Unified Types

In Scala, all values are instances of a class, including numerical values and functions. The diagram below illustrates the class hierarchy.

Scala Type Hierarchy

Scala Class Hierarchy

The superclass of all classes scala.Any has two direct subclasses: scala.AnyVal and scala.AnyRef.

scala.AnyVal represents value classes. All value classes are non-nullable and predefined; they correspond to the primitive types of Java-like languages. Note that the diagram above also shows implicit conversions between the value classes.

scala.AnyRef represents reference classes. All non-value classes are defined as reference class. Every user-defined class in Scala implicitly extends scala.AnyRef. If Scala is used in the context of a Java runtime environment, scala.AnyRef corresponds to java.lang.Object.

Here is an example that demonstrates that strings, integers, characters, boolean values, and functions are all objects 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 variable list of type List[Any]. The list is initialized with elements of various types, but they all are instance of scala.Any, so you can add them to the list.

Here is the output of the program:

a string
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