I pretty much exclusively use Kotlin for competitive programming, mostly because it's the language I'm currently most comfortable with. Here are some scattered notes and tidbits about my experience which I think might be useful to others; if you have any tips/suggestions, feel free to let me know.

Useful features

• A lot less boilerplate than Java. Type inference means a lot less "Pokémon speak". Variables and functions can be declared straight in the top-level of the file. (basically the equivalent of static functions)

• data classes – basically custom tuples. Allows convenient destructuring declarations too.

• Has access to the data structures in the Java standard library (TreeMap, HashMap, PriorityQueue etc.), and also can use BigInteger and BigDecimal if needed

• Functional idioms for collection manipulation; map, fold, filter, etc.

• inline classes – allows the creation of a new type that wraps over a base type, but that is represented by an underlying type at runtime. Especially useful for "modulo $$$10^9 + 7$$$" problems, as I keep code for a ModInt class that overloads the arithmetic operators appropriately, but is represented as a plain int in JVM runtime. Keep in mind that they are experimental as of Kotlin 1.3, but that's fine for CP in my opinion

• unsigned integer types in the standard library that use the inline class feature.

• run function – great way to write code that needs to shortcut (e.g. return@run "NO") without having to write a new function and pass every relevant argument

• functions in functions – functions can be defined within e.g. the main function, so again, no having to pass lots of arguments or global variables