Useful features

• A lot less boilerplate than Java
• 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 a ModInt template that overloads the arithmetic operators appropriately, but is represented as a plain int in JVM runtime
• run function – great way to write code that needs to shortcut (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