Id suggest rust, gets you a step closer to the hardware and a bit of a different paradigm than Java while still feeling high level.

Python seems to be a staple for as long as I can remember and it looks like it's still gonna be going strong for a good while yet!

I'm thinking of taking the dive and finally learning it myself soon.

C# is a great choice.

Incredibly versatile language and should be an easy jump from java.

I have a couple of suggestions based on what you want!

Ocaml: A good intro into functional programming

The languages you've used seem to be primarily imperative style. Ocaml is not commonly used in the industry (with some exceptions), but it has a vibrant small community, and more importantly, because it's a functional language, it will be different enough from what you know. Even if you don't use it in industry, it will teach you a lot that would be hard to learn using non-functional languages.

Ocaml is a compiled language, with a very fast compiler and very good type system.

Scala: the only semi-mainstream language that blends OOP and proper functional programming well.

Although scala is mostly a functional language, it still has most of the gestures of imperative OOP languages like Java. It is also far more mainstream than ocaml and has used in industry.

Scala also runs in JVM like Java, and thus gives you access to most JVM libraries.

Rust

Intro into low level programming Other answers spoke enough about Rust so I won't dive here

Zig

similar to Rust in low-levelness, but less mainstream, better C interop, and other interesting differences

Raku: the result of 15 years of programming language design

Raku, formerly perl 6, was almost redesigned from the ground up to resolve complaints of perl 5. It has a very interesting design, with a gradual type system, a blend of functional and OOP paradigms, plus interesting syntax features like sigils and grammars.

There are many interesting languages out there. I could talk forever about this, but I'll leave you with the above. I personally would recommend ocaml if you're interested in functional programming or care about performance. Otherwise, Raku, since its very interesting but doesn't perform well (Raku's VM is still immature and under development).

I can recommend Rust - I'm training a few people on it.

And a pure functional programming language like Haskell or Scheme, if you don't know what functional programming is, or are not comfortable with it. Functional programming needs a different mental approach to traditional (imperative) programming paradigm. Some of the more modern languages like Rust, JS and Python incorporate a lot of functional programming constructs. So it makes sense to learn them.

And a lisp - Common Lisp's popularity is a public secret. Scheme is also fine. This family is homoiconic (program and data are treated more less the same). The syntax is actually very close to its AST. This gives Lisp unparalleled metaprogramming capabilities - mostly through macros. Macros in traditional languages are nowhere near Lisp Macros.

If it interests you, study a stack based language like Forth or Factor. Though they feel very different from Lisps, they have similar underlying properties. And you get more or less the same advantages.

I advise you to learn something different and hard for you. Only this case will help you to grow and realize a lot of new.

• Rust for hard
• Nim for something different
• C for understanding how things work

All these languages are efficient and forget about hype and popularity. Language does not matter if you have what to write with it.

Learnyouahaskell.com is a good way to get started with Haskell. I'd recommend that as best for your mind expansion mission.

No jokes: pick a language that is in the market, but has a different design philosophy than your background. Your background includes compiled static, and loose scripting, with strong library tooling, so you have diversity there, so a language in which you have to think differently is the right choice.

I recommend:

1. Rust if you want something safe but that makes you work differently from java
2. Go if you want a real mental challenge (the coding approach is very different from java)
3. Lua if you want really see functional programming as a philosophy
4. Python if you want scripting, and are tired of the web

Personally, the language that's taught me the most to learn has been Haskell. It has a lot of very interesting ideas and a learning curve that plateaus after most other languages. There are several ideas that have trickled down from Haskell to other parts of the programming world and learning about them in the context Haskell is in my opinion better because you'll learn about them in a context where they fit in with the rest of the language very well instead of being late additions that offer an alternate way of doing things.

Coming from Java and JS, Haskell has a very different approach to a lot of things so you'll have to re-learn a lot before you get productive in it. This can be frustrating for some but you'll learn more if you get over that hump on the other hand.

Haskell doesn't see very much industry use and arguably isn't very well suited for industrial application (I haven't used it professionally so I don't know personally) so it might not directly help you land any new jobs but it is in my opinion it's a very good way to develop as a programmer.

Give Clojure a go.

It's a modern variant of lisp that runs on the JVM and has deep interoperability with Java, so you can leverage your existing knowledge of Java libraries.

But as it's a lisp, it will have you thinking about problems in a very different way.

I’m a huge fan of Haskell and (for pragmatic purposes) Purescript. Purescript is hard to find much in the way of documentation but it is so similar to Haskell in that the steep learning curve is worth it, IMO. I rarely find a project that I couldn’t accomplish with one or the other or both.

