10 Features I Wish Java Would Steal From the Kotlin Language

This article is overdue. After the hype around the release of Kotlin 1.0 has settled, let’s have a serious look at some Kotlin language features that we should have in Java as well. In this article, I’m not going to wish for unicorns. But there are some low hanging fruit (as far as I naively can see), which could be introduced into the Java language without great risk. While you’re reading this article, be sure to copy paste examples to http://try.kotlinlang.org, an online REPL for Kotlin

1. Data class

Language designers hardly ever agree on the necessity and the feature scope of what a class is. In Java, curiously, every class always has identity a concept that is not really needed in 80% – 90% of all real world Java classes. Likewise, a Java class always has a monitor on which you can synchronize. In most cases, when you write a class, you really just want to group values, like Strings, ints, doubles. For instance:

public class Person {
    final String firstName;
    final String lastName;
    public JavaPerson(...) {
    // Getters

    // Hashcode / equals

    // Tostring

    // Egh...

By the time you’ve finished typing all of the above, your fingers will no longer be. Java developers have implemented ugly workarounds for the above, like IDE code generation, or lombok, which is the biggest of all hacks. In a better Java, nothing in Lombok would really be needed. As, for instance, if Java had Kotlin’s data classes:

data class Person(
  val firstName: String,
  val lastName: String

The above is all we need to declare the equivalent of the previous Java code. Because a data class is used to store data (duh), i.e. values, the implementation of things like hashCode(), equals(), toString() is obvious and can be provided by default. Furthermore, data classes are first class tuples, so they can be used as such, e.g. to destructure them again in individual references:

val jon = Person("Jon", "Doe") 
val (firstName, lastName) = jon

In this case, we may hope. Valhalla / Java 10 is being designed and with it, value types. We’ll see how many features will be provided on the JVM directly, and in the Java language. This will certainly be an exciting addition. Notice how val is possible in Kotlin: Local variable type inference. This is being discussed for a future Java version right now.

2. Defaulted parameters

How many times do you overload an API like the following:

interface Stream<T> {
    Stream<T> sorted();
    Stream<T> sorted(Comparator<? super T> comparator);

The above are exactly the same JDK Stream operations. The first one simply applies Comparator.naturalOrder() to the second one. So we could write the following, in Kotlin:

fun sorted(comparator : Comparator<T> 
         = Comparator.naturalOrder()) : Stream<T>

The advantage of this isn’t immediately visible, when there is only one defaulted parameter. But imagine a function with tons of optional parameters:

fun reformat(str: String,
             normalizeCase: Boolean = true,
             upperCaseFirstLetter: Boolean = true,
             divideByCamelHumps: Boolean = false,
             wordSeparator: Char = ' ') {

Which can be called in any of the following ways:

reformat(str, true, true, false, '_')
  normalizeCase = true,
  upperCaseFirstLetter = true,
  divideByCamelHumps = false,
  wordSeparator = '_'

The power of defaulted parameters is that they are especially useful when passing arguments by name, rather than by index. This is currently not supported in the JVM, which until Java 8, doesn’t retain the parameter name at all (in Java 8, you can turn on a JVM flag for this, but with all of Java’s legacy, you shouldn’t rely on this yet). Heck, this feature is something I’m using in PL/SQL every day. Of course, in Java, you can work around this limitation by passing a parameter object.

3. Simplified instanceof checks

If you will, this is really an instanceof switch. Some people may claim that this stuff is evil, bad OO design. Nja nja. I say, this happens every now and then. And apparently, in Java 7, string switches were considered sufficiently common to modify the language to allow them. Why not instanceof switches?

val hasPrefix = when(x) {
  is String -> x.startsWith("prefix")
  else -> false

Not only is this doing an instanceof switch, it is doing it in the form of an assignable expression. Kotlin’s version of this when expression is powerful. You can mix any sort of predicate expressions, similar to SQL’s CASE expression. For instance, this is possible as well:

when (x) {
  in 1..10 -> print("x is in the range")
  in validNumbers -> print("x is valid")
  !in 10..20 -> print("x is outside the range")
  else -> print("none of the above")

Compare to SQL (not implemented in all dialects):

  WHEN BETWEEN 1 AND 10 THEN 'x is in the range'
  WHEN IN (SELECT * FROM validNumbers) THEN 'x is valid'
  WHEN NOT BETWEEN 10 AND 20 'x is outside the range'
  ELSE 'none of the above'

As you can see, only SQL is more powerful than Kotlin.

