使用地点差异

使用站点差异类似于 Java 通配符：

出投影：

  val takeList : MutableList<out SomeType> = ... // Java: List<? extends SomeType>

  val takenValue : SomeType = takeList[0] // OK, since upper bound is SomeType

  takeList.add(takenValue) // Error, lower bound for generic is not specified

在投影：

  val putList : MutableList<in SomeType> = ... // Java: List<? super SomeType>
  
  val valueToPut : SomeType = ...
  putList.add(valueToPut) // OK, since lower bound is SomeType

  putList[0] // This expression has type Any, since no upper bound is specified

明星投影

  val starList : MutableList<*> = ... // Java: List<?>

  starList[0] // This expression has type Any, since no upper bound is specified
  starList.add(someValue) // Error, lower bound for generic is not specified

也可以看看：

• 从 Java 调用 Kotlin 时的 Variant Generics 互操作性。