Rewrote traits tour

tutorials/tour/_posts/2017-02-13-traits.md

@@ -9,46 +9,74 @@ categories: tour
 num: 5
 next-page: mixin-class-composition
 previous-page: classes
+assumed-knowledge: expressions, classes, generics, objects, companion-objects
 ---
 
-Similar to interfaces in Java, traits are used to define object types by specifying the signature of the supported methods. Like in Java 8, Scala allows traits to be partially implemented; i.e. it is possible to define default implementations for some methods. In contrast to classes, traits may not have constructor parameters.
-Here is an example:
-
+Traits are used to share interfaces and fields between classes. They are similar to Java 8's interfaces. Classes and objects can extend traits but traits cannot be instantiated and therefore have no parameters.
+
+## Defining a trait
+A minimal trait is simply the keyword `trait` and an identifier:
+
+```tut
+trait HairColor
+```
+
+Traits become especially useful as generic types and with abstract methods.
 ```tut
-trait Similarity {
-  def isSimilar(x: Any): Boolean
-  def isNotSimilar(x: Any): Boolean = !isSimilar(x)
+trait Iterator[A] {
+  def hasNext: Boolean
+  def next(): A
 }
 ```
-
-This trait consists of two methods `isSimilar` and `isNotSimilar`. While `isSimilar` does not provide a concrete method implementation (it is abstract in the terminology of Java), method `isNotSimilar` defines a concrete implementation. Consequently, classes that integrate this trait only have to provide a concrete implementation for `isSimilar`. The behavior for `isNotSimilar` gets inherited directly from the trait. Traits are typically integrated into a [class](classes.html) (or other traits) with a [mixin class composition](mixin-class-composition.html):
-
+
+Extending the `trait Iterator[A]` requires a type `A` and implementations of the methods `hasNext` and `next`.
+
+## Using traits
+Use the `extends` keyword to extend a trait. Then implement any abstract members of the trait using the `override` keyword:
 ```tut
-class Point(xc: Int, yc: Int) extends Similarity {
-  var x: Int = xc
-  var y: Int = yc
-  def isSimilar(obj: Any) =
-    obj.isInstanceOf[Point] &&
-    obj.asInstanceOf[Point].x == x
+trait Iterator[A] {
+  def hasNext: Boolean
+  def next(): A
 }
-object TraitsTest extends App {
-  val p1 = new Point(2, 3)
-  val p2 = new Point(2, 4)
-  val p3 = new Point(3, 3)
-  val p4 = new Point(2, 3)
-  println(p1.isSimilar(p2))
-  println(p1.isSimilar(p3))
-  // Point's isNotSimilar is defined in Similarity
-  println(p1.isNotSimilar(2))
-  println(p1.isNotSimilar(p4))
+
+
+class IntIterator(to: Int) extends Iterator[Int] {
+  private var current = 0
+  override def hasNext: Boolean = current < to
+  override def next(): Int =  {
+    if (hasNext) {
+      val t = current
+      current += 1
+      t
+    } else 0
+  }
 }
-```
-
-Here is the output of the program:
 
+
+val iterator = new IntIterator(10)
+iterator.next()  // prints 0
+iterator.next()  // prints 1
 ```
-true
-false
-true
-false
+This `IntIterator` class takes a parameter `to` as an upper bound. It `extends Iterator[Int]` which means that the `next` method must return an Int.
+
+## Subtyping
+Subtypes of traits can be used where a the trait is required.
+```tut
+import scala.collection.mutable.ArrayBuffer
+
+trait Pet {
+  val name: String
+}
+
+class Cat(val name: String) extends Pet
+class Dog(val name: String) extends Pet
+
+val dog = new Dog("Harry")
+val cat = new Cat("Sally")
+
+val animals = ArrayBuffer.empty[Pet]
+animals.append(dog)
+animals.append(cat)
+animals.foreach(pet => println(pet.name))  // Prints Harry Sally
 ```
+The `trait Pet` has an abstract field `name` which gets implemented by Cat and Dog in their constructors. On the last line, we call `pet.name` which must be implemented in any subtype of the trait `Pet`.