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30 Object Equality

Sample run of chapter's interpreter examples

• 30.1 Equality in Scala
• 30.2 Writing an equality method
• 30.3 Defining equality for parameterized types
• 30.4 Recipes for equals and hashCode
• 30.5 Conclusion

30.1 Equality in Scala

  final def == (that: Any): Boolean = 
    if (null eq this) {null eq that} else {this equals that}


  

30.2 Writing an equality method

  var hashSet: Set[C] = new collection.immutable.HashSet
  hashSet += elem1
  hashSet contains elem2    // returns false!


  class Point(val x: Int, val y: Int) { ... }


// In file object-equality/Points1.scala

  // An utterly wrong definition of equals
  def equals(other: Point): Boolean =
    this.x == other.x && this.y == other.y


  scala> val p1, p2 = new Point(1, 2)
  p1: Point = Point@37d7d90f
  p2: Point = Point@3beb846d

  scala> val q = new Point(2, 3)
  q: Point = Point@e0cf182

  scala> p1 equals p2
  res0: Boolean = true

  scala> p1 equals q
  res1: Boolean = false


  scala> import scala.collection.mutable
  import scala.collection.mutable

  scala> val coll = mutable.HashSet(p1)
  coll: scala.collection.mutable.HashSet[Point] = 
  Set(Point@37d7d90f)

  scala> coll contains p2
  res2: Boolean = false


  scala> val p2a: Any = p2
  p2a: Any = Point@3beb846d


  scala> p1 equals p2a
  res3: Boolean = false


  def equals(other: Any): Boolean


// In file object-equality/Points2.scala

  // A better definition, but still not perfect
  override def equals(other: Any) = other match {
    case that: Point => this.x == that.x && this.y == that.y
    case _ => false
  }


  def ==(other: Point): Boolean = // Don't do this!


  scala> val p1, p2 = new Point(1, 2)
  p1: Point = Point@122c1533
  p2: Point = Point@c23d097

  scala> collection.mutable.HashSet(p1) contains p2
  res4: Boolean = false


// In file object-equality/Points3.scala

  class Point(val x: Int, val y: Int) {
    override def hashCode = (x, y).##
    override def equals(other: Any) = other match { 
      case that: Point => this.x == that.x && this.y == that.y 
      case _ => false 
    }
  }


// In file object-equality/Points5.scala

  class Point(var x: Int, var y: Int) { // Problematic
    override def hashCode = (x, y).##
    override def equals(other: Any) = other match { 
      case that: Point => this.x == that.x && this.y == that.y 
      case _ => false 
    }
  }


  scala> val p = new Point(1, 2)
  p: Point = Point@5428bd62

  scala> val coll = collection.mutable.HashSet(p)
  coll: scala.collection.mutable.HashSet[Point] = 
  Set(Point@5428bd62)

  scala> coll contains p
  res5: Boolean = true


  scala> p.x += 1

  scala> coll contains p
  res7: Boolean = false


  scala> coll.iterator contains p
  res8: Boolean = true


// In file object-equality/Points5.scala

  object Color extends Enumeration {
    val Red, Orange, Yellow, Green, Blue, Indigo, Violet = Value
  }


// In file object-equality/Points5.scala

  class ColoredPoint(x: Int, y: Int, val color: Color.Value) 
      extends Point(x, y) { // Problem: equals not symmetric

    override def equals(other: Any) = other match {
      case that: ColoredPoint => 
        this.color == that.color && super.equals(that)
      case _ => false
    }
  }


  scala> val p = new Point(1, 2)
  p: Point = Point@5428bd62

  scala> val cp = new ColoredPoint(1, 2, Color.Red)
  cp: ColoredPoint = ColoredPoint@5428bd62

  scala> p equals cp
  res9: Boolean = true

  scala> cp equals p
  res10: Boolean = false


  scala> collection.mutable.HashSet[Point](p) contains cp
  res11: Boolean = true

  scala> collection.mutable.HashSet[Point](cp) contains p
  res12: Boolean = false


// In file object-equality/Points6.scala

  class ColoredPoint(x: Int, y: Int, val color: Color.Value) 
      extends Point(x, y) { // Problem: equals not transitive

    override def equals(other: Any) = other match {
      case that: ColoredPoint =>
        (this.color == that.color) && super.equals(that)
      case that: Point =>
        that equals this
      case _ =>
        false
    }
  }


  scala> val redp = new ColoredPoint(1, 2, Color.Red)
  redp: ColoredPoint = ColoredPoint@5428bd62

  scala> val bluep = new ColoredPoint(1, 2, Color.Blue)
  bluep: ColoredPoint = ColoredPoint@5428bd62


  scala> redp == p
  res13: Boolean = true

  scala> p == bluep
  res14: Boolean = true


  scala> redp == bluep
  res15: Boolean = false


// In file object-equality/Points7.scala

  // A technically valid, but unsatisfying, equals method
  class Point(val x: Int, val y: Int) {
    override def hashCode = (x, y).##
    override def equals(other: Any) = other match {
      case that: Point => 
        this.x == that.x && this.y == that.y && 
        this.getClass == that.getClass
      case _ => false
    }
  }


