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 well-liked language and due to this fact higher suited to new cellular apps. However each Kotlin and Java supply many strengths as general-purpose languages, and it’s vital for builders to know the language variations, for functions equivalent 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 so much in widespread 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 may 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 well-liked language for Android growth in 2019. Exterior of Android, nevertheless, there isn’t any 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



Venture 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 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 (Venture 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 elements are usually vital issues for builders and shoppers.

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 techniques with restricted assets within the Nineties. The associated environmental necessities led to 2 major 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 assist over 6,000 encoded directions, relying on the counting technique.
  • Runtime (versus compile-time) operations: The multiplatform strategy (“Write once and run anywhere”) encourages runtime (as a substitute of compile-time) optimizations. In different phrases, the JVM interprets the majority of its bytecode into directions at runtime. Nevertheless, to enhance efficiency, you might use open-source implementations of the JVM, equivalent to 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 features keep away from a perform name, enhancing efficiency, whereas Java invokes further overhead reminiscence.
  • Kotlin’s higher-order features keep away from Java lambda’s particular name to InvokeDynamic, enhancing efficiency.
  • Kotlin’s generated bytecode accommodates 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 the usage of objects for base sorts (i.e., Kotlin’s implementation) requires extra allocation than primitive knowledge sorts (i.e., Java’s implementation). Nevertheless, 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 method 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. Chances are you’ll look at online benchmarks which present minor variations in micro-benchmarks, however these can’t be generalized to the dimensions of a full manufacturing utility.

Distinctive Function Comparability

Kotlin and Java have core similarities, however every language provides completely different, distinctive options. Since Kotlin turned Google’s most well-liked language for Android growth, I’ve discovered extension features and specific 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 look at a extra detailed breakdown of options accessible in Kotlin versus Java. Chances are you’ll comply with together with my examples utilizing the Kotlin Playground or a Java compiler for a extra hands-on studying strategy.





Extension features



Permits you to prolong a category or an interface with new functionalities equivalent to added properties or strategies with out having to create a brand new class:

class Instance 

// extension perform declaration
enjoyable Instance.printHelloWorld()  println("Whats up World!") 

// extension perform utilization

Good casts



Retains observe of circumstances inside if statements, protected casting robotically:

enjoyable instance(a: Any) 
  if (a is String) 
    println(a.size) // automated 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 features



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

Native assist for delegation



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

Sort aliases



Supplies shortened or customized names for current sorts, together with features and inside or nested courses: typealias ShortName = LongNameExistingType.

Non-private fields



Presents protected and default (also referred to as package-private) modifiers, along with public and non-public modifiers. Java has all 4 access 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;
  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.

Word: Checked exceptions had been meant to encourage builders to design strong software program. Nevertheless, they’ll 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 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 likely one of the best Kotlin options. This function saves time as a result of builders don’t should deal with NullPointerExceptions (that are RuntimeExceptions).

In Java, by default, you’ll be able to assign a null worth to any variable:

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

In Kotlin, however, 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'll be able to'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 reduce the usage of nullable variables for finest practices; these Kotlin versus Java examples are supposed to show variations within the languages. Kotlin inexperienced persons ought to keep away from the entice of setting variables to be nullable with out a function (this may additionally occur whenever you convert Java code to Kotlin).

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

State of affairs


You might be looking for an merchandise in a listing that isn’t there (normally when coping with the information layer).

val record: Listing<Int> = listOf(1,2,3)
val searchResultItem = record.firstOrNull  it == 0 
  // 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 acquired began with Kotlin. Ultimately, I finished utilizing them nearly fully, besides when defining view bindings and variable injections in Android:

@Inject // With the Hilt library, that is initialized robotically
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: Shifting 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 to be able to transition between the 2 languages. Let’s look at 4 widespread ideas that function in a different way in Kotlin and Java:




Knowledge 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 courses perform 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) comply with 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 comply with the syntax parameter1, parameter2 -> code and are all the time surrounded by curly braces:

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


Java threads make concurrency potential, and the java.util.concurrency package deal permits for straightforward multithreading via its utility courses. The Executor and ExecutorService classes are particularly helpful for concurrency. (Project 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 memory manager reduces earlier limitations on concurrency and multithreading for builders shifting between iOS and Android.

Static habits in courses

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

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

Kotlin companion objects supply static habits in courses, however the syntax just isn’t as easy:

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 provide you with an thought of the general scenario:

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")


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

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



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 Venture Planning: Further Issues

We’ve examined many vital elements to consider when deciding between Kotlin and Java in a general-purpose context. Nevertheless, 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 for those who ought to use Java or Kotlin? Select Kotlin, Google’s most well-liked Android language, surely.

Nevertheless, this query is moot for current Android functions. In my expertise throughout a variety of shoppers, the 2 extra vital questions are: How are you treating tech debt? and How are you taking good care of 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 should work round unstable code that’s difficult to refactor. Corporations can simply enter a unending cycle of tech debt and value. It could be price 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 taking good care of 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 stunning code.

Consideration to DX for senior builders is very vital since their experience trickles down and impacts all engineers. Senior builders like to study and experiment with the most recent applied sciences. Maintaining with newer developments and language releases will enable your group members to achieve their best potential. That is vital whatever the group’s language selection, although completely different languages have various timelines: With younger languages like Kotlin, an engineer engaged on legacy code can fall behind developments in lower than one yr; with mature languages like Java, it is going to 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 latest Android apps. Google has put all of its efforts into Kotlin, and its new applied sciences are Kotlin-first. Builders of current apps may contemplate integrating Kotlin into any new code—IntelliJ comes with an automated Java to Kotlin tool—and may look at elements that attain past our preliminary query of language selection.

The editorial group 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.

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