scala – val a：设置[Fruit] =设置[Apple]()不编译,为什么？怎

怎么解决这个问题？

package sandbox.stackOverFlow

class Fruit 
class Apple extends Fruit
class Banana extends Fruit

class Why{
  val a:Set[Fruit]=Set[Apple]() // Does not compile,why ?
  val b:List[Fruit]=List[Apple]() // Does compile.
}

产生编译错误：

type mismatch;
 found   : scala.collection.immutable.Set[sandbox.stackOverFlow.Apple]
 required: Set[sandbox.stackOverFlow.Fruit]
Note: sandbox.stackOverFlow.Apple <: sandbox.stackOverFlow.Fruit,but trait Set is invariant in type A.
You may wish to investigate a wildcard type such as `_ <: sandbox.stackOverFlow.Fruit`. (SLS 3.2.10)
  val a:Set[Fruit]=Set[Apple]()
                         ^

编辑：

正如Jatin的回答所指出的,这个问题已在这里得到解答：Why is Scala’s immutable Set not covariant in its type?