I disagree.
Java records are from Standard ML but sealed is not, it's from Scala with the improvement that subtypes of a sealed types are sealed too.
Java still not have a real syntax for ADT, Standard ML has datatype, Rust or Scala 3 have enums on steroid [1].
sealed interface Expr {}
record ConstantExpr(int i) implements Expr {}
record PlusExpr(Expr a, Expr b) implements Expr {}
record TimesExpr(Expr a, Expr b) implements Expr {}
record NegExpr(Expr e) implements Expr {}
Java misses the syntactic sugar to define ADT in one place like in Standard ML, OCaml or F#.
The closest you get is to declare records inside a sealed interface, but once you add generics in the mix, it starts to get messy.
I was not able to define a classical list with cons and nil in Java.
sealed interface List<T> {
record Cons<T>(T car, List<T> cdr) implements List<T> {}
record Nil() implements List {}
}
I don't think so.
> I was not able to define a classical list with cons and nil in Java.
If you want to restrict Nil to a single instance, don't make it a record class:
sealed interface List<T> {
static <T> List<T> cons(T car, List<? extends T> cdr) { return new Cons<T>(car, cdr); }
static <T> List<T> nil() { return (List<T>)Nil.INSTANCE; }
record Cons<T>(T car, List<? extends T> cdr) implements List<T> {}
enum Nil implements List<Void> { INSTANCE }
}
import static List.*;
var x = cons("a", nil());
var y = cons(1, x);
Although, in this particular case,
record List<T>(T car, List<? extends T> cdr){}
List<String> list = cons("a", nil());
switch(list) {
case Cons<String>(var car, var cdr) -> ...
case Nil -> ...
}
You may be able to use extractors as the inverse of the static methods cons() and nil() but that adds more boilerplate.
Disjunctive enums like in Rust or Scala 3 is a simpler way to define ADT.
Of course, Java could add rules for the special case of zero-argument records, making them more enum-like, but I really think the value in doing that is negative. Other than being able to directly translate ML examples, it would add complexity that isn't worth it.
In fact, in this particular case I'd do it like so:
record List<T>(T car, List<? extends T> cdr) {
public List { if (car == null) throw new IllegalArgumentException("null element"); }
}
switch(list) {
case List<String>(var car, var cdr) -> ...
case null -> ...
}
Sealed classes, data classes, multiline strings, coroutines -> project loom, switch statement improvements to resemble Kotlin when statement, improvements to Streams to match Sequences.
That's not accurate.
Popularity of a feature and enterprise demand drive it's adoption.
It doesn't really matter what the language team knows, it matters what the average developer knows since their collective wants are in large part what drives the language team.
There are hundreds of thousands of Java devs who only ever wrote Java and suddenly had to write Kotlin, and are now clambering for these feature improvements in Java.
2. Java addresses those topics (where relevant) pretty much in the opposite way from Kotlin. Java rejected coroutines and is going down a completely different path with virtual threads; Java rejected data classes and chose to go down algebraic data types. So if you think Java gets inspiration from Kotlin, then it is in the form of what not to do.
If anything, the acceptance of more parts of the FP paradigm into all major languages (like C#, JS/TS, C++) starting with lambdas likely have pushed Java, but to single out the all-around very niche Kotlin language as a source is imo a non sequitur.
It's also important to not be US centric. In many parts of the world Android is the dominant OS.
I really struggle to see Kotlin as being part of the debate here. These features are being driven by enterprise Java user pain points. And those people aren’t looking at Kotlin.
By Java devs who only ever wrote Java, and suddenly had to write Kotlin for Android. They're now aware of these features as a result and are demanding them in Java.