For me personally Kotlin does not bring that much to the table over a modern version of Java. Sure if you're stuck in Java 8 it can be a win, but compared to Java 21 it just doesn't bring enough to warrant the switch for me.
Tying down a value to a name only if you have something meaningful to say in the name, and only when the value is actually finished instead of being a work in progress, that gives me great piece of mind and frees up mental resources for other things. I wish every language had a "kotlinized" sibling that simply added the scope functions on syntax level (or a subset of the scope functions, not sure you need a zoo that size)
I think Kotlin is at risk, though, of having the JVM force them to support two competing models to accomplish the same thing - coroutines and virtual threads is a good example (although Java still doesn’t have language level structured concurrency). This could really make things messy.
I just don't think any feature of the language of Kotlin is that much better than Java's implementation. I'm also a big supporter of brevity != clarity when it comes to code.
What language feature in Kotlin do you think is that much better?
It's weird being on the other side of the fence now - hearing the same things I used to say.
Kotlin was a gateway drug out of not just Java, but OO/Solid and into FP for me (FP being another thing I used to not "get"). The beauty of Kotlin is you don't have to ever write FP code if you don't want to - but you will want to before too long.
I would recommend trying it out for yourself. It's Spring integration is amazing, and you'll have a great time. Write even a little Kotlin and you'll have an "ah ha!" moment and everything will forever be different.
It's one of those things where you need to experience it for yourself before you actually understand how great it is. There is no argument you will hear that will convince you, sadly. I was the same once...
- Having to remember the difference between "Function", "Consumer", "Predicate" and "Supplier" is weird. In Kotlin (and basically every other typed FP language that I know), I just write the type signature the natural way.
- Having to use streams in order to use map, etc., is noisy. Why weren't these APIs retrofitted to collections?
- Checked exceptions are a pain with functional code, e.g. map.
- Mutability is still the default behaviour of almost everything in Java, save records (and strings).
etc.
Sure, methods along the lines of the following could have been added:
public <R> List<R> map(Function<? super E, ? extends R> mapper) {
return stream().map(mapper).toList();
}
- I don’t think this is really that big of a deal.
- Using streams is actually preferred so you don’t cause a performance nightmare and kotlin you has to use asSequence anyway.
- I thought checked error handling was all the rage nowadays?
- Mutability is absolutely fine if you’re not going over a thread boundary.
And checked exceptions are widely seen as an anti-feature - I don't think any other language has them, with the possible exception of Swift (see below), and for good reason. Other languages encode errors (more or less ergonomically) in the type system (e.g. Rust, Haskell, and to an extent it's what Kotlin advocates as well), and that has the advantage of playing well together with other language mechanisms - e.g. no extra allowances need to be made so that map works with them.
But for the sake of the argument: Swift also has something similar to checked exceptions, but only in the sense that functions can either throw or not throw. However, Swift also at least has constructs like "rethrows", which lets you say that a function such as map should throw iff the higher order function it wraps throws.
Java simply decided that it doesn't need to deal with this problem at all (for whatever reason - they simply could have added something like "rethrows") and now it's up to callers to deal with the incredible awkwardness that is using FP with Java library code that makes heavy use of checked exceptions.
var, records, sealed types, and pattern matching make a huge difference in how Java is written. A lot has changed since 8.
I left the Java world for the C# world around 2008 and then returned to the Java world last year. C#/.NET changed much more significantly much faster. Almost all of the significant differences in day to day life with respect to the Java language and standard libraries that occurred between 2008 and 2022 are due to changes introduced with Java 8.
I don't think it's that good an example, though there's some merit to it. Kotlin coroutines, to me, target three main use cases:
1. An abstraction over an underlying threading model. Get threaded execution without having to explicitly deal with thread pools and such. I'd consider structured concurrency to be part of this use case.
2. A way to write non-blocking code that is structured as if it were blocking. (Incidentally: I love Kotlin's decision to make "await" opt-out instead of opt-in.)
3. A way to write code that generates sequences of values without having to either write an explicit class that maintains state across calls or use callbacks/CPS. Or in other words, a way to use the "yield" function.
Use case 1 still makes sense with virtual threads. Run your coroutines on an executor that uses a virtual thread pool instead of a platform thread pool. If you prefer the coroutines API, you can still use it for structured concurrency and such.
Use case 2 is much less interesting in the presence of virtual threads; you can just write blocking code directly, no need for the language to turn it into async code for you.
Use case 3 has nothing to do with threads to begin with, so it remains exactly as valuable as before and there's no reason to change any existing code at all.
Do coroutines get less useful in the presence of virtual threads? Absolutely. But they aren't competing with virtual threads; there are substantial non-overlapping use cases that make them a worthwhile language feature.