Elm

In short, it's ruined my expectations of languages. It's a functional language, like the style of Haskell, and transpiles to html or js (its meant for web). There's very little that it allows for going wrong, and for things that could fail, it either tells you to port that out to JS and bring it back when you're done, or you have to handle a Result type or Maybe type.

It sounds strict, yes, but not having to deal with issues later is so nice.

I would suggest Nim, I had a blast learning it and making a small project. It is not a mainstream language, nor is it a joke language.

https://nim-lang.org/

C or C++, specifically with the use of compiler explorer so you can get a feel for how code actually runs.

Common Lisp or Haskell to get a taste of something really different.

I’ll suggest Elixir. It’s a language that runs on the same virtual machine as Erlang, which has proven to be great for ultra-reliable and excellent at managing many, MANY concurrent processes.

Elixir itself builds upon this great foundation with a syntax similar to Ruby, but entirely functional. It’s a delightful language to read and write.

Rust, haskell, python, c++ are all interesting choices. I would argue that c# is too close to what you already know to be interesting.

If I were you though, I'd pick a project first, then decide what language makes sense for it.

Rust

Kotlin is by far my fav language. I've been astonished by its capabilities used with the "Exposed" library

How about COB-

No joke languages.

Oh...

C. Plain old low-level "portable assembly" C.

In particular I'd recommend a Game Boy game, because GBDK uses SDCC and is fairly low-bullshit. With scanline interrupts, you have a surprising amount of time to do clever visual effects, flexing the machines one-and-a-half background layers. The machine is sufficiently explained in about forty-five minutes via the Ultimate Game Boy Talk.

(NES is also an option, but - don't. I say this with my NES homebrew project open in the other monitor. It's a miracle this thing ever caught on. You need a decent understanding of the hardware, C, and 6502 assembly, just to put stuff onscreen efficiently. And then the fillrate still blows. The Game Boy can blast the whole tilemap in a fraction of a frame if you cheat hard enough.)

If you want more power, the GBA supports C and C++, and that little fucker runs Doom. It's intended as a super-flexible sprites-and-tilemaps affair, like an SNES with an upgraded graphics chip. (And a downgraded sound chip.) You can scale and rotate dozens of sprites at 60 Hz, atop two scaled-and-rotated backgrounds. Or use fullscreen bitmaps in 8-bit or 16-bit color. (Well, 15-bit.) The homebrew scene for the GBA started before the console even launched, because it has a pretty standard ARM chip and will do anything the Genesis could without breaking a sweat. There was nearly a voxel heightmap version of Banjo Pilot that looked pretty damn close to Diddy Kong Racing on N64.

I recommend C and retro consoles because they're suited to one another - and they offer control. You are in command of every single byte on these machines. C is the ideal language for that. It offers familiar high-level structures. It supports familiar syntax. But it runs very close to the metal, and if you want to get even closer, it will let you.

I recently picked up Rust, still a beginner, but it's very nice. And the compiler errors and tips are top. Since I used only Python before, Rust is my first compiled language and I'm glad it was Rust. Options and Result's are also something I appreciate tbh. Other than for AI, simple stuff and maybe prototypes, I would use Rust over Python. Definitely recommend it. The borrow checker is a pain in the ass though. I'll also be using it later for embedded systems when I become more fluent with it. I am also currently making a big project with Rust that I haven't been able to do with Python.

C++, or if you want to lean towards a more modern lang, C#. Kotlin is also a lot of fun

Kotlin is the language to beat imo. Very expressive and full interop with Java. Structured concurrency so multithreading Is a breeze. Build mobile apps easily with it for Android (and in theory iOS but that's still alpha). Desktop apps too!

Scala can also be compiled to JavaScript with really good interop with JS libraries, and to native with decent interop with C

Prolog.

It will free your mind.

Scheme, and work through SICP, watch the lectures along with the reading.

I prefer Chez Scheme but there's many implementations. Chez's fast and practical, C FFI, large standard library, nice REPL with editor.

I see you mentioned JS, but not TS. If you haven't tried TypeScript, you 100% should! It helps a lot at scale.

Also, I really do recommend Rust. It's pretty awesome having the errors actually make sense, and it's not as complicated as the hype makes it out to be (until you get into async rust lol).

As others have mentioned, C# is also awesome.

C.

Go. It'll be just different enough from what you have experience with to make you think about things differently (in a good way!) from now on. And it's also a fantastically well-designed language that's great for getting real work done. And it's lightning fast as languages go, and compiles to an actual executable. Really a pleasure to work with. It's my (no pun intended) go-to language for every new project I start. (Excluding what I write specifically for a paycheck. I don't have a choice there.)