Sample run of chapter's interpreter examples
elem(s: String): Element
// In file composition-and-inheritance/LayoutElement.scala val column1 = elem("hello") above elem("***") val column2 = elem("***") above elem("world") column1 beside column2
hello *** *** world
abstract class Element { def contents: Array[String] }
abstract class Element ...
scala> new Element <console>:5: error: class Element is abstract; cannot be instantiated new Element ^
// In file composition-and-inheritance/Ex3.scala abstract class Element { def contents: Array[String] def height: Int = contents.length def width: Int = if (height == 0) 0 else contents(0).length }
def width(): Int
def width: Int
// In file composition-and-inheritance/Ex2.scala abstract class Element { def contents: Array[String] val height = contents.length val width = if (height == 0) 0 else contents(0).length }
// In file composition-and-inheritance/Ex2.scala Array(1, 2, 3).toString "abc".length
// In file composition-and-inheritance/Ex2.scala "hello".length // no () because no side-effect println() // better to not drop the ()
// In file composition-and-inheritance/Ex11.scala class ArrayElement(conts: Array[String]) extends Element { def contents: Array[String] = conts }
... extends Element ...
scala> val ae = new ArrayElement(Array("hello", "world")) ae: ArrayElement = ArrayElement@39274bf7 scala> ae.width res0: Int = 5
// In file composition-and-inheritance/Ex2.scala val e: Element = new ArrayElement(Array("hello"))
// In file composition-and-inheritance/Ex3.scala class ArrayElement(conts: Array[String]) extends Element { val contents: Array[String] = conts }
// This is Java class CompilesFine { private int f = 0; public int f() { return 1; } }
// In file composition-and-inheritance/Ex4.scala.err class WontCompile { private var f = 0 // Won't compile, because a field def f = 1 // and method have the same name }
// In file composition-and-inheritance/Ex6.scala class ArrayElement( val contents: Array[String] ) extends Element
class ArrayElement(x123: Array[String]) extends Element { val contents: Array[String] = x123 }
// In file composition-and-inheritance/Ex8.scala class Cat { val dangerous = false } class Tiger( override val dangerous: Boolean, private var age: Int ) extends Cat
// In file composition-and-inheritance/Ex8.scala class Tiger(param1: Boolean, param2: Int) extends Cat { override val dangerous = param1 private var age = param2 }
// In file composition-and-inheritance/Ex12.scala class LineElement(s: String) extends ArrayElement(Array(s)) { override def width = s.length override def height = 1 }
... extends ArrayElement(Array(s)) ...
$ scalac LineElement.scala .../LineElement.scala:50: error: method hight overrides nothing override def hight = 1 ^
def hidden(): Boolean
.../Shapes.scala:6: error: error overriding method hidden in class Shape of type ()Boolean; method hidden needs `override' modifier def hidden(): Boolean = ^
// In file composition-and-inheritance/Ex12.scala class UniformElement( ch: Char, override val width: Int, override val height: Int ) extends Element { private val line = ch.toString * width def contents = Array.fill(height)(line) }
// In file composition-and-inheritance/Ex9.scala val e1: Element = new ArrayElement(Array("hello", "world")) val ae: ArrayElement = new LineElement("hello") val e2: Element = ae val e3: Element = new UniformElement('x', 2, 3)
abstract class Element { def demo() = { println("Element's implementation invoked") } } class ArrayElement extends Element { override def demo() = { println("ArrayElement's implementation invoked") } } class LineElement extends ArrayElement { override def demo() = { println("LineElement's implementation invoked") } } // UniformElement inherits Element's demo class UniformElement extends Element
def invokeDemo(e: Element) = { e.demo() }
scala> invokeDemo(new ArrayElement) ArrayElement's implementation invoked
scala> invokeDemo(new LineElement) LineElement's implementation invoked
scala> invokeDemo(new UniformElement) Element's implementation invoked
class ArrayElement extends Element { final override def demo() = { println("ArrayElement's implementation invoked") } }
elem.scala:18: error: error overriding method demo in class ArrayElement of type ()Unit; method demo cannot override final member override def demo() = { ^
final class ArrayElement extends Element { override def demo() = { println("ArrayElement's implementation invoked") } }
elem.scala: 18: error: illegal inheritance from final class ArrayElement class LineElement extends ArrayElement { ^
// In file composition-and-inheritance/Ex11.scala class LineElement(s: String) extends Element { val contents = Array(s) override def width = s.length override def height = 1 }
// In file composition-and-inheritance/Ex10.scala def above(that: Element): Element = new ArrayElement(this.contents ++ that.contents)
