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 a 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 system type as...