second-most-loved programming language. More developers want to continue working with Kotlin.

What is Java?

Released in 1995, Java is an object-oriented statically-typed programming language and computing platform. Acquired by Oracle Corporation, Java is employed in the development of Android apps, server apps, web apps, websites, and embedded systems. Many developers still prefer Java for Android app development. It boasts the title of the second-most active language on Github. And, there is no wonder, Java has been around for over 20 years, and its popularity only seems to grow.

Advantages of Java

  • Easy to learn, understand & flexible
  • A good choice for cross-platform apps
  • Java has a large open-source ecosystem
  • Java apps are more compact as compared to Kotlin. Java apps tend to be lighter.
  • Java possesses faster build process than Kotlin.

Disadvantages of Java

  • Java comes with limitations such as it causes problems with Android API design.
  • Being a verbose language, Java requires writing more code as it carries a higher risk of bugs and errors.
  • This programming language is slower in comparison to many other languages since it requires a lot of memory.

Where it is Used?

Check out the following examples of prominent open source apps, which are written in Java:

What is Kotlin?

Designed by programmers from JetBrains, Kotlin,  is a cross-platform, statistically typed and general-purpose programming language with type interference. It targets the JVM, Android, JavaScript, and Native. An open source language developed under the Apache 2.0 license, Kotlin combines both object-oriented and functional programming features. Kotlin possesses the support of all major Java IDEs including Android Studio, Eclipse, and NetBeans.

Advantages of Kotlin

  • It is easier to switch from Java to Kotlin.
  • Kotlin includes smart extension functions to help developers build clean APIs.
  • It has null in its type system. Nullability problems are a common pain point in Java. Since Android often uses null to represent the absence of a value. Kotlin solves it by placing null directly in its type system.
  • Kotlin is more concise than Java. It means fewer opportunities for errors.
  • Adopting Kotlin does not cost anything.
  • Kotlin is compatible with all Java libraries and frameworks, the JVM (Java Virtual Machine) and can be integrated with Gradle or Maven build systems.

Disadvantages of Kotlin

  • Slower compilation speed than Java. Although, Kotlin was shown to beat Java.
  • It has a smaller developer community. This means limited learning resources and difficulty in finding answers to questions.
  • Experienced Kotlin developers are still a rarity. So, finding a mentor for your team might be tricky.
  • The Android Studio’s auto-complete and compilation inclines to run slower in comparison to pure Java project.

Where it is Used?

Check out two popular apps written in Kotlin:

Kotlin and Java Syntax Comparison

Like Java, C and C++, Kotlin is statically typed programming language. Statically typed programming languages are those languages in that variables do not need to be defined before they are used. It means that static typing has to do with explicit declaration or initialization of variables before they are employed.

In static typing, it does not mean that it is mandatory to declare all the variables first before we use. Variables may be initialized anywhere in the program and we as developers have to do so, to use those variables in the program when there is a requirement. Let’s take an example -

* Java Code */

static int num1, num2; //explicit declaration

num1 = 60; //use the variables anywhere

num2 = 55;

/* Kotlin Code*/

val a: Int

val b: Int

a = 78

b = 100

Additionally, Kotlin also supports procedural programming with the use of functions. Like in Java, C and C++, the entry point to a Kotlin program is a function named “main”. Basically, it passed an array containing any command line arguments.

According to Google Trends, you can see the popularity of programming languages, Java and Kotlin, which is alive among Android developers. This is what you can check in the below graph.

Difference Between Kotlin and Java

  • Null Safety: Kotlin avoids NullPointerException. Kotlin fails at compile-time whenever a NullPointerException may be thrown.
  • Data Classes: In Kotlin, there are Data Classes that leads to autogeneration of boilerplate such as equals, hashCode, toString, getters/setters.

/* Java Code */

Class Book {

private String title;

private Author author;

public String getTitle () {

return title;


public void setTitle(String title) {

 this.title = title;


public Author getAuthor() {

return author;


public void setAuthor (Author author) {

this.author = author;



On the other hand, the above same class in Kotlin can define concisely in one line -

/* kotlin Code */

data class Book(var title: String,

Var author: Author)

It also allows us to make copies of data classes with the help of copy()   —

Val book = Book(“Kotlin”, “JetBrains”)

Val copy = book.copy()

