It’s true that Java misplaced the Android battle to Kotlin, which is now Google’s most well-liked language and due to this fact higher suited to new cell apps. However each Kotlin and Java supply many strengths as general-purpose languages, and it’s necessary for builders to know the language variations, for functions reminiscent of migrating from Java to Kotlin. On this article, we are going to break down Kotlin’s and Java’s variations and similarities so you can also make knowledgeable selections and transfer seamlessly between the 2.

Are Kotlin and Java Related?

Certainly, the 2 languages have rather a lot in widespread from a high-level perspective. Each Kotlin and Java run on the Java Digital Machine (JVM) as an alternative of constructing on to native code. And the 2 languages can name into one another simply: You possibly can name Java code from Kotlin and Kotlin code from Java. Java can be utilized in server-side functions, databases, internet front-end functions, embedded methods and enterprise functions, cell, and extra. Kotlin is equally versatile: It targets the JVM , Android, JavaScript, and Kotlin/Native, and will also be used for server-side, internet, and desktop growth.

Java is a way more mature language than Kotlin, with its first launch in 1996. Although Kotlin 1.0 was launched a lot later, in 2016, Kotlin rapidly turned the official most well-liked language for Android growth in 2019. Exterior of Android, nevertheless, there isn’t any advice to interchange Java with Kotlin.

12 months




JDK Beta, JDK 1.0, JDK 1.1, J2SE 1.2, J2SE 1.3, J2SE 1.4, J2SE 5.0, Java SE 6



Challenge Loom first commit




Kotlin growth began


Java SE 7

Kotlin challenge introduced



Kotlin open sourced


Java SE 8 (LTS)




Kotlin 1.0


Java SE 9

Kotlin 1.2; Kotlin assist for Android introduced


Java SE 10, Java SE 11 (LTS)

Kotlin 1.3 (coroutines)


Java SE 12, Java SE 13

Kotlin 1.4 (interoperability for Goal-C and Swift); Kotlin introduced as Google’s most well-liked language for builders


Java SE 14, Java SE 15



Java SE 16, Java SE 17 (LTS)

Kotlin 1.5, Kotlin 1.6


Java SE 18, JDK 19 EAB (Challenge Loom)

Kotlin 1.7 (alpha model of Kotlin K2 compiler)

Kotlin vs. Java: Efficiency and Reminiscence

Earlier than detailing Kotlin’s and Java’s options, we’ll look at their efficiency and reminiscence consumption as these components are typically necessary issues for builders and purchasers.

Kotlin, Java, and the opposite JVM languages, though not equal, are pretty related when it comes to efficiency, no less than when in comparison with languages in different compiler households like GCC or Clang. The JVM was initially designed to focus on embedded methods with restricted assets within the Nineties. The associated environmental necessities led to 2 predominant constraints:

  • Easy JVM bytecode: The present model of JVM, through which each Kotlin and Java are compiled, has solely 205 directions. Compared, a contemporary x64 processor can simply assist over 6,000 encoded directions, relying on the counting methodology.
  • Runtime (versus compile-time) operations: The multiplatform strategy (“Write as soon as and run wherever”) encourages runtime (as an alternative of compile-time) optimizations. In different phrases, the JVM interprets the majority of its bytecode into directions at runtime. Nonetheless, to enhance efficiency, you might use open-source implementations of the JVM, reminiscent of HotSpot, which pre-compiles the bytecode to run sooner via the interpreter.

With related compilation processes and runtime environments, Kotlin and Java have solely minor efficiency variations ensuing from their distinct options. For instance:

  • Kotlin’s inline capabilities keep away from a operate name, enhancing efficiency, whereas Java invokes extra overhead reminiscence.
  • Kotlin’s higher-order capabilities keep away from Java lambda’s particular name to InvokeDynamic, enhancing efficiency.
  • Kotlin’s generated bytecode comprises assertions for nullity checks when utilizing exterior dependencies, slowing efficiency in comparison with Java.

