Variance provides a way to constrain parameterized types. It defines a subtyping relationship between parameterized types based on the subtyping relationship of their component types.
Imagine that you have the following class hierarchy:
class Tool
class HandTool extends Tool
class PowerTool extends Tool
class Hammer extends HandTool
class Screwdriver extends HandTool
class Driller extends PowerTool
If we define a generic box:
trait Box[T] {
def get: T
}How can
Box of
Tools relate to one another? Scala provides three ways:
Covariant:
Box[Hammer] <: Box[Tool] if Hammer <: ToolContravariant:
Box[Tool] <: Box[Hammer] if Tool <: HammerInvariant: There's no subtyping relationship between
Box[Tool]andBox[Hammer]independently of the subtyping relationship ofToolandHammer