Kotlin vs Java: [2019] Most Important Differences That You Must Know

Kotlin Vs Java


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Kotlin is a statically typed language developed by JetBrains. Similar to Java, Kotlin has become a top choice for developing Android applications. This is evident from the fact that Android Studio comes with inbuilt support for Kotlin like it has for Java.

Kotlin vs Java

So, the question is whether one should switch to Kotlin from Java or not? Well, it depends on preferences. However, before making the switch it’s important to understand the distinction between the two programming languages.

Checked Exceptions

One major difference between Java and Kotlin is that the latter has no provision for checked exceptions. Therefore, there is no need to catch or declare any exceptions.

If a developer working in Java finds it infuriating to use try/catch blocks in the code then the omission made by Kotlin can be considered a welcome change. However, it’s the opposite if the developer believes that checked exceptions encourage error recovery and the creation of robust code.

Code Conciseness

Comparing a Java class with an equivalent Kotlin class demonstrates the conciseness of Kotlin code. For performing the same operation that the Java class does, a Kotlin class necessitates for less code.

For example, a particular segment where Kotlin can significantly reduce the total amount of boilerplate code is findViewByIds.

Kotlin Android Extensions permit importing a reference to a View into the Activity file. This allows for working with that View as if it was part of the Activity.


CPU-intensive work and network I/O are long-running operations. The calling thread is blocked until the whole operation completes. As Android is single-threaded by default, an app’s UI gets completely frozen as soon as the main thread is blocked.

The traditional solution for the problem in Java is to create a background thread for the long-running or intensive work. However, managing multiple threads leads to an increase in the complexity as well as errors in the code.

Kotlin also allows the creation of additional threads. However, there is a better way of managing intensive operations in Kotlin, known as coroutines. Coroutines are stackless, which means they demand lower memory usage as compared to threads.

Coroutines are able to perform long-running and intensive tasks by suspending execution without blocking the thread and then resuming the execution at some later time. It allows the creation of non-blocking asynchronous code that appears to be synchronous.

The code using coroutines is not only clear but concise too. Moreover, coroutines allow for creating elegant additional styles of asynchronous non-block programming such as async/await.

Data Classes

Full-size projects have several classes that are solely meant to hold data. Though these classes have very little to no functionality, a developer needs to write a lot of boilerplate code in Java.

Usually, a developer needs to define a constructor, several fields to store the data, getter and setter functions for each of the fields, and equals(), hashCode(), and toString() functions.

Kotlin has a very simple way of creating such classes. The developer needs to only include the data keyword in the class definition, and voila! The compiler will take care of the entire task on its own.

Extension Functions

Kotlin allows developers to extend a class with new functionality via extension functions. These functions, although available in other programming languages like C#, aren’t available in Java.

Creating an extension function is easy in Kotlin. It is done by prefixing the name of the class that needs to be extended to the name of the function being created. In order to call the function on the instances of the extended class, one needs to use the ‘.’ notation.

Higher-Order Functions and Lambdas

A higher-order function is one that takes functions as parameters or returns a function. Also, Kotlin functions are first-class. This means that they can be stored in data structures and variables, which can be passed as arguments to and returned from other higher-order functions.

The whole thing simply means that functions can be operated in all the ways that are possible for other non-function values.

As a statically typed programming language, Kotlin makes use of a range of function types for representing functions. Moreover, it comes with a set of specialized language constructs, such as the lambda expressions.

Anonymous functions and lambda expressions are also known as function literals. These are functions not declared but passed immediately as an expression.

Implicit Widening Conversions

There is no support for implicit widening conversions for numbers in Kotlin. So, smaller types aren’t able to get converted to bigger types. Whereas Java has support for implicit conversions, Kotlin requires to perform an explicit conversion in order to achieve the conversion.

Inline Functions

Variables that are accessed in the body of the function are known as closures. Making use of higher-order functions can impose several runtime penalties. Every function in Kotlin is an object and it captures a closure.

