协变covariant[+T] 逆变contravariant[-T]

协变covariant[+T] 逆变contravariant[-T]在spark中用的不多，作了解。

 

含义	Scala 标记
协变covariant	C[T’]是 C[T] 的子类	[+T]
逆变contravariant	C[T] 是 C[T’]的子类	[-T]
不变invariant	C[T] 和 C[T’]无关	[T]

scala> classCovariant[+A]

defined class Covariant

 

scala> val cv:Covariant[AnyRef] = new Covariant[String]

cv: Covariant[AnyRef] =Covariant@4035acf6

 

scala> val cv:Covariant[String] = new Covariant[AnyRef]

<console>:6:error: type mismatch;

 found  : Covariant[AnyRef]

 required: Covariant[String]

       val cv: Covariant[String] = newCovariant[AnyRef]

                                   ^

scala> classContravariant[-A]

defined classContravariant

 

scala> val cv:Contravariant[String] = new Contravariant[AnyRef]

cv:Contravariant[AnyRef] = Contravariant@49fa7ba

 

scala> val fail:Contravariant[AnyRef] = new Contravariant[String]

<console>:6:error: type mismatch;

 found  : Contravariant[String]

 required: Contravariant[AnyRef]

       val fail: Contravariant[AnyRef] = newContravariant[String]

 

 

 

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