// In file composition-and-inheritance/Ex10.scala def beside(that: Element): Element = { val contents = new Array[String](this.contents.length) for (i <- 0 until this.contents.length) contents(i) = this.contents(i) + that.contents(i) new ArrayElement(contents) }
// In file composition-and-inheritance/Ex10.scala new ArrayElement( for ( (line1, line2) <- this.contents zip that.contents ) yield line1 + line2 )
// In file composition-and-inheritance/Ex10.scala Array(1, 2, 3) zip Array("a", "b")
// In file composition-and-inheritance/Ex10.scala Array((1, "a"), (2, "b"))
// In file composition-and-inheritance/Ex10.scala override def toString = contents mkString "\n"
// In file composition-and-inheritance/Ex11.scala abstract class Element { def contents: Array[String] def width: Int = if (height == 0) 0 else contents(0).length def height: Int = contents.length def above(that: Element): Element = new ArrayElement(this.contents ++ that.contents) def beside(that: Element): Element = new ArrayElement( for ( (line1, line2) <- this.contents zip that.contents ) yield line1 + line2 ) override def toString = contents mkString "\n" }
// In file composition-and-inheritance/Ex12.scala object Element { def elem(contents: Array[String]): Element = new ArrayElement(contents) def elem(chr: Char, width: Int, height: Int): Element = new UniformElement(chr, width, height) def elem(line: String): Element = new LineElement(line) }
// In file composition-and-inheritance/Ex12.scala import Element.elem abstract class Element { def contents: Array[String] def width: Int = if (height == 0) 0 else contents(0).length def height: Int = contents.length def above(that: Element): Element = elem(this.contents ++ that.contents) def beside(that: Element): Element = elem( for ( (line1, line2) <- this.contents zip that.contents ) yield line1 + line2 ) override def toString = contents mkString "\n" }
new ArrayElement(Array("hello")) above new ArrayElement(Array("world!"))
new ArrayElement(Array("one", "two")) beside new ArrayElement(Array("one"))
// In file composition-and-inheritance/LayoutElement.scala object Element { private class ArrayElement( val contents: Array[String] ) extends Element private class LineElement(s: String) extends Element { val contents = Array(s) override def width = s.length override def height = 1 } private class UniformElement( ch: Char, override val width: Int, override val height: Int ) extends Element { private val line = ch.toString * width def contents = Array.fill(height)(line) } def elem(contents: Array[String]): Element = new ArrayElement(contents) def elem(chr: Char, width: Int, height: Int): Element = new UniformElement(chr, width, height) def elem(line: String): Element = new LineElement(line) }
// In file composition-and-inheritance/LayoutElement.scala import Element.elem abstract class Element { def contents: Array[String] def width: Int = contents(0).length def height: Int = contents.length def above(that: Element): Element = { val this1 = this widen that.width val that1 = that widen this.width elem(this1.contents ++ that1.contents) } def beside(that: Element): Element = { val this1 = this heighten that.height val that1 = that heighten this.height elem( for ((line1, line2) <- this1.contents zip that1.contents) yield line1 + line2) } def widen(w: Int): Element = if (w <= width) this else { val left = elem(' ', (w - width) / 2, height) val right = elem(' ', w - width - left.width, height) left beside this beside right } def heighten(h: Int): Element = if (h <= height) this else { val top = elem(' ', width, (h - height) / 2) val bot = elem(' ', width, h - height - top.height) top above this above bot } override def toString = contents mkString "\n" }
// In file composition-and-inheritance/Spiral.scala import Element.elem object Spiral { val space = elem(" ") val corner = elem("+") def spiral(nEdges: Int, direction: Int): Element = { if (nEdges == 1) elem("+") else { val sp = spiral(nEdges - 1, (direction + 3) % 4) def verticalBar = elem('|', 1, sp.height) def horizontalBar = elem('-', sp.width, 1) if (direction == 0) (corner beside horizontalBar) above (sp beside space) else if (direction == 1) (sp above space) beside (corner above verticalBar) else if (direction == 2) (space beside sp) above (horizontalBar beside corner) else (verticalBar above corner) beside (space above sp) } } def main(args: Array[String]) = { val nSides = args(0).toInt println(spiral(nSides, 0)) } }
$ scala Spiral 6 $ scala Spiral 11 $ scala Spiral 17 +----- +---------- +---------------- | | | | +-+ | +------+ | +------------+ | + | | | | | | | | | | | +--+ | | | +--------+ | +---+ | | | | | | | | | | | | ++ | | | | | +----+ | | | | | | | | | | | | | | +----+ | | | | | ++ | | | | | | | | | | | | | +--------+ | | | +--+ | | | | | | | | | | | +------+ | | | | | | | +----------+ | | | +--------------+
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