Kotlin

input.toList().asSequence().groupingBy { it }.eachCount()

val -> immutable variable - can’t be changed once set (same as final in Java)

var -> mutable variable - can be changed during its lifetime

Unused variables can be replaced with _ (similar to Golang), e.g.:

mapOfSomething.forEach { _, value -> ... }

All classes are final (cannot be extended), unless prefixed with open operator., e.g.

open class Dog { ... }

Implement an interface via the colon : operator, e.g.

open class Dog : Animal { ... }

Extend an existing class via color : operator but also call the super constructor, e.g.:

class ShaggyDog(val hairColor: String) : Dog() { .. }

Declare private variables in a class, e.g.:

class Dog {
    private val animal: Boolean = true
}

Constructor variables that are also private variables, e.g.:

class Dog(val hairColor: String)

Entity classes (similar to POJOs in Java) can be simplified via data class (getters, setters not needed):

data class Car(val type: String, val color: String)