Now let’s flip to reminiscence. It’s true in principle that the usage of objects for base sorts (i.e., Kotlin’s implementation) requires extra allocation than primitive knowledge sorts (i.e., Java’s implementation). Nonetheless, in apply, Java’s bytecode makes use of autoboxing and unboxing calls to work with objects, which may add computational overhead when utilized in extra. For instance, Java’s String.format methodology solely takes objects as enter, so formatting a Java int will field it in an Integer object earlier than the decision to String.format.

On the entire, there are not any vital Java and Kotlin variations associated to efficiency and reminiscence. It’s possible you’ll look at on-line benchmarks which present minor variations in micro-benchmarks, however these can’t be generalized to the dimensions of a full manufacturing utility.

Distinctive Characteristic Comparability

Kotlin and Java have core similarities, however every language gives totally different, distinctive options. Since Kotlin turned Google’s most well-liked language for Android growth, I’ve discovered extension capabilities and specific nullability to be essentially the most helpful options. Alternatively, when utilizing Kotlin, the Java options that I miss essentially the most are the protected key phrase and the ternary operator.

From left to right are shown a white Variable oval, an equals sign, a green First Expression box, a question mark, a dark blue Second Expression box, a colon, and a light blue Third Expression box. The First Expression box has two arrows: one labeled “Is True” points to the Second Expression box, and the second labeled “Is False” points to the Third Expression box. Second Expression and Third Expression each have their own Return Value arrow pointing to the Variable oval.
The Ternary Operator

Let’s look at a extra detailed breakdown of options accessible in Kotlin versus Java. It’s possible you’ll observe together with my examples utilizing the Kotlin Playground or a Java compiler for a extra hands-on studying strategy.





Extension capabilities



Lets you lengthen a category or an interface with new functionalities reminiscent of added properties or strategies with out having to create a brand new class:

class Instance {}

// extension operate declaration
enjoyable Instance.printHelloWorld() { println("Hey World!") }

// extension operate utilization

Good casts



Retains monitor of circumstances inside if statements, protected casting routinely:

enjoyable instance(a: Any) {
  if (a is String) {
    println(a.size) // automated forged to String

Kotlin additionally gives protected and unsafe forged operators:

// unsafe "as" forged throws exceptions
val a: String = b as String
// protected "as?" forged returns null on failure
val c: String? = d as? String

Inline capabilities



Reduces overhead reminiscence prices and improves pace by inlining operate code (copying it to the decision web site): inline enjoyable instance().

Native assist for delegation



Helps the delegation design sample natively with the usage of the by key phrase: class Derived(b: Base) : Base by b.

Kind aliases



Gives shortened or customized names for current sorts, together with capabilities and inside or nested lessons: typealias ShortName = LongNameExistingType.

Non-private fields



Affords protected and default (also referred to as package-private) modifiers, along with public and non-public modifiers. Java has all 4 entry modifiers, whereas Kotlin is lacking protected and the default modifier.

Ternary operator



Replaces an if/else assertion with easier and extra readable code:

if (firstExpression) { // if/else
  variable = secondExpression;
} else {
  variable = thirdExpression;

// ternary operator
variable = (firstExpression) ? secondExpression : thirdExpression;

Implicit widening conversions



Permits for automated conversion from a smaller knowledge sort to a bigger knowledge sort:

int i = 10;
lengthy l = i; // first widening conversion: int to lengthy
float f = l; // second widening conversion: lengthy to drift

Checked exceptions



Requires, at compile time, a technique to catch exceptions with the throws key phrase or handles exceptions with a try-catch block.

Be aware: Checked exceptions had been meant to encourage builders to design sturdy software program. Nonetheless, they’ll create boilerplate code, make refactoring tough, and result in poor error dealing with when misused. Whether or not this characteristic is a professional or con is dependent upon developer choice.

There’s one subject I’ve deliberately excluded from this desk: null security in Kotlin versus Java. This subject warrants a extra detailed Kotlin to Java comparability.

