在Scala中，参数类型_>：和<：是什么？

Question

I have a parameterized Generic type, where the generic type is from Case classes I defined separately.

These are

case class TypeA(user_id: String,
                   device_id: String) extends Serializable


case class TypeB(user_id: String,
                   device_id: String,
                   org_id: String,
                   app_name: String) extends Serializable

I use a Map function to read in the datasets, based on the Generic type schema

val res = this.readHandlersMap.map(s => s._1 match {

case "a" => {
  (s._1, s._2.read[TypeA]((Encoders.product[TypeA].schema)).asInstanceOf[Dataset[TypeA]])
}

case "b" => {
   (s._1, s._2.read[TypeB]((Encoders.product[TypeB].schema)).asInstanceOf[Dataset[TypeB]])    
}
});

After checking the output of res , I get that it is of type

Map[String, Dataset[_ >: TypeA with TypeB <: Serializable with Product]]

I don't get the meaning of this type. What do the _>: and with s do in this type? And when I try to declare res as of type Dataset[Serializable] , I get an error. What will this type be if I add a TypeC ?

Answer 1

These are basically upper and lower bound for the defined generic type:

For example:

Car <: Vehicle //it means car is subtype of vehicle 
Fruit >: Mango //fruit is super type of the mango

That means we can put an instance of car on vehicle stack. And an instance of mango on the stack of Fruit.

Map[String, Dataset[_ >: TypeA with TypeB <: Serializable with Product]]

This line means that Dataset is of type where. _ is super type of TypeA with TypeB is the subtype of the Serializable.