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12 Traits

Sample run of chapter's interpreter examples

• 12.1 How traits work
• 12.2 Thin versus rich interfaces
• 12.3 Example: Rectangular objects
• 12.4 The Ordered trait
• 12.5 Traits as stackable modifications
• 12.6 Why not multiple inheritance?
• 12.7 To trait or not to trait?
• 12.8 Conclusion

12.1 How traits work

// In file traits/Philosophical.scala

  trait Philosophical {
    def philosophize() = {
      println("I consume memory, therefore I am!")
    }
  }


  class Frog extends Philosophical {
    override def toString = "green"
  }


  scala> val frog = new Frog
  frog: Frog = green

  scala> frog.philosophize()
  I consume memory, therefore I am!


  scala> val phil: Philosophical = frog  
  phil: Philosophical = green

  scala> phil.philosophize()
  I consume memory, therefore I am!


// In file traits/Ex1.scala

  class Animal

  class Frog extends Animal with Philosophical {
    override def toString = "green"
  }


// In file traits/Frog.scala

  class Animal
  trait HasLegs

  class Frog extends Animal with Philosophical with HasLegs {
    override def toString = "green"
  }


// In file traits/Ex2.scala

  class Animal

  class Frog extends Animal with Philosophical {
    override def toString = "green"
    override def philosophize() = {
      println("It ain't easy being " + toString + "!")
    }
  }


  scala> val phrog: Philosophical = new Frog
  phrog: Philosophical = green

  scala> phrog.philosophize()
  It ain't easy being green!


// In file traits/Ex2.scala

  class Point(x: Int, y: Int)


  trait NoPoint(x: Int, y: Int) // Does not compile


  

12.2 Thin versus rich interfaces

// In file traits/Ex2.scala

  trait CharSequence {
    def charAt(index: Int): Char
    def length: Int
    def subSequence(start: Int, end: Int): CharSequence
    def toString(): String
  }


  

12.3 Example: Rectangular objects

// In file traits/Point.scala

  class Point(val x: Int, val y: Int)

  class Rectangle(val topLeft: Point, val bottomRight: Point) {
    def left = topLeft.x
    def right = bottomRight.x
    def width = right - left
    // and many more geometric methods...
  }


  abstract class Component {
    def topLeft: Point
    def bottomRight: Point

    def left = topLeft.x
    def right = bottomRight.x
    def width = right - left
    // and many more geometric methods...
  }


// In file traits/Rectangular.scala

  trait Rectangular {
    def topLeft: Point
    def bottomRight: Point

    def left = topLeft.x
    def right = bottomRight.x
    def width = right - left
    // and many more geometric methods...
  }


  abstract class Component extends Rectangular {
    // other methods...
  }


// In file traits/Rectangle.scala

  class Rectangle(val topLeft: Point, val bottomRight: Point)
      extends Rectangular {

    // other methods...
  }


  scala> val rect = new Rectangle(new Point(1, 1),
       |     new Point(10, 10))
  rect: Rectangle = Rectangle@5f5da68c

  scala> rect.left
  res2: Int = 1

  scala> rect.right
  res3: Int = 10

  scala> rect.width
  res4: Int = 9


  

12.4 The Ordered trait

  class Rational(n: Int, d: Int) {
    // ...
    def < (that: Rational) = 
      this.numer * that.denom < that.numer * this.denom
    def > (that: Rational) = that < this
    def <= (that: Rational) = (this < that) || (this == that)
    def >= (that: Rational) = (this > that) || (this == that)
  }


// In file traits/Rational.scala

  class Rational(n: Int, d: Int) extends Ordered[Rational] {
    // ...
    def compare(that: Rational) =
      (this.numer * that.denom) - (that.numer * this.denom)
  }


  scala> val half = new Rational(1, 2)
  half: Rational = 1/2

  scala> val third = new Rational(1, 3)
  third: Rational = 1/3

  scala> half < third
  res5: Boolean = false

  scala> half > third
  res6: Boolean = true


// In file traits/Ex2.scala

  trait Ordered[T] {
    def compare(that: T): Int

    def <(that: T): Boolean = (this compare that) < 0
    def >(that: T): Boolean = (this compare that) > 0
    def <=(that: T): Boolean = (this compare that) <= 0
    def >=(that: T): Boolean = (this compare that) >= 0
  }


  

12.5 Traits as stackable modifications

// In file traits/IntQueue.scala

  abstract class IntQueue {
    def get(): Int
    def put(x: Int): Unit
  }


// In file traits/BasicIntQueue.scala

  import scala.collection.mutable.ArrayBuffer

  class BasicIntQueue extends IntQueue {
    private val buf = new ArrayBuffer[Int]
    def get() = buf.remove(0)
    def put(x: Int) = { buf += x }
  }


  scala> val queue = new BasicIntQueue
  queue: BasicIntQueue = BasicIntQueue@23164256

  scala> queue.put(10)

  scala> queue.put(20)

  scala> queue.get()
  res9: Int = 10

  scala> queue.get()
  res10: Int = 20


// In file traits/Ex2.scala

  trait Doubling extends IntQueue {
    abstract override def put(x: Int) = { super.put(2 * x) }
  }


  scala> class MyQueue extends BasicIntQueue with Doubling
  defined class MyQueue

  scala> val queue = new MyQueue
  queue: MyQueue = MyQueue@44bbf788

  scala> queue.put(10)

  scala> queue.get()
  res12: Int = 20


  scala> val queue = new BasicIntQueue with Doubling
  queue: BasicIntQueue with Doubling = $anon$1@141f05bf

  scala> queue.put(10)

  scala> queue.get()
  res14: Int = 20


// In file traits/Incrementing.scala

  trait Incrementing extends IntQueue {
    abstract override def put(x: Int) = { super.put(x + 1) }
  }
  trait Filtering extends IntQueue {
    abstract override def put(x: Int) = {
      if (x >= 0) super.put(x)
    }
  }


  scala> val queue = (new BasicIntQueue
       |     with Incrementing with Filtering)
  queue: BasicIntQueue with Incrementing with Filtering...

  scala> queue.put(-1); queue.put(0); queue.put(1)

  scala> queue.get()
  res16: Int = 1


  scala> queue.get()
  res17: Int = 2


  scala> val queue = (new BasicIntQueue
       |     with Filtering with Incrementing)
  queue: BasicIntQueue with Filtering with Incrementing...

  scala> queue.put(-1); queue.put(0); queue.put(1)

  scala> queue.get()
  res19: Int = 0

  scala> queue.get()
  res20: Int = 1

  scala> queue.get()
  res21: Int = 2


  

12.6 Why not multiple inheritance?

// In file traits/Ex2.scala

  // Multiple inheritance thought experiment
  val q = new BasicIntQueue with Incrementing with Doubling
  q.put(42)  // which put would be called?


  // Multiple inheritance thought experiment
  trait MyQueue extends BasicIntQueue
      with Incrementing with Doubling {

    def put(x: Int) = {
      Incrementing.super.put(x) // (Not real Scala)
      Doubling.super.put(x)
    }
  }


// In file traits/Ex3.scala

  class Animal 
  trait Furry extends Animal
  trait HasLegs extends Animal
  trait FourLegged extends HasLegs
  class Cat extends Animal with Furry with FourLegged


  

12.7 To trait or not to trait?

12.8 Conclusion