Must override val variable in scala - scala

Question

I meet a weird problem in scala. Following is my code, class Employee extends class Person
But this piece of code can not been compiled, I have explicit define firstName and lastName as val variable. Why is that ? Does it mean I have to override val variable in base class ? And what is the purpose ?
class Person( firstName: String, lastName: String) {
}
class Employee(override val firstName: String, override val lastName: String, val depart: String)
extends Person(firstName,lastName){
}

Answer 1

The input parameters for the constructor are not vals unless you say they are. And if they are already, why override them?
class Person(val firstName: String, val lastName: String) {}
class Strange(
override val firstName: String, override val lastName: String
) extends Person("John","Doe") {}
class Employee(fn: String, ln: String, val depart: String) extends Person(fn,ln) {}
If they're not vals and you want to make vals, you don't need to override:
class Person(firstName: String, lastName: String) {}
class Employee(
val firstName: String, val lastName: String, val depart: String
) extends Person(firstName,lastName) {}

Answer 2

Since the constructor arguments have no val/var declaration in Person, and as Person is no case class, the arguments will not be members of class Person, merely constructor arguments. The compiler is telling you essentially: hey, you said, that firstName and lastName are members, which override/redefine something inherited from a base class - but there is nothing as far as I can tell...
class Person(val firstName: String, val lastName: String)
class Employee(fn: String, ln: String, val salary: BigDecimal) extends Person(fn, ln)
You do not need to declare firstName/lastName as overrides here, btw. Simply forwarding the values to the base class' constructor will do the trick.

Answer 3

You might also consider redesigning your super classes as traits as much as possible. Example:
trait Person {
def firstName: String
def lastName: String
}
class Employee(
val firstName: String,
val lastName: String,
val department: String
) extends Person
or even
trait Employee extends Person {
def department: String
}
class SimpleEmployee(
val firstName: String,
val lastName: String,
val department: String
) extends Employee

Answer 4

Unless I've misunderstood your intention, here's how to extend Person.
Welcome to Scala version 2.8.0.final (Java HotSpot(TM) Client VM, Java 1.6.0_21).
Type in expressions to have them evaluated.
Type :help for more information.
scala> class Person( firstName: String, lastName: String)
defined class Person
scala> class Employee(firstName: String, lastName: String, depart: String) extends Person(firstName, lastName)
defined class Employee