Any and Nothing Classes

Some methods like equals() and toString() are pervasive. In Java you’d expect to find them in the Object base class. In Kotlin, these methods are included in the Any class, along with a number of other methods that are useful on instances of any class. Any is Kotlin’s counterpart of Java’s Object class, except Any has a lot of special methods that come in through extension functions. Kotlin also has a class named Nothing that serves to stand in as type when a function literally is expected to return nothing—this is useful for type-checking methods when one or more branches is expected to return nothing. Nothing in Java is equivalent to Kotlin’s Nothing. In this section you’ll learn about the facilities offered by the ubiquitous Any and the purpose of Nothing.

Any, the base class

All classes in Kotlin inherit from Any, which maps to Object in Java. If a function will take objects of different types as a parameter, then you can specify its type as Any. Likewise, if you can’t put your finger on a specific type to return, you may return Any. The Any class gives you the maximum—often too much—flexibility from the type point of view, so use it sparingly.

The purpose of Any isn’t to let us define variables, parameters, or return types as Any, though occasionally we may want to, but to provide some common methods that are available on all Kotlin types. For example, methods like equals(), hashCode(), and toString() may be called on any type in Kotlin because those methods are implemented in Any.

Even though Any maps to Object in Java in the bytecode, they’re not identical. Also, Any offers some special methods through extension functions. For example, the to() extension function that we saw in Using Pair and Triple, is an excellent example. Since creating a Pair of different objects is such a common operation and Pair is used widely with helper functions to create maps, Kotlin decided to make the to() method, which creates a Pair of objects of any type, universally available on objects of every single type.

Likewise, executing a block of code in the context of an object can remove a lot of verbose and repetitive code. To facilitate this, Any has extension functions like let(), run(), apply(), and also()—we’ll explore these in Fluency with Any Object. By using these methods you can remove a lot of clutter. These are also useful for creating highly fluent internal DSLs in Kotlin, as you’ll see in Chapter 14, Creating Internal DSLs.

Nothing is deeper than void

In languages like Java we use void to indicate that a method returns nothing. In Kotlin we use Unit, instead, to tell us when functions, which are expressions, return nothing useful. But there are situations where a function truly returns nothing…nada; that’s where the Nothing class comes in. The class Nothing has no instances and it represents a value or result that will never exist. When used as a return type of a method it means that the function never returns—the function call will only result in an exception.

One unique capability of Nothing is that it can stand in for anything—that is, Nothing is substitutable for any class, including Int, Double, String, and so on. For example, take a look at the following code: