Swift’s extensions system has spoiled me, and I feel the pain of this whenever I have to write Java
You should take a look at kotlin, pretty similar to swift and fully interoperable with java.
Ditto, but Rust's traits. God those are so fun. It's like duck typing a la Python but you can just slap whatever methods you want on a foreign type without worrying about breaking anything because they're only visible to the current crate (or other crates that import the Trait)
Swift’s extensions system has spoiled me, and I feel the pain of this whenever I have to write Java
You should take a look at kotlin, pretty similar to swift and fully interoperable with java.
Ditto, but Rust's traits. God those are so fun. It's like duck typing a la Python but you can just slap whatever methods you want on a foreign type without worrying about breaking anything because they're only visible to the current crate (or other crates that import the Trait)