In Kotlin, the uppermost type is called Any. This is analogous to Java's object type. The Any type defines the well-known toString, hashCode, and equals methods. It also defines the extension methods apply, let, and to, among others. These methods will be described in more detail in Chapter 5, Higher Order Functions and Functional Programming.

The Unit type is the equivalent of void in Java. Having a Unit type is common in a functional programming language, and the distinction between void and Unit is subtle. Void is not a type, but a special edge case that is used to indicate to the compiler that a function returns no value. Unit is a proper type, with a singleton instance, also referred to as Unit or (). When a function is defined as a returning Unit, then it will return the singleton unit instance.

This results in greater soundness of the type system as now all functions can be defined as having a return value, even if it's just the Unit type, and functions that have no arguments...