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)
reformat(str, true, true, false, '_')
reformat(str,
  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):

CASE x
  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'
END

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)
println(map["a"])

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)
   .from(a)
   .join(b).on(a.ID.eq(b.AUTHOR_ID))
   .orderBy(1, 2, 3)
   .forEach {
       println("""${it[b.TITLE]} 
               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:

list.stream()
    .zipWithIndex()
    .forEach(System.out::println);

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

seq(list.stream())
    .zipWithIndex()
    .forEach(System.out::println);

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

list.zipWithIndex()
    .forEach(System.out::println);

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)
         .findFirst();

// Agressive programming ahead
result.orElse(OR_ELSE);

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
    .flatMap(Person::getDepartment)
    .map(Department::getHead)
    .flatMap(Person::getName);

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");
else
    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 {
    count()
} catch (e: ArithmeticException) {
    throw IllegalStateException(e)
}

Beautiful!

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 {
    @AliasFor("attribute")
    String value() default "";
    @AliasFor("value")
    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<out T> : 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

Conclusion

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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Java 8 Friday: Optional Will Remain an Option in Java


At Data Geekery, we love Java. And as we’re really into jOOQ’s fluent API and query DSL, we’re absolutely thrilled about what Java 8 will bring to our ecosystem.

Java 8 Friday

Every Friday, we’re showing you a couple of nice new tutorial-style Java 8 features, which take advantage of lambda expressions, extension methods, and other great stuff. You’ll find the source code on GitHub.

Optional: A new Option in Java

So far, we’ve been pretty thrilled with all the additions to Java 8. All in all, this is a revolution more than anything before. But there are also one or two sore spots. One of them is how Java will never really get rid of

Null: The billion dollar mistake tweet this

In a previous blog post, we have explained the merits of NULL handling in the Ceylon language, which has found one of the best solutions to tackle this issue – at least on the JVM which is doomed to support the null pointer forever. In Ceylon, nullability is a flag that can be added to every type by appending a question mark to the type name. An example:

void hello() {
    String? name = process.arguments.first;
    String greeting;
    if (exists name) {
        greeting = "Hello, ``name``!";
    }
    else {
        greeting = "Hello, World!";
    }
    print(greeting);
}

That’s pretty slick. Combined with flow-sensitive typing, you will never run into the dreaded NullPointerException again:

Recently in the Operating Room. By Geek and Poke

Recently in the Operating Room. By Geek and Poke

Other languages have introduced the Option type. Most prominently: Scala. Java 8 now also introduced the Optional type (as well as the OptionalInt, OptionalLong, OptionalDouble types – more about those later on)

How does Optional work?

The main point behind Optional is to wrap an Object and to provide convenience API to handle nullability in a fluent manner. This goes well with Java 8 lambda expressions, which allow for lazy execution of operations. An example:

Optional<String> stringOrNot = Optional.of("123");

// This String reference will never be null
String alwaysAString =
    stringOrNot.orElse("");

// This Integer reference will be wrapped again
Optional<Integer> integerOrNot = 
    stringOrNot.map(Integer::parseInt);

// This int reference will never be null
int alwaysAnInt = stringOrNot
        .map(s -> Integer.parseInt(s))
        .orElse(0);

There are certain merits to the above in fluent APIs, specifically in the new Java 8 Streams API, which makes extensive use of Optional. For example:

Arrays.asList(1, 2, 3)
      .stream()
      .findAny()
      .ifPresent(System.out::println);

The above piece of code will print any number from the Stream onto the console, but only if such a number exists.

Old API is not retrofitted

For obvious backwards-compatibility reasons, the “old API” is not retrofitted. In other words, unlike Scala, Java 8 doesn’t use Optional all over the JDK. In fact, the only place where Optional is used is in the Streams API. As you can see in the Javadoc, usage is very scarce:

http://docs.oracle.com/javase/8/docs/api/java/util/class-use/Optional.html

This makes Optional a bit difficult to use. We’ve already blogged about this topic before. Concretely, the absence of an Optional type in the API is no guarantee of non-nullability. This is particularly nasty if you convert Streams into collections and collections into streams.

