I need help creating a program using Scala.

object subtyping {

// Instances of Counter have a integer field that can be incremented, decremented, or read. class Counter { private var n = 0 def increment () = { n = n + 1 } def decrement () = { n = n - 1 } def get () : Int = n }

// EXERCISE 1: complete the following function. // The observeCounter function has one parameter f: a function that accepts (a reference to) a Counter instance but returns nothing. // The observeCounter function should call f with (a reference to) an object (of a class extending Counter). // Your class that extends Counter must keep track of the total number of times that increment/decrement have been called. // I.e., if the increment method is called 3 times on an instance, and the decrement method is called 2 times on the same instance, then it should store 5 (somewhere other than the existing field n). // observeCounter should call f, and then return the total number of times that increment/decrement were called on the instance by f. def observeCounter (f : Counter => Unit) : Int = { // TODO: Provide definition here. -1 }

// EXERCISE 2: complete the following function. // It is the same as observeCounter except that f has a parameter of type List[Counter] not Counter. // f will insist that the List[Counter] has length 3. // You must return a List[Int] not an Int. // The first element of the result List[Int] must correspond to the number of times that increment/decrement were called on the first element of type List[Counter], similarly for the second and third elements. def observeCounterList (f : List[Counter] => Unit) : List[Int] = { // TODO: Provide definition here. List (-1, -1, -1) }

// EXERCISE 3: complete the following function. // It is the same as observeCounterList except that f has a parameter of type Array[Counter] not List[Counter]. // f will insist that the Array[Counter] has length 3. // You must return a Array[Int] not a List[Int]. // The first element of the result Array[Int] must correspond to the number of times that increment/decrement were called on the first element of type Array[Counter], similarly for the second and third elements. def observeCounterArray (f : Array[Counter] => Unit) : Array[Int] = { // TODO: Provide definition here. List (-1, -1, -1).toArray } }