Both classes and function objects call for memory allocations. These along with virtual calls introduce runtime overhead. Such an additional overhead can be avoided by inlining the lambda expressions in Kotlin. One such example is the lock() function.

Unlike Kotlin, Java doesn’t provide support for inline functions. Nonetheless, the Java compiler is capable of performing inlining using the final method. This is so because final methods cannot be overridden by sub-classes. Also, call to a final method is resolved at compile time.

Native Support for Delegation

In programming terminology, Delegation represents the process where a receiving object delegates operations to a second delegate object. Kotlin supports composition over inheritance design pattern by means of the first-class delegation, also known as implicit delegation.

Class delegation is an alternative to inheritance in Kotlin. This makes it possible to use multiple inheritances. Also, Kotlin’s delegated properties prevent the duplication of code.

Non-private Fields

Encapsulation is essential in any program for achieving a desirable level of maintainability.

By means of encapsulating the representation of an object, it can be enforced how callers interact with it. Moreover, it is possible to change the representation without the need to modify callers, provided the public API remains unchanged.

Non-private fields or public fields in Java are useful in scenarios where the callers of an object need to change accordingly to its representation. It simply means that such fields expose the representation of an object to the callers. Kotlin doesn’t have non-private fields.

Null Safety

One of the most infuriating issues concerning Java for developers is the NullPointerExceptions. Java lets developers assign a null value to any variable. However, if they try to use an object reference that has a null value, there comes the NullPointerException!

Unlike Java, all types are non-nullable in Kotlin by default. If developers try to assign or return null in the Kotlin code, it’ll fail at compile time. However, there’s a way around. In order to assign a null value to a variable in Kotlin, it is required to explicitly mark that variable as nullable. This is done by adding a question mark after the type, for example:

val number: Int? = null

Thus, there are no NullPointerExceptions in Kotlin. If you do encounter such an exception in Kotlin then it is most likely that either you explicitly assigned a null value or it is due to some external Java code.

Primitive Types

There are 8 primitive data types, including char, double, float, and int. Unlike Kotlin, variables of a primitive type aren’t objects in Java. This means that they aren’t an object instantiated from a class or a struct.

Smart Casts

Before an object can be cast in Java, it is mandatory to check type. This is also true in scenarios where it’s obvious to cast the object.

Unlike Java, Kotlin comes with the smart casts feature, which automatically handles such redundant casts. You don’t need to cast inside a statement provided it is already checked with the ‘is operator’ in Kotlin.

Static Members

Kotlin has no provision for static members. However, in Java programming language the keyword static reflects that the particular member with which the keyword is used belongs to a type itself instead to an instance of that type.

It simply means that one and only one instance of that static member is created and shared across all instances of the class.

Support for Constructors

A Kotlin class, unlike a Java class, can have one or more secondary constructors in addition to a primary constructor. This is done by including these secondary constructors in the class declaration.

Ternary Operator

Unlike Kotlin, Java has a ternary operator. The Java ternary operator simply works like a basic if statement. It consists of a condition that evaluates to true or false.

Moreover, the Java ternary operator has two values. Only one of them is returned depending on whether the condition is true or false. The syntax for the Java ternary operator is:

(condition) ? (value1) : (value 2)

Wildcard Types

In generic code, ‘?’ represents an unknown type. It is known as the wildcard. There are several uses of a wildcard, including as the type of a field, local variable, or parameter.

While Java’s type system offers wildcard types, Kotlin doesn’t. However, it has two different things; declaration-site variance and type projections as an alternative to wildcard types.