  • Extension Functions: Kotlin allows developers to extend the functionality of exciting classes without inheriting from them. That means Kotlin provides the ability to extend a class with new functionality without having to inherit from the class. It is done by extension functions. For declaring an extension function, the developer needs to prefix its name with receiver type, i.e. the type being extended. The following example adds a swap function to MutableList<List>  —

fun MutableList <Int> .swap(index1: Int, index2: Int) {

 val tmp = this[index]

 this[index1] = this[index2]

 this[index2] = tmp


The ‘this’ keyword inside an extension function corresponds to the receiver object, the one which is passed before the dot. We can now call such a function on any MutableList<Int>  —

Val abc = mutableListOf (1, 2, 3)

abc. swap (0,2)

  • Smart Casts: When it comes to casts, Kotlin compiler is intelligent. In many cases, you do not need to use explicit cast operators in Kotlin, but in Kotlin there is “is-checks” for immutable values and inserts cast automatically when needed -

fun demo(x: Any) {

if (x is String) {

 print(x.length) // x is automatically cast to string



  • Type Inference: A great thing about Kotlin is that you don’t have to specify the type of each variable explicitly (in clear and detailed manner). However, if you want to define a data type explicitly, you can also do this. Let’s check the following example -

/* not explicitly defined */

fun main (args: Array <String> ) {

val text = 20



/* explicitly defined */

fun main(args: Array < String > ) {

val text: Int = 20



  • Functional Programming: In Kotlin, the important thing is a functional programming language. Basically, Kotlin consists of various useful methods that include higher-order functions, lambda expressions, operator overloading, lazy evaluation, operator overloading, and so many things. Functional programming makes Kotlin handy when it comes to collections -

fun main(args: Array < String > )  {

val numbers = arrayListOf (20, -5, 33, -39)

val nonNegativeNumbers = numbers.filter {

 it  >=  0


println (nonNegativeNumbers)


Output is - 20, 33

        Higher-order functions are those functions which take functions as a parameter. It also returns a function. Let’s check out the following code:

fun alphaNum(fun: () ->Unit) {}

        In the above code, “func” is the name of the parameter and “ () -> Unit” is the function type. In that case, we can say func will be a function, which does not receive any parameters and does not return any value.

    Lambda expression or an anonymous function is a “function literal” that means a function, which is not declared but passed immediately as an expression.

Here’s an example of a Lambda Expression -

Val sum: (Int, Int) - > Int = {


y - > x+ y


In the above Lambda Expression, we simply declare a variable ‘sum’ that take two integers and adds them together and returns total as an integer.

Anonymous Function is a function that allows developers to specify the return type and in this, the function name is omitted. Let us the following example -

Either this way -

fun(x: Int, y: Int) : Int = x + y

Or, this way -

fun(X: Int, y: Int) : Int {

return a + b


Features of Java and Kotlin Language

1. Easy Learning Curve


  • The developers who know Java can easily learn Kotlin. Switching from Java to Kotlin does not have much overhead. Plus, adoption of Kotlin does not cost anything.
  • With Kotlin, there is much faster and playful Android app development process.


2. Security


The debate on Kotlin and Java proved that Kotlin is more secure so that there is a smaller chance of crashes. Kotlin is more flexible and it provides more possibilities, which help to reduce the number of lines written, as a result, fewer bugs and errors occur.


Java is labeled as a “secure language”. When these byte codes are executed, the JVM (Java Virtual Machine) can take care of security. Java security is enabled in each stage. JVM is an interpreter that is installed in each client machine and it is updated with the latest security updates on the internet. When these byte codes are executed, the JVM takes care of security.

Java is said to be more secure than other programming languages.  

  • As Java program runs inside its own virtual machine sandbox.
  • Data hiding in Java (OOPs) makes it one of the secured languages.
  • No use of pointers preventing unauthorized access to the memory block
  • No access to the memory management
  • Access Control Functionality
  • Exception Handling
  • Use of final keyword

3. Interoperability

Kotlin looks and feels slightly different than Java while writing and reading it. But, it does not mean that it is different in its functionality. Kotlin is 100% compatible with existing Java code. Kotlin compiles to the exact same bytecode and making it interoperable in the JVM or on Android. Kotlin programs have complete access to all existing Java frameworks and libraries. It even applies to frameworks, which rely on annotation processing or other advanced prerequisites.  There are no wrappers or adapter layers involved in using Kotlin code without the need for you to do any rewriting by hand. Kotlin’s core purpose is interoperability. Developers can simply write modules in Kotlin, which works flawlessly within existing Java code. By eliminating the Bytecode, a Kotlin compiler allows two languages to work in unison in the same project.  You can also continue to work on your old Java projects using Kotlin. As all your favorite Java frameworks are still available and whatever framework you will write in Kotlin is easily adopted by the one who loves Java.

