A new hope for the JVM: Kotlin

A new hope for the JVM: Kotlin

Giacomo Bresciani



Java is an old programming language. Version 1.0 was released in 1996 by Sun Microsystems and even though it has evolved and grown a lot over the past twenty years it is still carrying on some bad design choices such as null (ask Tony Hoare), primitive types or lack of a proper function type. With the last version of the language (Java 8) Java tried to address some of these problems introducing concepts such as Optional or lambda expression. Although these additions clearly represent a step forward for the language I still have the feeling that they are just patches applied to mitigate problems and not to solve them at their very source. For example Optional could be used to reduce NPE (Null Pointer Exception) but it is clearly not designed for this purpose and lambda expressions, implemented in Java 8 with SAM types, still force you to write an interface only to define a function.

The Android world

All the above concerns about Java are even more problematic within the Android world where, due to the notorious fragmentation (a huge amount of devices are stuck with an outdated VM), you are forced to target lower Java versions (6 and 7). Google is addressing the problem with its new compiler Jack that enables some of the features of Java 8 maintaining backward compatibility with older OS versions. But still it lets us deal with the verbosity of the language and it doesn’t truly solve the problem.

A new hope

Kotlin Hello World!

Back in 2011 the JetBrains team (the guys behind IntelliJ and so Android Studio) unveiled Kotlin, a new programming language that targets the JVM (and can also be compiled to JavaScript). Kotlin is a statically-typed language that combines Object Oriented and functional features enforcing no particular philosophy of programming, and it introduces a whole new set of concepts and tools that helps making the code safer, cleaner and much more readable. Thanks to its nature it works everywhere Java do and it is also interoperable with it, meaning it will not force you to rewrite the entire codebase to taste it: you can add it to your project a little at a time (maybe starting with tests 😉). It also features a REPL kotlinc-jvm that allows you to test language features with no effort (see the doc form more info). I am going to rapidly cover some features of Kotlin that address the previously mentioned Java limitations.


In Kotlin a variable cannot be null. If you want or need a variable to be nullable you have to add ? to the variable type:

val x: Int = null // compile error
val y: Int? = null // ok

Thanks to this information the compiler sees Int and Int? as two completely different types and can therefore enforce the null-safety of your variables. The ?. allows you to safe call methods on nullable variables without throwing NPE but simply returning null at the end of the call chain:

val x: Int? = null
x?.toString()?.substring(2) // no NPE, returns null

The ?: operator (Elvis operator) allows you to provide a “default” value when the variable is null:

// The two expressions are semantically equivalent:
text?.length ?: -1
(text.length != null) ? text.length  : -1

Higher-Order Functions and Lambdas

In Kotlin is possible to declare a method (or more generally a function) that returns or takes another function as parameter. The syntax to define the function type is similar to other languages such as Scala or Swift and is very intuitive:

val function: (T1, T2) -> R = {t1, t2 -> r}

Data classes

data class City(val name: String, val state: String)

Data classes address the verbosity of Java when dealing with classes that have the only purpose to hold data. With a single line you get equals()/hashCode(), toString() and getters/setters for free (and if you are a Java developer you already knows the benefits!)


Java has to maintain backward compatibility with previous versions and still has to support the huge amount of developers and codebases present all around the world; therefore it is natural that every new feature and design change is to be considered, weighted and reasoned really carefully, inevitably slowing down its evolution. But this does not have to mean that us, as Android developers, “tied” to the JVM, should not try more modern and advanced languages such as Kotlin. At bottom, a part of our job (one of the best!) is to try and experiment new technologies and to learn new concepts and techniques that improve our ability to address problems in the best possible way (and of course, to have some fun 😄).

I think that it is fundamental for a software engineer to be exposed to more than a single programming language: learning new patterns, exploring other programming paradigms or simply using and understanding a never-seen syntax has an immeasurable value for our growth and most of the times it turns out to be unexpectedly useful even when coding with ”our” language. So why not do it with a language that allows us to continue working on projects targeting our beloved JVM?