4. Map key / value traversal

Now this could really be done very easily only with syntax sugar. Granted, having local variable type inference would already be a plus, but check this out

val map: Map<String, Int> = ...

And now, you can do:

for ((k, v) in map) {

After all, most of the time when traversing a map, it’ll be by Map.entrySet(). Map could have been enhanced to extend Iterable<Entry<K, V>> in Java 5, but hasn’t. That’s really a pity. After all, it has been enhanced in Java 8 to allow for internal iteration over the entry set in Java 8 via Map.forEach():

map.forEach((k, v) -> {

It’s not too late, JDK gods. You can still let Map<K, V> extend Iterable<Entry<K, V>>

5. Map access literals

This one is something that would add tons and tons of value to the Java language. We have arrays, like most other languages. And like most other languages, we can access array elements by using square brackets:

int[] array = { 1, 2, 3 };
int value = array[0];

Note also the fact that we have array initialiser literals in Java, which is great. So, why not also allow for accessing map elements with the same syntax?

val map = hashMapOf<String, Int>()
map.put("a", 1)

In fact, x[y] is just syntax sugar for a method call backed by x.get(y). This is so great, we have immediately proceeded with renaming our Record.getValue() methods in jOOQ to Record.get() (leaving the old ones as synonyms, of course), such that you can now dereference your database record values as such, in Kotlin

ctx.select(a.FIRST_NAME, a.LAST_NAME, b.TITLE)
   .orderBy(1, 2, 3)
   .forEach {
               by ${it[a.FIRST_NAME]} ${it[a.LAST_NAME]}""")

Since jOOQ holds all column type information on individual record columns, you can actually know in advance that it[b.TITLE] is a String expression. Great, huh? So, not only can this syntax be used with JDK maps, it can be used with any library that exposes the basic get() and set() methods. Stay tuned for more jOOQ and Kotlin examples here: https://github.com/jOOQ/jOOQ/blob/master/jOOQ-examples/jOOQ-kotlin-example/src/main/kotlin/org/jooq/example/kotlin/FunWithKotlinAndJOOQ.kt

6. Extension functions

This one is a controversial topic, and I can perfectly understand when language designers stay clear of it. But every now and then, extension functions are very useful. The Kotlin syntax here is actually just for a function to pretend to be part of the receiver type:

fun MutableList<Int>.swap(index1: Int, index2: Int) {
  val tmp = this[index1] // 'this' corresponds to the list
  this[index1] = this[index2]
  this[index2] = tmp

This will now allow for swapping elements in a list:

val l = mutableListOf(1, 2, 3)
l.swap(0, 2)

This would be very useful for libraries like jOOλ, which extends the Java 8 Stream API by wrapping it in a jOOλ type (another such library is StreamEx, with a slightly different focus). The jOOλ Seq wrapper type is not really important, as it pretends to be a Stream on steroids. It would be great, if jOOλ methods could be put onto Stream artificially, just by importing them:


The zipWithIndex() method isn’t really there. The above would just translate to the following, less readable code:


In fact, extension methods would even allow to bypass wrapping everything explicitly in a stream(). For instance, you could then do:


As all of jOOλ’s method could be designed to also be applied to Iterable. Again, this is a controversial topic. For instance, because
While giving the illusion of being virtual, extension functions really are just sugared static methods. It’s a significant risk for object oriented application design to engage in that trickery, which is why this feature probably won’t make it into Java.

7. Safe-call operator (and also: Elvis operator)

Optional is meh. It’s understandable that an Optional type needed to be introduced in order to abstract over the absence of primitive type values, which cannot be null. We now have things like OptionalInt, e.g. to model things like:

OptionalInt result =
IntStream.of(1, 2, 3)
         .filter(i -> i > 3)

// Agressive programming ahead

Optional is a monad
Yes. It allows you to flatMap() the absent value. o_O Sure, if you want to do sophisticated functional programming, you’ll start typing map() and flatMap() everywhere. Like today, when we’re typing getters and setters. Along will come lombok generating flatmapping calls, and Spring will add some @AliasFor style annotation for flatmapping. And only the enlightened will be able to decipher your code. When all we needed was just a simple null safety operator before getting back to daily business. Like:

String name = bob?.department?.head?.name

I really like this type of pragmatism in Kotlin. Or do you prefer (flat)mapping?