4. Multithreading


Creating too many threads can make an application slow. Threads are objects that impose overhead during object allocation and garbage collection. In Kotlin, multi-threading is a feature, which allows concurrent execution of two or more parts of a program for maximum utilization of CPU. Each part of such a program is called a thread. Threads are light-weight processes within a process. In Kotlin, there are multiple ways of performing multithreading just like Java such as:

  • Lambda expressions
  • By extending thread class
  • By implementing Runnable interface


A thread is a lightweight subprocess and the smallest unit of processing. Threads are independent. If there occurs an exception in one thread, it does not affect other threads. However, it uses a shared memory area.

Image Credits: javatpoint

As shown in the image, a thread is executed inside the process. There is context-switching between the threads. There can also be multiple processes inside the OS and one process can have multiple threads. In Java, multithreading is a process of executing multiple threads at the same time. A thread is a lightweight sub-process and the smallest unit of processing. Multithreading and multiprocessing, both are used to achieve multitasking. Here are a few advantages of multithreading in Java:

  • It does not block the user because threads are independent.
  • You can perform multiple operations together as it saves time.
  • Threads are independent, so it does not affect other threads if an exception occurs in a single thread.

5. Performance


Java build takes time an average of 15.5 seconds, while Kotlin average 18.5 seconds; an increase of 17% Kotlin is not off to a great start, but it is how most people will compile their code. Java compiles 10-15% faster for clean faster. For incremental compilation, Kotlin shows identical to Java or even slightly better results. The decreased compile means less unproductive time for a developer.


In comparison with Java, the code compilation process is slower.

According to Statistics ~

For clean builds with no Gradle daemon, Java compiles 17% faster than Kotlin.

On the other hand, for consecutive incremental builds with no files or one isolated/core file changed, the compilation time is almost similar. Kotlin is a bit slower than Java.

When Should Make You Switch to Kotlin?

Undoubtedly, when it comes to Android development, the first thing came in mind is - Java. Kotlin appeared as a new language for JVM (Java Virtual Machine) in 2011. Kotlin is easy to understand and making easier to develop apps for developers. Here we are going to compare the features of Java and Kotlin and there are some of the reasons when should you prefer Kotlin over Java:

Developers can interoperate Kotlin with Java that means they can carry on their work with old java projects with Kotlin. Whatever the developer can write in Kotlin is easily adopted by Java. The best thing is -  the most of the Java frameworks are available in Kotlin.

For instance, if you are working with outsource team and your projects demand an in-house team all of a sudden, how difficult it is to learn a completely new programming language? In this case, switching from Java to Kotlin can offer a perfect solution. JetBrains also created Java-Kotlin converter, which helps app developers resolve such issues.

The possibility of switching from one language to another depends on the size of your project. Here are a few cases -

  • If your project is small (around 160-200 hours) and there is no complex architecture, you should consider switching to Kotlin.
  • If your project is medium (up to 5-6 months), it is pretty convenient impossible to switch into Kotlin.
  • If your project is large (more than 12 months), it is completely impossible to switch into Kotlin.

And, Why Java is Bad?

By using Kotlin as a programming language, the risk is minimal. It is also possible to integrate new features built in Java to your codebase in later stages. Due to the fact that the complete Java interop of Kotlin programming language.

With Kotlin, code quality improves along with less code, and less bugs. Null-safety improves the stability of your projects in most cases. On the other hand, in Java, you’ll not get all these advantages, which you would get from Kotlin.

When Should Make You Switch to Java?

Kotlin has a lot to offer to Android developers, there are still some drawbacks, which developers need to keep in mind. Kotlin may have excellent support in Android studio, but it is not officially endorsed by Google. Plus, Android Studio’s auto-compilation tends to run a bit slower when working with Kotlin. If you need all this, it is better to switch to Java over Kotlin.

Over to You: The Final Verdict

Both Android programming languages, Kotlin and Java, have their own strong points. Choosing the right one, you will need to consider the long-term strategic aspects of both platforms, including the fact that Google is moving towards to Kotlin and slowly moving away from Java.

Making the right choice for your Android mobile app development can be tricky if there is a lacking of a complete range of skills and team. We highly recommend you take professional advice before hiring a mobile app development company or partner.

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