Scala Type Hierarchy

In Scala, there are no primitive types. All basic types are also classes and they are part of the class hierarchy. Scala type hierarchy is divided into two categories which are AnyRef and AnyVal.

Scala Type Hierarchy

Any is the supertype of all types, also called the top type. It defines certain universal methods such as equals, hashCode, and toString. Any has two direct subclasses: AnyVal and AnyRef.

AnyVal represents value types. There are nine predefined value types and they are non-nullable: Double, Float, Long, Int, Short, Byte, Char, Unit, and Boolean. Unit is a value type that carries no meaningful information. There is exactly one instance of Unit which can be declared literally like so: (). All functions must return something so sometimes Unit is a useful return type.

AnyRef represents reference types. All non-value types are defined as reference types. Every user-defined type in Scala is a subtype of AnyRef. If Scala is used in the context of a Java runtime environment, AnyRef corresponds to java.lang.Object

Here is an example that demonstrates that strings, integers, characters, boolean values, and functions are all of the types Any just like every other object:

val list: List[Any] = List(
  "a string",
  732,  // an integer
  'c',  // a character
  true, // a boolean value
  () => "an anonymous function returning a string"
)
list.foreach(element => println(element))

It defines a value list of the type List[Any]. The list is initialized with elements of various types, but each is an instance of scala. Any, so you can add them to the list.

Nothing and Null

There are two special types at the bottom of the hierarchy. Nothing is the type of throw expression, and Null is the type of the value null. These special types are subtypes of everything else, which helps us assign types to throw and null while keeping other types in our code sane. The following code illustrates this:

def badness = throw new Exception("Error") // badness: Nothing
null
// res: Null = null
val bar = if(true) 123 else badness // bar: Int = 123
val baz = if(false) "it worked" else null // baz: String = null

Although the types of badness and res are Nothing and Null respectively, the types of bar and baz are still sensible. This is because Int is the least common supertype of Int and Nothing, and String is the least common supertype of String and Null.