Optional<String> name = bob

Can you read this? I cannot. Neither can I write this. If you get this wrong, you’ll get boxoxed.
Of course, Ceylon is the only language that got nulls right. But Ceylon has tons of features that Java will not get before version 42, and I’m not wishing for unicorns. I’m wishing for the safe-call operator (and also the elvis operator, which is slightly different), which could be implemented in Java too. The above expression is just syntax sugar for:

String name = null;
if (bob != null) {
    Department d = bob.department
    if (d != null) {
        Person h = d.head;
        if (h != null)
            name = h.name;

What can possibly be wrong with that simplification?

8. Everything is an expression

Now this might just be a unicorn. I don’t know if there is a JLS / parser limitation that will forever keep us in the misery of prehistoric distinction between statement and expression. At some point in time, people have started using statements for things that yield side-effects, and expressions for more functional-ish things. It is thus not surprising, that all String methods are really expressions, operating on an immutable string, returning a new string all the time. This doesn’t seem to go well with, for instance, if-else in Java, which is expected to contain blocks and statements, each possibly yielding side-effects. But is that really a requirement? Can’t we write something like this in Java as well?

val max = if (a > b) a else b

OK, we have this weird conditional expression using ?:. But what about Kotlin’s when (i.e. Java’s switch)?

val hasPrefix = when(x) {
  is String -> x.startsWith("prefix")
  else -> false

Isn’t that much more useful than the following equivalent?

boolean hasPrefix;

if (x instanceof String)
    hasPrefix = x.startsWith("prefix");
    hasPrefix = false;

(yes, I know about ?:. I just find if-else easier to read, and I don’t see why that should be a statement, not an expression. Heck, in Kotlin, even try is an expression, not a statement:

val result = try {
} catch (e: ArithmeticException) {
    throw IllegalStateException(e)


9. Single expression functions

Now this. This would save so much time reading and writing simple glue code. And in fact, we already have the syntax in annotations. Check out Spring’s magical @AliasFor annotation, for instance. It yields:

public @interface AliasFor {
    String value() default "";
    String attribute() default "";

Now, if you squint really hard, these are just methods yielding constant values, because annotations are just interfaces with generated byte code for their implementations. We can discuss syntax. Of course, this irregular usage of default is weird, given that it was not re-used in Java 8 for default methods, but I guess Java always needs the extra syntax so developers feel alive as they can better feel their typing fingers. That’s OK. We can live with that. But then again, why do we have to? Why not just converge to the following?

public @interface AliasFor {
    String value() = "";
    String attribute() = "";

And the same also for class / interface default methods?

// Stop pretending this isn't an interface
public interface AliasFor {
    String value() = "";
    String attribute() = "";

Now that would look nice. But given Java’s existing syntax, this might just be a unicorn, so let’s move on to…

10. Flow-sensitive typing

Now this. THIS! We’ve blogged about sum types before. Java has sum types with exceptions since Java 7:

try {
catch (IOException | SQLException e) {
    // e can be of type IOException and/or SQLException
    // within this scope

But Java, unfortunately, doesn’t have flow-sensitive typing. Flow-sensitive typing is of the essence in a language that supports sum types, but it is also useful otherwise. For instance, in Kotlin:

when (x) {
    is String -> println(x.length)

We don’t need to cast, obviously, because we already checked that x is String. Conversely, in Java:

if (x instanceof String)
    System.out.println(((String) x).length());

Aaagh, all this typing. IDE autocompletion is smart enough to offer a contextual type’s methods already and then generate the unnecessary cast for you. But it would be great if this was never needed, every time we explicitly narrow a type using control flow structures. For more info, see this wikipedia entry about flow sensitive typing. A feature that could absolutely be added to the Java language. After all, we already got flow-sensitive final local variables since Java 8.

11. (Bonus) Declaration site variance

Last but not least, better generics via declaration site variance. Many other languages know this, for instance also C#’s IEnumerable:
public interface IEnumerable : IEnumerable
The keyword out here means that the generic type T is produced from the type IEnumerable (as opposed to in, which stands for consumption). In C#, Scala, Ceylon, Kotlin, and many other languages, we can declare this on the type declaration, rather than on its usage (although, many languages allow for both). In this case, we say that IEnumerable is covariant with its type T, which means again that IEnumerable<Integer> is a subtype of IEnumerable<Object> In Java, this isn’t possible, which is why we have a bazillion question by Java newbies on Stack Overflow. Why can’t I…

Iterable<String> strings = Arrays.asList("abc");
Iterable<Object> objects = strings; // boom

In languages like Kotlin, the above would be possible. After all, why shouldn’t it? A thing that can produce strings can also produce objects, and we can even use it in this way in Java:

Iterable<String> strings = Arrays.asList("abc");
for (Object o : strings) {
    // Works!

The lack of declaration site variance has made a lot of APIs very intelligible. Consider Stream:

<R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);

This is just noise. A function is contravariant with its argument type and covariant with its result type by nature a better definition of Function or Stream would be:

interface Function<in T, out R> {}
interface Stream<out T> {}

If this were possible, all that ? super and ? extends garbage could be removed without losing any functionality. In case you’re wondering what I’m even talking about? :)
The great news is, this is being discussed for a (near) future version of Java: http://openjdk.java.net/jeps/8043488


Kotlin is a promising language, even if it is very late to a game that already seems to have been decided, not in favour of alternative languages on the JVM. Nonetheless, it is a very interesting language to learn from, and with a lot of very good decisions made about some simple things. Some of these decisions will hopefully be picked up by the Java language gods and integrated into Java. This list here shows some features that might be “easy” to add.
More info about Kotlin idioms: https://kotlinlang.org/docs/reference/idioms.html

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20 thoughts on “10 Features I Wish Java Would Steal From the Kotlin Language

  1. “?.” is only called the Elvis operator in C# while in Kotlin it’s called the safe-call operator. In Kotlin, “?:” is the Elvis operator and it has the same number of eyes as the actual Elvis, unlike the C# one.

      1. The paragraph is still titled “Elvis operator” (which is also a neat feature, Java could steal).

  2. What I really like about point 3 is that you can use it with enums, and, most importantly, the compiler will refuse to compile if one of the enum constants is not handled. This allows adding an enum constant safely. Sure, we can add a method to the enum itself, but it’s not always possible, or even desirable.

  3. A lot of this is covered in Project Lombok – Can’t use Java without it anymore

      1. lombok, which is the biggest of all hacks. In a better Java, nothing in Lombok would really be needed.

        A better world’s Java would include lombok. I’d also prefer something build-in to all the insanity lombok has to do in order to keep me sane and save my fingers.

        While giving the illusion of being virtual, extension functions really are just sugared static methods.

        Sure, but AFAIK they could be made virtual if someone really wanted. Just let the compiler generate a top-level extension method doing virtual dispatch using an instanceof switch to one of the applicable user-defined extension methods. This could probably lead to too many surprises as the methods reside in other source files and can get forgotten.

        1. A better world’s Java would include lombok. I’d also prefer something build-in to all the insanity lombok has to do in order to keep me sane and save my fingers.

          One might be tempted to claim that all very popular libraries should be rendered obsolete by new Java versions. Much of Guava would qualify too, for instance.

          Sure, but AFAIK they could be made virtual if someone really wanted. Just let the compiler generate a top-level extension method doing virtual dispatch using an instanceof switch to one of the applicable user-defined extension methods.

          Hmm, there’s a lot of potential for conflicts, this way. Let’s assume that there are two completely independent void x(); extension methods on, say, java.lang.Object. If they were actually added to Object using virtual dispatch, which implementation would win in general?

          With static extension methods, it doesn’t matter. Object is not modified, and the method call sugar is only locally applied to the individual call sites. Or, perhaps, I’m missing something?

          1. One might be tempted to claim that all very popular libraries should be rendered obsolete by new Java versions. Much of Guava would qualify too, for instance.

            Sure, and this is actually happening, albeit rather slowly and not always in the best possible way (e.g., com.google.common.base.Objects.equal can be statically imported, unlike java.util.Objects.equals).

            Let’s assume that there are two completely independent void x(); extension methods on, say, java.lang.Object.

            Can you do this in a single project? I see, you “import” just the one you want.

            With static extension methods, it doesn’t matter. Object is not modified

            Dynamic extension methods would not modify existing classes either. They’d just get aggregated into a dispatching extension method. Somehow.

            and the method call sugar is only locally applied to the individual call sites.

            Now, I can see the big mess resulting from my idea. On every call site, I’d collect all “imported” extension methods and aggregate them. You forget one and get a nasty surprise.

