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.

Primer

• Kotlin has an official primer for competitive programming. However, the IO code suggested here is only so-so; it's definitely better than Scanner, but you definitely can save a lot of runtime in heavy input problems by using the classic Java combination of BufferedReader + StringTokenizer

Useful features

• A lot less boilerplate than Java. Members are public by default. 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). Fields have implicit getters and setters that can easily be overridden when necessary.

• 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.

• inline functions – tells the compiler to inline the function to call sites. Useful for higher-order functions (JVM won't need to create a new object for the lambda) as well as small functions that are called very often; basically, anything you might use a macro for in C++, you probably want to use an inline fun or inline val for

• tailrec fun – tail recursion optimization

• 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

Potential pitfalls

• Generic wrappers for JVM primitive types can cause TLE for some problems. Use primitive arrays (IntArray etc.) whenever possible to avoid this, but see next point

• Inherits the hack-vulnerable quicksort from Java for primitive arrays. Easiest solution is to use generic arrays or lists instead, but due to the performance benefit of primitive arrays, I've took the trouble to write code that shuffles them randomly before sorting