每个“setter”方法都需要Scala中的“getter”方法？

class Person{
   var id  = 0 
}
var p = new Person 
p.id 
p.id = 2

等于

class Person{
private var _id = 0 
def id = _id
def id_=(i: Int) = _id = i
}
val p = new Person 
p.id // be equal to invoke id method of class Person
p.id = 2   // be equal to p.id_=(2)

有效.但是如果你对getter方法def id = _id发表评论,那么p.id = 2会导致编译错误

error: value key is not a member of Person

谁有人解释为什么？