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11 Scala's Hierarchy

Sample run of chapter's interpreter examples

• 11.1 Scala's class hierarchy
• 11.2 How primitives are implemented
• 11.3 Bottom types
• 11.4 Defining your own value classes
• 11.5 Conclusion

11.1 Scala's class hierarchy

  final def ==(that: Any): Boolean
  final def !=(that: Any): Boolean
  def equals(that: Any): Boolean
  def ##: Int
  def hashCode: Int
  def toString: String


  scala> new Int


  <console>:5: error: class Int is abstract; cannot be 
  instantiated
         new Int
         ^


  scala> 42.toString
  res1: String = 42

  scala> 42.hashCode
  res2: Int = 42

  scala> 42 equals 42
  res3: Boolean = true


  scala> 42 max 43
  res4: Int = 43

  scala> 42 min 43
  res5: Int = 42

  scala> 1 until 5
  res6: scala.collection.immutable.Range = Range 1 until 5

  scala> 1 to 5
  res7: scala.collection.immutable.Range.Inclusive
    = Range 1 to 5

  scala> 3.abs
  res8: Int = 3

  scala> (-3).abs
  res9: Int = 3


  

11.2 How primitives are implemented

// In file scalas-hierarchy/Ex1.java

  // This is Java
  boolean isEqual(int x, int y) {
    return x == y;
  }
  System.out.println(isEqual(421, 421));


// In file scalas-hierarchy/Ex2.java

  // This is Java
  boolean isEqual(Integer x, Integer y) {
    return x == y;
  }
  System.out.println(isEqual(421, 421));


  scala> def isEqual(x: Int, y: Int) = x == y
  isEqual: (x: Int, y: Int)Boolean

  scala> isEqual(421, 421)
  res10: Boolean = true

  scala> def isEqual(x: Any, y: Any) = x == y
  isEqual: (x: Any, y: Any)Boolean

  scala> isEqual(421, 421)
  res11: Boolean = true


  scala> val x = "abcd".substring(2)
  x: String = cd

  scala> val y = "abcd".substring(2)
  y: String = cd

  scala> x == y
  res12: Boolean = true


  scala> val x = new String("abc")
  x: String = abc

  scala> val y = new String("abc")
  y: String = abc

  scala> x == y
  res13: Boolean = true

  scala> x eq y
  res14: Boolean = false

  scala> x ne y
  res15: Boolean = true


  

11.3 Bottom types

  scala> val i: Int = null
                      ^
         error: an expression of type Null is ineligible for 
  implicit conversion


  def error(message: String): Nothing =
    throw new RuntimeException(message)


// In file scalas-hierarchy/Ex3.scala

  def divide(x: Int, y: Int): Int = 
    if (y != 0) x / y 
    else sys.error("can't divide by zero")


  

11.4 Defining your own value classes

  class Dollars(val amount: Int) extends AnyVal {
    override def toString() = "$" + amount
  }


  scala> val money = new Dollars(1000000)
  money: Dollars = $1000000
  scala> money.amount
  res16: Int = 1000000


  class SwissFrancs(val amount: Int) extends AnyVal {
    override def toString() = s"$amount CHF"
  }


  scala> val dollars = new Dollars(1000)
  dollars: Dollars = $1000
  scala> val francs = new SwissFrancs(1000)
  francs: SwissFrancs = 1000 CHF


  def title(text: String, anchor: String, style: String): String =
    s"<a id='$anchor'><h1 class='$style'>$text</h1></a>"


  scala> title("chap:vcls", "bold", "Value Classes")
  res17: String = <a id='bold'><h1 class='Value 
      Classes'>chap:vcls</h1></a>


  class Anchor(val value: String) extends AnyVal
  class Style(val value: String) extends AnyVal
  class Text(val value: String) extends AnyVal
  class Html(val value: String) extends AnyVal


  def title(text: Text, anchor: Anchor, style: Style): Html =
    new Html(
      s"<a id='${anchor.value}'>" +
          s"<h1 class='${style.value}'>" +
          text.value +
          "</h1></a>"
    )


  scala> title(new Anchor("chap:vcls"), new Style("bold"),
       |     new Text("Value Classes"))
               ^
  error: type mismatch;
   found   : Anchor
   required: Text
                                        ^
  error: type mismatch;
   found   : Style
   required: Anchor
      new Text("Value Classes"))
      ^
  On line 2: error: type mismatch;
   found   : Text
   required: Style


  

11.5 Conclusion