Class Any is the root of the Scala class hierarchy.

Class Any is the root of the Scala class hierarchy. Every class in a Scala execution environment inherits directly or indirectly from this class.

Starting with Scala 2.10 it is possible to directly extend Any using universal traits. A universal trait is a trait that extends Any, only has defs as members, and does no initialization.

The main use case for universal traits is to allow basic inheritance of methods for value classes. For example,

trait Printable extends Any {
  def print(): Unit = println(this)
}
class Wrapper(val underlying: Int) extends AnyVal with Printable

val w = new Wrapper(3)
w.print()

See the Value Classes and Universal Traits for more details on the interplay of universal traits and value classes.

Nothing is - together with scala.Null - at the bottom of Scala's type hierarchy.

Nothing is - together with scala.Null - at the bottom of Scala's type hierarchy.

Nothing is a subtype of every other type (including scala.Null); there exist no instances of this type. Although type Nothing is uninhabited, it is nevertheless useful in several ways. For instance, the Scala library defines a value scala.collection.immutable.Nil of type List[Nothing]. Because lists are covariant in Scala, this makes scala.collection.immutable.Nil an instance of List[T], for any element of type T.

Another usage for Nothing is the return type for methods which never return normally. One example is method error in scala.sys, which always throws an exception.

Null is - together with scala.Nothing - at the bottom of the Scala type hierarchy.

Null is - together with scala.Nothing - at the bottom of the Scala type hierarchy.

Null is a subtype of all reference types; its only instance is the null reference. Since Null is not a subtype of value types, null is not a member of any such type. For instance, it is not possible to assign null to a variable of type scala.Int.