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Kotlin vs. Java: All-purpose Makes use of and Android Apps

It’s true that Java misplaced the Android battle to Kotlin, which is now Google’s most popular language and subsequently higher suited to new cellular apps. However each Kotlin and Java provide many strengths as general-purpose languages, and it’s necessary for builders to know the language variations, for functions similar to 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 choices and transfer seamlessly between the 2.

Are Kotlin and Java Comparable?

Certainly, the 2 languages have lots in frequent from a high-level perspective. Each Kotlin and Java run on the Java Digital Machine (JVM) as a substitute 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, net front-end functions, embedded techniques and enterprise functions, cellular, and extra. Kotlin is equally versatile: It targets the JVM , Android, JavaScript, and Kotlin/Native, and may also be used for server-side, net, 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 shortly turned the official most popular language for Android growth in 2019. Exterior of Android, nevertheless, there isn’t a suggestion to switch 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 undertaking introduced



Kotlin open sourced


Java SE 8 (LTS)




Kotlin 1.0


Java SE 9

Kotlin 1.2; Kotlin help 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 popular 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 study their efficiency and reminiscence consumption as these components are usually necessary issues for builders and shoppers.

Kotlin, Java, and the opposite JVM languages, though not equal, are pretty comparable by way of 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 techniques with restricted sources within the Nineties. The associated environmental necessities led to 2 essential constraints:

  • Easy JVM bytecode: The present model of JVM, wherein each Kotlin and Java are compiled, has solely 205 directions. As compared, a contemporary x64 processor can simply help over 6,000 encoded directions, relying on the counting technique.
  • Runtime (versus compile-time) operations: The multiplatform method (“Write as soon as and run anyplace”) encourages runtime (as a substitute 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, similar to HotSpot, which pre-compiles the bytecode to run quicker by means of the interpreter.

With comparable 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 further 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 idea that using objects for base varieties (i.e., Kotlin’s implementation) requires extra allocation than primitive information varieties (i.e., Java’s implementation). Nonetheless, in follow, Java’s bytecode makes use of autoboxing and unboxing calls to work with objects, which might add computational overhead when utilized in extra. For instance, Java’s String.format technique 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 aren’t any vital Java and Kotlin variations associated to efficiency and reminiscence. You might study on-line benchmarks which present minor variations in micro-benchmarks, however these can’t be generalized to the size of a full manufacturing utility.

Distinctive Function Comparability

Kotlin and Java have core similarities, however every language provides totally different, distinctive options. Since Kotlin turned Google’s most popular language for Android growth, I’ve discovered extension capabilities and express nullability to be probably the most helpful options. Then again, when utilizing Kotlin, the Java options that I miss probably 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 study a extra detailed breakdown of options out there in Kotlin versus Java. You might observe together with my examples utilizing the Kotlin Playground or a Java compiler for a extra hands-on studying method.





Extension capabilities



Lets you lengthen a category or an interface with new functionalities similar to 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 observe of situations inside if statements, protected casting mechanically:

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

Kotlin additionally supplies protected and unsafe solid operators:

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

Inline capabilities



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

Native help for delegation



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

Sort aliases



Gives shortened or customized names for current varieties, together with capabilities and internal or nested courses: typealias ShortName = LongNameExistingType.

Non-private fields



Provides protected and default (often known as package-private) modifiers, along with public and personal 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 computerized conversion from a smaller information sort to a bigger information 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 way to catch exceptions with the throws key phrase or handles exceptions with a try-catch block.

Word: Checked exceptions had been supposed to encourage builders to design strong software program. Nonetheless, they will create boilerplate code, make refactoring troublesome, and result in poor error dealing with when misused. Whether or not this function is a professional or con will depend on developer desire.

There may be 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

In my view, non-nullability is among the biggest Kotlin options. This function saves time as a result of builders don’t must deal with NullPointerExceptions (that are RuntimeExceptions).

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

String x = null;
// Working 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, we have now 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 possibly can'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 take advantage of non-nullable variables by default, and decrease using nullable variables for finest practices; these Kotlin versus Java examples are supposed to show variations within the languages. Kotlin freshmen ought to keep away from the lure of setting variables to be nullable with out a function (this will additionally occur once you convert Java code to Kotlin).

Nonetheless, there are a number of instances the place you’d use nullable variables in Kotlin:

State of affairs


You’re trying to find an merchandise in a listing that’s not there (normally when coping with the information layer).

val checklist: Checklist<Int> = listOf(1,2,3)
val searchResultItem = checklist.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 after I first received began with Kotlin. Finally, I finished utilizing them nearly utterly, besides when defining view bindings and variable injections in Android:

@Inject // With the Hilt library, that is initialized mechanically
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 supplies added flexibility and an improved developer expertise in comparison with Java.

Shared Function Variations: Transferring Between Java and Kotlin

Whereas every language has distinctive options, Kotlin and Java share many options too, and it’s crucial to know their peculiarities in an effort to transition between the 2 languages. Let’s study 4 frequent ideas that function in another way in Kotlin and Java:




Knowledge switch objects (DTOs)

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

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

Kotlin information courses operate equally to Java data, with toString, equals, and copy strategies out there:

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

Lambda expressions

Java lambda expressions (out there 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 all the time surrounded by curly braces:

var p: Checklist<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 by means of its utility courses. The Executor and ExecutorService courses are particularly helpful for concurrency. (Challenge Loom additionally provides 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 transferring between iOS and Android.

Static habits in courses

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

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

Kotlin companion objects provide static habits in courses, however the syntax isn’t as simple:

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

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

Loop Sort



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")


Checklist<String> checklist = Arrays.asList("first", "second");

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

checklist.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: Further Concerns

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 should you ought to use Java or Kotlin? Select Kotlin, Google’s most popular Android language, definitely.

Nonetheless, this query is moot for current Android functions. In my expertise throughout a variety of shoppers, 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 a substitute 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 must work round unstable code that’s difficult to refactor. Firms can simply enter a unending cycle of tech debt and value. It might 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 by means of DX? Builders require help throughout all ranges of their careers:

  • Junior builders profit from correct sources.
  • Mid-level builders develop by means of alternatives to guide and train.
  • Senior builders require the ability 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 tendencies and language releases will permit your staff members to achieve their biggest 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 tendencies in lower than one 12 months; with mature languages like Java, it can 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 may think about integrating Kotlin into any new code—IntelliJ comes with an computerized Java to Kotlin device—and will study 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 introduced on this article.

Additional Studying on the Toptal Engineering Weblog:



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