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 subsequently higher suited to new cell apps. However each Kotlin and Java provide many strengths as general-purpose languages, and it’s essential 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 Related?

Certainly, the 2 languages have quite a bit 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’ll be able to name Java code from Kotlin and Kotlin code from Java. Java can be utilized in server-side purposes, databases, internet front-end purposes, embedded programs and enterprise purposes, cell, and extra. Kotlin is equally versatile: It targets the JVM , Android, JavaScript, and Kotlin/Native, and may 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 a suggestion to interchange Java with Kotlin.





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 challenge 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 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 components are usually essential issues for builders and shoppers.

Kotlin, Java, and the opposite JVM languages, though not equal, are pretty comparable by way of efficiency, not less than when in comparison with languages in different compiler households like GCC or Clang. The JVM was initially designed to focus on embedded programs with restricted sources within the Nineteen Nineties. The associated environmental necessities led to 2 principal constraints:

  • Easy JVM bytecode: The present model of JVM, through which 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 once and run anywhere”) encourages runtime (as an alternative of compile-time) optimizations. In different phrases, the JVM interprets the majority of its bytecode into directions at runtime. Nevertheless, to enhance efficiency, you could use open-source implementations of the JVM, equivalent to HotSpot, which pre-compiles the bytecode to run sooner by way 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 features keep away from a perform name, enhancing efficiency, whereas Java invokes extra 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 using objects for base sorts (i.e., Kotlin’s implementation) requires extra allocation than primitive information sorts (i.e., Java’s implementation). Nevertheless, in follow, 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 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 aren’t any important 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 software.

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 essentially the most helpful options. Then again, 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. Chances are you’ll observe together with my examples utilizing the Kotlin Playground or a Java compiler for a extra hands-on studying method.





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("Hey World!") 

// extension perform utilization

Sensible 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 offers 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 help for delegation



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

Kind aliases



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

Non-private fields



Provides protected and default (also called 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 computerized conversion from a smaller information kind to a bigger information kind:

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.

Notice: Checked exceptions had been supposed 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

In my view, non-nullability is among the best 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’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, then again, we’ve 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 using nullable variables for greatest practices; these Kotlin versus Java examples are supposed to show variations within the languages. Kotlin novices ought to keep away from the entice of setting variables to be nullable and not using a objective (this may additionally occur while you convert Java code to Kotlin).

Nevertheless, there are just a few circumstances the place you’d use nullable variables in Kotlin:

State of affairs


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

val record: Checklist<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 obtained began with Kotlin. Finally, I finished utilizing them virtually 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 offers 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 needed to know their peculiarities with a purpose to transition between the 2 languages. Let’s look at 4 widespread ideas that function in another way in Kotlin and Java:




Information switch objects (DTOs)

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

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

Kotlin information courses perform equally to Java data, with toString, equals, and copy strategies accessible:

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


Java threads make concurrency doable, and the java.util.concurrency bundle permits for simple multithreading by way of 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 embody 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 situations and be sure that solely a single copy of an merchandise is created. The static key phrase may be utilized to variables, features, blocks, and extra:

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

Kotlin companion objects provide static habits in courses, 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 offer you an concept of the general scenario:

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")


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

for (String worth: record) 
var record: Checklist<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 essential components 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 software from scratch and questioning should you ought to use Java or Kotlin? Select Kotlin, Google’s most well-liked Android language, definitely.

Nevertheless, this query is moot for current Android purposes. In my expertise throughout a variety of shoppers, the 2 extra essential 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 spanking 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 must work round unstable code that’s difficult to refactor. Firms can simply enter a endless cycle of tech debt and value. 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 by way of DX? Builders require help throughout all ranges of their careers:

  • Junior builders profit from correct sources.
  • Mid-level builders develop by way of alternatives to steer and train.
  • Senior builders require the facility to architect and implement lovely code.

Consideration to DX for senior builders is very essential 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 tendencies and language releases will enable your workforce members to achieve their best potential. That is essential whatever the workforce’s language selection, although completely 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 is going to take longer.

Kotlin and Java: Two Highly effective Languages

Whereas Java has a variety of purposes, 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 take into account integrating Kotlin into any new code—IntelliJ comes with an computerized Java to Kotlin tool—and may look at components that attain past our preliminary query of language selection.

The editorial workforce 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.

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