can we have methods as arguments in a class definition? - scala

Question

Following is the snippet of code I was going thru a scala book.
One of the parameter of the Car class is "def color: String"
I didn't understand. "def" is a keyword to define a method. how can that be used in paramenters?
scala> abstract class Car {
| val year: Int
| val automatic: Boolean = true
| def color: String
| }

Answer 1

A function that takes other functions are arguments is known as a higher-order function, here is an example of that:
// A function that takes a list of Ints, and a function that takes an Int and returns a boolean.
def filterList(list: List[Int], filter: (Int) => Boolean) = { /* implementation */ }
// You might call it like this
def filter(i: Int) = i > 0
val list = List(-5, 0, 5, 10)
filterList(list, filter)
// Or shorthand like this
val list = List(-5, 0, 5, 10)
filterList(list, _ > 0)
However that is not what is happening in your example. In your example the Car class has three class members, two of them are variables, and one of them is a function. If you were to extend your abstract class, you could test the values out:
abstract class Car {
val year: Int
val automatic: Boolean = true
def color: String
}
case class Sedan(year: Int) extends Car {
def color = "red"
}
val volkswagen = Sedan(2012)
volkswagen.year // 2012
volkswagen.automatic // true
volkswagen.color // red
Here, having color as a function (using def) doesn't make too much sense, because with my implementation, the color is always going to be "red".
A better example for using a function for a class member would be for some value that is going to change:
class BrokenClock {
val currentTime = DateTime.now()
}
class Clock {
def currentTime = DateTime.now()
}
// This would always print the same time, because it is a value that was computed once when you create a new instance of BrokenClock
val brokenClock = BrokenClock()
brokenClock.currentTime // 2017-03-31 22:51:00
brokenClock.currentTime // 2017-03-31 22:51:00
brokenClock.currentTime // 2017-03-31 22:51:00
// This will be a different value every time, because each time we are calling a function that is computing a new value for us
val clock = Clock()
clock.currentTime // 2017-03-31 22:51:00
clock.currentTime // 2017-03-31 22:52:00
clock.currentTime // 2017-03-31 22:53:00