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Chapter17.scala
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Chapter17.scala
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/**
* Task 1:
*
* Define an immutable `class Pair[T, S]` with a method `swap` that returns a new pair with
* the components swapped.
*/
package task1701 {
class Pair[T, S](val first: T, val second: S) {
def swap(): Pair[S, T] = new Pair(second, first)
}
}
/**
* Task 2:
*
* Define a mutable `class Pair[T]` with a method `swap` that swaps the components of the pair.
*/
package task1702 {
class Pair[T](var first: T, var second: T) {
def swap(): Unit = {
val tmp = first
first = second
second = tmp
}
}
}
/**
* Task 3:
*
* Given a `class Pair[T, S]`, write a generic method `swap` that takes a pair as its argument
* and returns a new pair with the components swapped.
*/
object Chapter17Task03 {
import task1701._
def swap[T, S](pair: Pair[T, S]): Pair[S, T] = new Pair(pair.second, pair.first)
}
/**
* Task 4:
*
* Why don't we need a lower bound for the `replaceFirst` method in Section 17.3,
* "Bounds for Type Variables”, on page 232 if we want to replace the first component of
* a `Pair[Person]` with a `Student`?
*
* Solution:
*
* We don't need a lower bound because we replacing with a sub-class, which is OK, since
* the result type is still `Pair[Person]`.
*/
package task1704 {
class Person(val name: String)
class Student(name: String) extends Person(name)
class Pair[T](val first: T, val second: T) {
def replaceFirst(newFirst: T): Pair[T] = new Pair(newFirst, second)
}
}
/**
* Task 5:
*
* Why does `RichInt` implement `Comparable[Int]` and not `Comparable[RichInt]`?
*
* Solution:
*
* To be able to use view bounds, like
* {{{
* class Pair[T <% Comparable[T]](val first: T, val second: T)
* }}}
* which than can be used with `Int` types, like
* {{{
* new Pair(1, 2)
* }}}
* we need to have implicit conversion from `T` to `Comparable[T]`. `RichInt` class
* implements `Comparable[Int]` and there is implicit conversion from `Int` to `RichInt`.
* So, we don't use `RichInt` class directly.
*/
/**
* Task 6:
*
* Write a generic method `middle` that returns the middle element from any `Iterable[T]`.
* For example, `middle("World")` is 'r'.
*/
object Chapter17Task06 {
//def middle[T](xs: Iterable[T]): Option[T] = {
//def middle[A, C](xs: C)(implicit ev: C <:< Iterable[A]): Option[A] = {
//def middle[A, C <% Iterable[A]](xs: C): Option[A] = {
def middle[A, C](xs: C)(implicit ev: C => Iterable[A]): Option[A] = {
val size = xs.size
if (size % 2 == 0) {
return None
}
var distance = size / 2
xs.find { _ =>
val found = if (distance == 0) true else false
distance -= 1
found
}
}
}
/**
* Task 7:
*
* Look through the methods of the `Iterable[+A]` trait. Which methods use the type parameter `A`?
* Why is it in a covariant position in these methods?
*
* Solution:
*
* Parameter `A` used in the following methods, for example: `head`, `last`, `min`, `max`.
* Its used in a covariant position since its defined with the covariant variance annotation (+A).
*/
/**
* Task 8:
*
* In Section 17.10, "Co- and Contravariant Positions", on page 238, the `replaceFirst` method
* has a type bound. Why can't you define an equivalent method on a mutable `Pair[T]`?
* {{{
* def replaceFirst[R >: T](newFirst: R) { first = newFirst } // Error
* }}}
*
* Solution:
*
* It is an error because the used type bound (`R >: T`) allows passing supper type instances.
* But we can change the type bound definition to allow passing sub type instances:
* {{{
* def replaceFirst[R <: T](newFirst: R) { first = newFirst }
* }}}
*/
/**
* Task 9:
*
* It may seem strange to restrict method parameters in an immutable `class Pair[+T]`. However,
* suppose you could define
* {{{
* def replaceFirst(newFirst: T)
* }}}
* in a `Pair[+T]`. The problem is that this method can be overridden in an unsound way.
* Construct an example of the problem. Define a subclass `NastyDoublePair` of `Pair[Double]`
* that overrides `replaceFirst` so that it makes a pair with the square root of `newFirst`.
* Then construct the call `replaceFirst("Hello")` on a `Pair[Any]` that is actually
* a `NastyDoublePair`.
*/
object Chapter17Task09 {
class Pair[+T](val first: T, val second: T) {
//def replaceFirst(newFirst: T): Pair[T] = new Pair(newFirst, second)
def replaceFirst[R >: T](newFirst: R): Pair[R] = new Pair(newFirst, second)
}
class NastyDoublePair(first: Double, second: Double) extends Pair[Double](first, second) {
//override def replaceFirst[R >: Double](newFirst: R) = new Pair(math.sqrt(newFirst), second)
override def replaceFirst[R >: Double](newFirst: R) = new Pair(math.sqrt(first), second)
}
def check(pair: Pair[Any]): Pair[Any] = pair.replaceFirst("Hello")
}
/**
* Task 10:
*
* Given a mutable `Pair[S, T]` class, use a type constraint to define a `swap` method that can
* be called if the type parameters are the same.
*/
package task1710 {
class Pair[S, T](var first: S, var second: T) {
def swap(implicit ev: T =:= S): Unit = {
val tmp = first.asInstanceOf[T]
first = second
second = tmp
}
}
}