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Rewrote upper type bounds article #749

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May 12, 2017
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Rewrote upper type bounds article #749

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17 changes: 8 additions & 9 deletions tutorials/tour/_posts/2017-02-13-upper-type-bounds.md
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Original file line number Diff line number Diff line change
Expand Up @@ -33,18 +33,17 @@ class Lion extends Animal {
override def name: String = "Lion"
}

class Cage[P <: Pet](p: P) {
class PetContainer[P <: Pet](p: P) {
def pet: P = p
}

object Main extends App {
var dogCage = new Cage[Dog](new Dog)
var catCage = new Cage[Cat](new Cat)
/* Cannot put Lion in a cage as Lion is not a Pet. */
// var lionCage = new Cage[Lion](new Lion)
}
val dogContainer = new PetContainer[Dog](new Dog)
val catContainer = new PetContainer[Cat](new Cat)
// val lionContainer = new PetContainer[Lion](new Lion)
// ^this would not compile
```
The `class PetContainer` take a type parameter `P` which must be a subtype of `Pet`. `Dog` and `Cat` are subtypes of `Pet` so we can create a new `PetContainer[Dog]` and `PetContainer[Cat]`. However, if we tried to create a `PetContainer[Lion]`, we would get the following Error:

An instance of class `Cage` may contain an animal with upper bound `Pet`. An animal of type `Lion` is not a pet and therefore cannot be put into a cage.
`type arguments [Lion] do not conform to class PetContainer's type parameter bounds [P <: Pet]`

The usage of lower type bounds is discussed [here](lower-type-bounds.html).
This is because `Lion` is not a subtype of `Pet`.