The Java 8 Optional type is treacherous tweet this

Parametric polymorphism

The worst implication of Optional on its “infected” API is parametric polymorphism, or simply: generics. When you reason about types, you will quickly understand that:

// This is a reference to a simple type:
Number s;

// This is a reference to a collection of
// the above simple type:
Collection<Number> c;

Generics are often used for what is generally accepted as composition. We have a Collection of String. With Optional, this compositional semantics is slightly abused (both in Scala and Java) to “wrap” a potentially nullable value. We now have:

// This is a reference to a nullable simple type:
Optional<Number> s;

// This is a reference to a collection of 
// possibly nullable simple types
Collection<Optional<Number>> c;

So far so good. We can substitute types to get the following:

// This is a reference to a simple type:
T s;

// This is a reference to a collection of
// the above simple type:
Collection<T> c;

But now enter wildcards and use-site variance. We can write

// No variance can be applied to simple types:
T s;

// Variance can be applied to collections of
// simple types:
Collection<? extends T> source;
Collection<? super T> target;

What do the above types mean in the context of Optional? Intuitively, we would like this to be about things like Optional<? extends Number> or Optional<? super Number>. In the above example we can write:

// Read a T-value from the source
T s = source.iterator().next();

// ... and put it into the target
target.add(s);

But this doesn’t work any longer with Optional

Collection<Optional<? extends T>> source;
Collection<Optional<? super T>> target;

// Read a value from the source
Optional<? extends T> s = source.iterator().next();

// ... cannot put it into the target
target.add(s); // Nope

… and there is no other way to reason about use-site variance when we have Optional and subtly more complex API.

If you add generic type erasure to the discussion, things get even worse. We no longer erase the component type of the above Collection, we also erase the type of virtually any reference. From a runtime / reflection perspective, this is almost like using Object all over the place!

Generic type systems are incredibly complex even for simple use-cases. Optional makes things only worse. It is quite hard to blend Optional with traditional collections API or other APIs. Compared to the ease of use of Ceylon’s flow-sensitive typing, or even Groovy’s elvis operator, Optional is like a sledge-hammer in your face.

Be careful when you apply it to your API!

Primitive types

One of the main reasons why Optional is still a very useful addition is the fact that the “object-stream” and the “primitive streams” have a “unified API” by the fact that we also have OptionalInt, OptionalLong, OptionalDouble types.

In other words, if you’re operating on primitive types, you can just switch the stream construction and reuse the rest of your stream API usage source code, in almost the same way. Compare these two chains:

// Stream and Optional
Optional<Integer> anyInteger = 
Arrays.asList(1, 2, 3)
      .stream()
      .filter(i -> i % 2 == 0)
      .findAny();
anyInteger.ifPresent(System.out::println);

// IntStream and OptionalInt
OptionalInt anyInt =
Arrays.stream(new int[] {1, 2, 3})
      .filter(i -> i % 2 == 0)
      .findAny();
anyInt.ifPresent(System.out::println);

In other words, given the scarce usage of these new types in JDK API, the dubious usefulness of such a type in general (if retrofitted into a very backwards-compatible environment) and the implications generics erasure have on Optional we dare say that

The only reason why this type was really added is to provide a more unified Streams API for both reference and primitive types tweet this

That’s tough. And makes us wonder, if we should finally get rid of primitive types altogether.

Oh, and…

Optional isn’t Serializable.

Nope. Not Serializable. Unlike ArrayList, for instance. For the usual reason:

Making something in the JDK serializable makes a dramatic increase in our maintenance costs, because it means that the representation is frozen for all time. This constrains our ability to evolve implementations in the future, and the number of cases where we are unable to easily fix a bug or provide an enhancement, which would otherwise be simple, is enormous. So, while it may look like a simple matter of “implements Serializable” to you, it is more than that. The amount of effort consumed by working around an earlier choice to make something serializable is staggering.

Citing Brian Goetz, from:

http://mail.openjdk.java.net/pipermail/jdk8-dev/2013-September/003276.html

Want to discuss Optional? Read these threads on reddit:

Stay tuned for more exciting Java 8 stuff published in this blog series.

More on Java 8

In the mean time, have a look at Eugen Paraschiv’s awesome Java 8 resources page