// In file object-equality/Points7.scala

  class ColoredPoint(x: Int, y: Int, val color: Color.Value) 
      extends Point(x, y) {

    override def equals(other: Any) = other match {
      case that: ColoredPoint =>
        (this.color == that.color) && super.equals(that)
      case _ => false
    }
  }


  scala> val pAnon = new Point(1, 1) { override val y = 2 }
  pAnon: Point = $anon$1@5428bd62


  def canEqual(other: Any): Boolean


// In file object-equality/Points8.scala

  class Point(val x: Int, val y: Int) {
    override def hashCode = (x, y).##
    override def equals(other: Any) = other match {
      case that: Point =>
        (that canEqual this) &&
        (this.x == that.x) && (this.y == that.y)
      case _ =>
        false
    }
    def canEqual(other: Any) = other.isInstanceOf[Point]
  }


// In file object-equality/Points8.scala

  class ColoredPoint(x: Int, y: Int, val color: Color.Value) 
      extends Point(x, y) {

    override def hashCode = (super.hashCode, color).##
    override def equals(other: Any) = other match {
      case that: ColoredPoint =>
        (that canEqual this) &&
        super.equals(that) && this.color == that.color
      case _ =>
        false
    }
    override def canEqual(other: Any) =
      other.isInstanceOf[ColoredPoint]
  }


  scala> val p = new Point(1, 2)
  p: Point = Point@5428bd62

  scala> val cp = new ColoredPoint(1, 2, Color.Indigo)
  cp: ColoredPoint = ColoredPoint@e6230d8f

  scala> val pAnon = new Point(1, 1) { override val y = 2 }
  pAnon: Point = $anon$1@5428bd62

  scala> val coll = List(p)
  coll: List[Point] = List(Point@5428bd62)

  scala> coll contains p 
  res16: Boolean = true

  scala> coll contains cp
  res17: Boolean = false

  scala> coll contains pAnon
  res18: Boolean = true


  

30.3 Defining equality for parameterized types

// In file object-equality/Tree3.scala

  trait Tree[+T] {
    def elem: T
    def left: Tree[T]
    def right: Tree[T]
  }

  object EmptyTree extends Tree[Nothing] {
    def elem =
      throw new NoSuchElementException("EmptyTree.elem")
    def left =
      throw new NoSuchElementException("EmptyTree.left")
    def right =
      throw new NoSuchElementException("EmptyTree.right")
  }

  class Branch[+T](
    val elem: T,
    val left: Tree[T],
    val right: Tree[T]
  ) extends Tree[T]


// In file object-equality/Tree3.scala

  class Branch[T](
    val elem: T,
    val left: Tree[T],
    val right: Tree[T]
  ) extends Tree[T] {

    override def equals(other: Any) = other match {
      case that: Branch[T] => this.elem == that.elem && 
                              this.left == that.left &&
                              this.right == that.right
      case _ => false
    }
  }


   $ fsc -unchecked Tree.scala
  Tree.scala:14: warning: non variable type-argument T in type 
  pattern is unchecked since it is eliminated by erasure
      case that: Branch[T] => this.elem == that.elem && 
                 ^


  scala> val b1 = new Branch[List[String]](Nil,
      |      EmptyTree, EmptyTree)
  b1: Branch[List[String]] = Branch@9d5fa4f

  scala> val b2 = new Branch[List[Int]](Nil,
      |      EmptyTree, EmptyTree)
  b2: Branch[List[Int]] = Branch@56cdfc29

  scala> b1 == b2
  res19: Boolean = true


// In file object-equality/Tree3.scala

  case that: Branch[t] => this.elem == that.elem && 
                          this.left == that.left &&
                          this.right == that.right


  case that: Branch[t] => 


  case that: Branch[_] =>


// In file object-equality/Tree3.scala

  override def hashCode: Int = (elem, left, right).##


// In file object-equality/Tree3.scala

  def canEqual(other: Any) = other match {
    case that: Branch[_] => true
    case _ => false
  }


// In file object-equality/Tree3.scala

    def canEqual(other: Any) = other.isInstanceOf[Branch[_]]


// In file object-equality/Tree4.scala

  class Branch[T](
    val elem: T,
    val left: Tree[T],
    val right: Tree[T]
  ) extends Tree[T] {

    override def equals(other: Any) = other match {
      case that: Branch[_] => (that canEqual this) && 
                              this.elem == that.elem && 
                              this.left == that.left &&
                              this.right == that.right
      case _ => false
    }

    def canEqual(other: Any) = other.isInstanceOf[Branch[_]]

    override def hashCode: Int = (elem, left, right).##
  }


  

30.4 Recipes for equals and hashCode

    def canEqual(other: Any): Boolean =


      other.isInstanceOf[Rational]


    override def equals(other: Any): Boolean =


  other match {
    // ...
  }


        case that: Rational =>


          super.equals(that) &&


          (that canEqual this) &&


          numer == that.numer &&
          denom == that.denom


  case _ => false


// In file object-equality/Rational.scala

  class Rational(n: Int, d: Int) {
  
    require(d != 0)
  
    private val g = gcd(n.abs, d.abs)
    val numer = (if (d < 0) -n else n) / g
    val denom = d.abs / g
  
    private def gcd(a: Int, b: Int): Int =
      if (b == 0) a else gcd(b, a % b)
  
    override def equals(other: Any): Boolean =
      other match {
  
        case that: Rational =>
          (that canEqual this) &&
          numer == that.numer &&
          denom == that.denom

        case _ => false
      }

    def canEqual(other: Any): Boolean =
      other.isInstanceOf[Rational]

    override def hashCode: Int = (numer, denom).##
  
    override def toString =
      if (denom == 1) numer.toString else numer + "/" + denom
  }


  override def hashCode: Int = (a, b, c, d, e).##


  override def hashCode: Int = (super.hashCode, numer, denom).##


  override val hashCode: Int = (numer, denom).##


  

30.5 Conclusion