Kotlin vs Java: Head to Head Comparison

Checked ExceptionsAvailableUnavailable
Code ConcisenessCan’t be labeled as conciseBetter than Java
Data ClassesRequired to write a lot of boilerplate codeRequires adding only the data keyword in the class definition
Extension FunctionsUnavailableAvailable
Higher-Order Functions and LambdasHigher-order functions are implemented using Callables. Lambdas expressions are introduced in the Java 8Comes as one of the prebuilt features
Implicit Widening ConversionsAvailableUnavailable
Inline FunctionsUnavailableAvailable
Native Support for DelegationUnavailableAvailable
Non-private FieldsAvailableUnavailable
Primitive TypesVariables of a primitive type aren’t objectsVariables of a primitive type are objects
Smart CastsUnavailableAvailable
Static MembersAvailableUnavailable
Support for ConstructorsCan’t have secondary constructors. Although, can have multiple constructors (constructor overloading)Can have one or more secondary constructors
Ternary OperatorAvailableUnavailable
Wildcard TypesAvailableUnavailable, has declaration-site variance and type projects as an alternative

Annotation Processing Libraries with Kotlin

In addition to providing support for existent Java frameworks and libraries, Kotlin also has a provision for advanced Java frameworks relying on annotation processing.

However, using a Java library that makes use of annotation processing in Kotlin requires adding the same to the Kotlin project in a bit different way than what is required for a Java library that doesn’t use annotation processing.

It is required to specify the dependency using the kotlin-kapt plugin. Afterwards, the Kotlin Annotation processing tool needs to be used in place of the annotationProcessor.

Still Confused? Here’s the Solution – Interchangeability!

Obviously, some features are better in Kotlin while for others Java is advantageous to use. For those not willing to ditch any of the two leading programming languages for Android development, thankfully, there’s another way around.

Irrespective of all the dissimilarities between the two programming languages, they are fully interoperable. Both Java and Kotlin compile to bytecode. This means it is possible to call Java code from Kotlin and vice-versa.

This flexibility has two advantages. First, it facilitates getting started with Kotlin by incrementally introducing the Kotlin code in a Java project. Second, both languages can be used simultaneously in any Android application development project.

To Summarize

For general-purpose programming, Java gains the upper hand. On the flip side, more and more developers and organizations are adopting Kotlin for rapidly developing Android applications.

Both Java and Kotlin have their own advantages over the other. The debate about which of the two is the greatest has just started, and it is not likely to be over anytime soon.

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26 Comments, RSS

  1. Avatar

    Edith Mckenzie February 7, 2019 @ 4:09 pm

    Is kotlin as fast as Java?

    • Avatar

      Dong Bone February 20, 2019 @ 7:59 pm

      This is not meant to say anyone is faster than others. Both generate byte code that runs on the JVM. Usually, they supposed to be the same.

      • Avatar

        Amit Kumar June 22, 2019 @ 5:43 pm

        That is not true. It certainly depends on the compiler how compact and efficient class file it generates.

    • Avatar

      Daniel June 5, 2019 @ 12:31 pm

      Kotlin is slower than Java at least at compile time

  2. Avatar

    Della Clayton February 7, 2019 @ 4:09 pm

    Is kotlin a programming language?

    • Avatar

      Alphonse Cano February 20, 2019 @ 8:00 pm

      Kotlin is statically typed general purpose programming language. Kotlin mainly targets JVM but also complies to JavaScript or native code via LLVM.

  3. Avatar

    Leland Burke February 7, 2019 @ 4:10 pm

    Does kotlin have a future?

    • Avatar

      Noe Gilley February 20, 2019 @ 8:00 pm

      Yes, Kotlin is going to be the future of Android App Development. Kotlin requires less code to write and fewer crashes occur on Kotlin and there are many more features that proves Kotlin’s influence is going to get bigger and bigger in the future.

  4. Avatar

    Cathy Jennings February 7, 2019 @ 4:11 pm

    Should I learn Java or Kotlin in order to develop Android apps?

    • Avatar

      Clair Lake February 20, 2019 @ 8:01 pm

      There are many advantages of using Kotlin over Java but to use Kotlin you need to learn Java as Kotlin has arrived from Java. When you work with Kotlin in the android studio you will have to use Java libraries and there you need to use the knowledge of Java to write the code on Kotlin.

