案例类对象上的Scala groupBy

Question

I want to group by on class instance(object)

// Below code fails to compile

  case class Employee(name:String,division:Int)

    val employees = List(Employee("Sumeet",1),Employee("Mahesh",2),Employee("Rhoit",1),Employee("Nexus",2))

     def groupByEmployee(list:List[Employee]):Map[Employee,List[Employee]] = {
        list groupBy(_)
      }

groupByEmployee(employees)

// But this works when i use specific attibute

case class Employee(name:String,division:Int)

val employees = List(Employee("Sumeet",1),Employee("Mahesh",2),Employee("Rhoit",1),Employee("Nexus",2))

 def groupByEmployee(list:List[Employee]):Map[Int,List[Employee]] = {
    list groupBy(_.division)
  }

groupByEmployee(employees)

Answer 1

groupBy takes a function that returns the value to group by. _.division is Scala short hand for (x) => x.division

If you want to group by the identity of a case class instance (eg, Employee("Sumeet", 1) and Employee("Sumeet", 1) group together, but Employee("Sumeet", 2) does not), then you'd want to use hashCode in your groupBy. Eg, groupBy(_.hashCode)