            When you imported these two dynamic extension methods

            class ExtObj1 {
               static void x(Object o) {...}
            class ExtStr1 {
               static void x(String o) {...}

            you’d get this static extension method generated and used instead of them

            static void x(Object o) {
               if (o instanceof String) {
                  ExtStr1.x((String) o);
               } else {

            On a different call site with different extensions, you could get something else. Pretty strange. Not really dynamic.

            I guess, it could work with some restrictions… but I’m not sure if these restrictions can be sane.

            1. Hmm, I’m not so sure how these are “dynamic”, or different from “ordinary” extension methods. I suspect that with the right design, such behaviour can still be achieved with classic “static” extension methods. But I might still be missing your point :)

          2. They’re different by the dynamic dispatch. What I wrote was a possible implementation of the dynamic dispatch using ordinary extension methods and one generated method per method signature.

            If there were a single non-conflicting set extension methods used globally, it’d be a “perfect dynamic dispatch”, working like if the methods were defined in the class itself (except for visibility restrictions, but let’s call it a feature).

            This could be usable. Instead of using

            @ExtensionMethods(ExtObj1::x, ExtStr1::x)
            class ExtendedByX {...}

            you’d configure your project by specifying a set of (classes specifying) extension methods to be used everywhere. Let’s say in extension-info.java. I hope, the “biggest of all hacks” is gonna offer such a thing soon (albeit with static dispatch).

            Your example of “two completely independent void x();” would be as illegal as defining void x(); in a class twice. This should be no real problem, given that you want x to mean the same thing in the whole project. Just choose one x and stick with it.

  4. About point 8. Not everything is expression. Assignment is not. An old legacy of C/C++ got fixed.

    1. Really? Interesting, thanks for pointing this out. Well, I guess if val is the default, then you can’t have assignment expressions. I do find that quite useful at times, though, in Java (although I agree it doesn’t really add to readability)

      1. IMO using an assignment as expression is a code smell (or a lack of the right language features for the job). For things like reading from a stream where you typically do `while (len = r.read(chars)) >= 0)` in java, you can use higher order functions like `Reader.forEachLine{ line -> /* do something with line */ }` in Kotlin.

        1. IMO using an assignment as expression is a code smell

          Not sure… This is a typical idiom how to read / write streams in Java:

          ByteArrayOutputStream result = new ByteArrayOutputStream();
          byte[] buffer = new byte[1024];
          int length;
          while ((length = inputStream.read(buffer)) != -1) {
              result.write(buffer, 0, length);
          return result.toString("UTF-8");

          I don’t think there’s anything particularly smelly about this example, given the historic context of special return values being encoded into primitive type ints.

          or a lack of the right language features for the job

          That, I completely agree with.

          you can use higher order functions like `Reader.forEachLine{ line -> /* do something with line */ }` in Kotlin.

          You can do that as well in Java:

          try (Stream<String> lines = Files.lines(path)) {
              lines.forEach(line -> { ... });

          So, I guess we agree. Given historic context, assignment expressions made sense. But with more modern APIs, we don’t really need that anymore.

  5. I agree with a lot of these, but not the null coalescing thing.
    Dealing with nulls SHOULD be painful so that people who return nulls will be gently encouraged by other developers to not return nulls.
    In SQL you don’t have nulls as return values. You can have empty results, but you don’t get null. (Yeah, you can have them as a value. It’s annoying there too. Pattern anyone?)

    The problem is that we see null, see that it’s annoying and we want some sugar for it. But I think that a better solution would be to do what Common Lisp did with the null. If we can’t get there, then nulls must be painful. Nulls must HURT. That way, people don’t write return null, because people will gently tie them up and beat them up until they stop doing that. Returning null is evil. Returning Optional is good. Even if people just do Optional.getOrThrow(() -> throw new ReasonableExceptionType(complaintString));
    They will deal with no value returns immediately, and then we can move on with our lives… :p

    1. SQL NULL isn’t really annoying, it’s quite elegant (apart from the fact that it is a bit inconsistent across the language) and has nothing to do with Java’s null. It means UNKNOWN with all the nice properties of three-valued logic associated with that. But yes, I agree. Otherwise, the fact that SQL operates on sets which can never be null is quite convenient. There are programming languages, where everything is an array, too…

      Anyway, I think what Kotlin did is quite reasonable on the JVM. As long as we’re running on the JVM, we have to live with null.

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