Kotlin vs. Java: Null Security

For my part, non-nullability is among the best Kotlin options. This characteristic saves time as a result of builders don’t should deal with NullPointerExceptions (that are RuntimeExceptions).

In Java, by default, you may assign a null worth to any variable:

String x = null;
// Operating this code throws a NullPointerException
strive {
    System.out.println("First character: " + x.charAt(0));
} catch (NullPointerException e) {
    System.out.println("NullPointerException thrown!");

In Kotlin, then again, now we have two choices, making a variable nullable or non-nullable:

var nonNullableNumber: Int = 1

// This line throws a compile-time error as a result of you may't assign a null worth
nonNullableNumber = null

var nullableNumber: Int? = 2

// This line doesn't throw an error since we used a nullable variable
nullableNumber = null

I exploit non-nullable variables by default, and decrease the usage of nullable variables for finest practices; these Kotlin versus Java examples are supposed to exhibit variations within the languages. Kotlin inexperienced persons ought to keep away from the entice of setting variables to be nullable and not using a goal (this may additionally occur if you convert Java code to Kotlin).

Nonetheless, there are a couple of circumstances the place you’ll use nullable variables in Kotlin:

State of affairs


You might be looking for an merchandise in a listing that’s not there (often when coping with the info layer).

val listing: Listing<Int> = listOf(1,2,3)
val searchResultItem = listing.firstOrNull { it == 0 }
searchResultItem?.let { 
  // Merchandise discovered, do one thing 
} ?: run { 
  // Merchandise not discovered, do one thing

You need to initialize a variable throughout runtime, utilizing lateinit.

lateinit var textual content: String

enjoyable runtimeFunction() { // e.g., Android onCreate
  textual content = "First textual content set"
  // After this, the variable can be utilized

I used to be responsible of overusing lateinit variables once I first bought began with Kotlin. Finally, I ended utilizing them virtually fully, besides when defining view bindings and variable injections in Android:

@Inject // With the Hilt library, that is initialized routinely
lateinit var supervisor: SomeManager

lateinit var viewBinding: ViewBinding

enjoyable onCreate() { // i.e., Android onCreate

  binding = ActivityMainBinding.inflate(layoutInflater, parentView, true)
  // ...

On the entire, null security in Kotlin gives added flexibility and an improved developer expertise in comparison with Java.

Shared Characteristic Variations: Transferring Between Java and Kotlin

Whereas every language has distinctive options, Kotlin and Java share many options too, and it’s needed to know their peculiarities in an effort to transition between the 2 languages. Let’s look at 4 widespread ideas that function in a different way in Kotlin and Java:




Information switch objects (DTOs)

Java data, which maintain details about knowledge or state and embrace toString, equals, and hashCode strategies by default, have been accessible since Java SE 15:

public file Worker(
  int id,
  String firstName,
  String lastName

Kotlin knowledge lessons operate equally to Java data, with toString, equals, and copy strategies accessible:

knowledge class Worker(
  val id: Int,
  val firstName: String,
  val lastName: String

Lambda expressions

Java lambda expressions (accessible since Java 8) observe a easy parameter -> expression syntax, with parentheses used for a number of parameters: (parameter1, parameter2) -> { code }:

ArrayList<Integer> ints =
  new ArrayList<>();
ints.forEach( (i) ->
  { System.out.println(i); } );

Kotlin lambda expressions observe the syntax { parameter1, parameter2 -> code } and are at all times surrounded by curly braces:

var p: Listing<String> =
  listOf("firstPhrase", "secondPhrase")
val isShorter = { s1: String,
  s2: String -> s1.size < s2.size }


Java threads make concurrency potential, and the java.util.concurrency package deal permits for straightforward multithreading via its utility lessons. The Executor and ExecutorService lessons are particularly helpful for concurrency. (Challenge Loom additionally gives light-weight threads.)

