Tour of Scala

Unified Types


In Scala, all values have a type, including numerical values and functions. The diagram below illustrates a subset of the type hierarchy.

Scala Type Hierarchy

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 which 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 type 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 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.

Here is the output of the program:

a string

Type Casting

Value types can be cast in the following way: Scala Type Hierarchy

For example:

val x: Long = 987654321
val y: Float = x.toFloat  // 9.8765434E8 (note that some precision is lost in this case)

val face: Char = '☺'
val number: Int = face  // 9786

Casting is unidirectional. This will not compile:

val x: Long = 987654321
val y: Float = x.toFloat  // 9.8765434E8
val z: Long = y  // Does not conform

You can also cast a reference type to a subtype. This will be covered later in the tour.

Nothing and Null

Nothing is a subtype of all types, also called the bottom type. There is no value that has type Nothing. A common use is to signal non-termination such as a thrown exception, program exit, or an infinite loop (i.e., it is the type of an expression which does not evaluate to a value, or a method that does not return normally).

Null is a subtype of all reference types (i.e. any subtype of AnyRef). It has a single value identified by the keyword literal null. Null is provided mostly for interoperability with other JVM languages and should almost never be used in Scala code. We’ll cover alternatives to null later in the tour.

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