  5. Avatar

    Tommy Goodman February 7, 2019 @ 4:18 pm

    What are the advantages of Kotlin over Java?

    • Avatar

      Chance Willey February 20, 2019 @ 8:02 pm

      Advantages of using Kotlin over Java:
      • Kotlin is more concise.
      • Kotlin will be safer than Java in the future.
      • Kotlin is inter-operable with the existing android programming language.
      • Kotlin codes in Android studio and build by Jet Brains. Android studio is also built by Jet Brains and built upon the IntelliJ IDEA. Therefore, it supports more features and tools for developers.
      • Kotlin requires less code to write.
      • Less crashes occur on Kotlin.
      • Kotlin is a type-safety language.
      • Kotlin saves your time.

  6. Avatar

    Warren Allison February 7, 2019 @ 4:21 pm

    Can Kotlin achieve the success of Java in server side programming?

    • Avatar

      Raymon Haller February 20, 2019 @ 8:02 pm

      When Java was launched there were fewer language in the competition. These days competition is more. Kotlin is a decent language with so many features but its main selling point is only those people who crow about how crap Java is. The most important thing that Java has built up enormous momentum over the years.

  7. Avatar

    Carroll Weber February 7, 2019 @ 4:34 pm

    Which one is better for developing an an Android application? React Native vs. Java vs. Kotlin.

    • Avatar

      Jess Pullen February 20, 2019 @ 8:03 pm

      Java is no doubt the first preference for android app development, after that Kotlin and React Native as React Native is a framework. Else, to choose a programming language its depend on different aspects such as usability, portability, reliability etc.

  8. Avatar

    Melinda Fuller February 7, 2019 @ 4:35 pm

    Is Flutter better than Kotlin/Java if you want to make Android apps only?

    • Avatar

      Johnie Stein February 20, 2019 @ 8:03 pm

      If it is only about Android apps then you should go with Java as Flutter is still in beta. You can think about Flutter when you are going to develop for both Android and iOS. Flutter is better as a cross-platform app development tool.

  9. Avatar

    Curtis Vasquez February 7, 2019 @ 4:36 pm

    Why did Google replace Java with Kotlin, and why only Kotlin but not other languages?

    • Avatar

      Branden Beach February 20, 2019 @ 8:04 pm

      Kotlin also runs on JVM and you can easily convert all your existing code into Kotlin with just one click inside an android studio. Java is very strong but it became too old now and there is a fact that technology will always be something new, replacing old stuff and you have to be ready to learn it.

  10. Avatar

    Henrietta Hopkins February 7, 2019 @ 4:51 pm

    Can we compare Xamarin Android to Java and Kotlin?

    • Avatar

      Tyron Lightfoot February 20, 2019 @ 8:05 pm

      Xamarin used for cross-platform mobile app development for Hybrid Apps using C# language while Java and Kotlin used to build Native android apps.
      Java and Kotlin are official languages for Android App development.

  11. Avatar

    Liamson March 11, 2019 @ 2:14 pm

    Android Studio will be avoid gradle library on Java 9.

  12. Avatar

    Jethro Acosta May 25, 2019 @ 1:41 pm

    I think you got some wrong points on this article.

    1. Higher order functions – you stated that only kotlin can do this but this can be achieved in Java using Callables.

    2. Secondary constructor, maybe you just did not elaborate this more but in java you can declare different constructors with different parameters.

  13. Avatar

    Nikolas August 26, 2019 @ 12:47 am

    Hi, I’m an experience programmer but totally new to Android/Java. For a friend I need to do a fairly simple apk (hope so) that needs to access BT & wifi functions, is Kotlin appropriated for that? (generally speaking with so few details) or should I start the tortuous path of learning Java?

  14. Avatar

    Mike September 6, 2019 @ 1:07 pm