Kotlin coroutines, from the kotlinx.coroutines library, facilitate concurrency and embrace a separate library department for multithreading. Kotlin 1.7.20’s new reminiscence supervisor reduces earlier limitations on concurrency and multithreading for builders shifting between iOS and Android.

Static conduct in lessons

Java static members facilitate the sharing of code amongst class situations and make sure that solely a single copy of an merchandise is created. The static key phrase might be utilized to variables, capabilities, blocks, and extra:

class Instance {
    static void f() {/*...*/}

Kotlin companion objects supply static conduct in lessons, however the syntax will not be as easy:

class Instance {
    companion object {
        enjoyable f() {/*...*/}

In fact, Kotlin and Java even have various syntaxes. Discussing each syntax distinction is past our scope, however a consideration of loops ought to provide you with an thought of the general state of affairs:

Loop Kind



for, utilizing in

for (int i=0; i<=5; i++) {
  System.out.println("printed 6 occasions");
for (i in 0..5) {
  println("printed 6 occasions")

for, utilizing till

for (int i=0; i<5; i++) {
  System.out.println("printed 5 occasions");
for (i in 0 till 5) {
  println("printed 5 occasions")


Listing<String> listing = Arrays.asList("first", "second");

for (String worth: listing) {
var listing: Listing<String> =
  listOf("first", "second")

listing.forEach {


int i = 5;
whereas (i > 0) {
  System.out.println("printed 5 occasions");
var i = 5
whereas (i > 0) {
  println("printed 5 occasions")

An in-depth understanding of Kotlin options will help in transitions between Kotlin and Java.

Android Challenge Planning: Extra Issues

We’ve examined many necessary components to consider when deciding between Kotlin and Java in a general-purpose context. Nonetheless, no Kotlin versus Java evaluation is full with out addressing the elephant within the room: Android. Are you making an Android utility from scratch and questioning in the event you ought to use Java or Kotlin? Select Kotlin, Google’s most well-liked Android language, indubitably.

Nonetheless, this query is moot for current Android functions. In my expertise throughout a variety of purchasers, the 2 extra necessary questions are: How are you treating tech debt? and How are you caring for your developer expertise (DX)?

So, how are you treating tech debt? In case your Android app is utilizing Java in 2022, your organization is probably going pushing for brand new options as an alternative of coping with tech debt. It’s comprehensible. The market is aggressive and calls for a quick turnaround cycle for app updates. However tech debt has a hidden impact: It causes elevated prices with every replace as a result of engineers should work round unstable code that’s difficult to refactor. Firms can simply enter a unending cycle of tech debt and price. It could be value pausing and investing in long-term options, even when this implies large-scale code refactors or updating your codebase to make use of a contemporary language like Kotlin.

And the way are you caring for your builders via DX? Builders require assist throughout all ranges of their careers:

  • Junior builders profit from correct assets.
  • Mid-level builders develop via alternatives to guide and educate.
  • Senior builders require the facility to architect and implement lovely code.

Consideration to DX for senior builders is very necessary since their experience trickles down and impacts all engineers. Senior builders like to study and experiment with the newest applied sciences. Maintaining with newer traits and language releases will permit your staff members to achieve their best potential. That is necessary whatever the staff’s language alternative, although totally different languages have various timelines: With younger languages like Kotlin, an engineer engaged on legacy code can fall behind traits in lower than one 12 months; with mature languages like Java, it should take longer.

Kotlin and Java: Two Highly effective Languages

Whereas Java has a variety of functions, Kotlin has undeniably stolen its thunder as the popular language for the event of recent Android apps. Google has put all of its efforts into Kotlin, and its new applied sciences are Kotlin-first. Builders of current apps would possibly contemplate integrating Kotlin into any new code—IntelliJ comes with an automated Java to Kotlin device—and will look at components that attain past our preliminary query of language alternative.

The editorial staff of the Toptal Engineering Weblog extends its gratitude to Thomas Wuillemin for reviewing the code samples and different technical content material offered on this article.

Additional Studying on the Toptal Engineering Weblog:

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