Файл: Programming_in_Scala,_2nd_edition.pdf

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)

}

Listing 10.12 · Hiding implementation with private classes.

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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)

var 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)

var bot = elem(' ', width, h - height - top.height) top above this above bot

}

override def toString = contents mkString "\n"

}

Listing 10.13 · Element with widen and heighten methods.

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10.15Putting it all together

A fun way to exercise almost all elements of the layout library is to write a program that draws a spiral with a given number of edges. This Spiral program, shown in Listing 10.14, will do just that:

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))

}

}

Listing 10.14 · The Spiral application.

Because Spiral is a standalone object with a main method with the proper signature, it is a Scala application. Spiral takes one command-line

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argument, an integer, and draws a spiral with the specified number of edges. For example, you could draw a six-edge spiral as shown below on the left, and larger spirals as shown to the right:

10.16Conclusion

In this section, you saw more concepts related to object-oriented programming in Scala. Among others, you encountered abstract classes, inheritance and subtyping, class hierarchies, parametric fields, and method overriding. You should have developed a feel for constructing a non-trivial class hierarchy in Scala. We’ll work with the layout library again in Chapter 14.

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The == method is essentially the same as equals and != is always the negation of equals.1 So individual classes can tailor what == or != means by overriding the equals method. We’ll show an example later in this chapter.

The root class Any has two subclasses: AnyVal and AnyRef. AnyVal is the parent class of every built-in value class in Scala. There are nine such value classes: Byte, Short, Char, Int, Long, Float, Double, Boolean, and

Unit. The first eight of these correspond to Java’s primitive types, and their values are represented at run time as Java’s primitive values. The instances of these classes are all written as literals in Scala. For example, 42 is an instance of Int, 'x' is an instance of Char, and false an instance of Boolean. You cannot create instances of these classes using new. This is enforced by the “trick” that value classes are all defined to be both abstract and final. So if you were to write:

scala> new Int

you would get:

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

new Int

ˆ

The other value class, Unit, corresponds roughly to Java’s void type; it is used as the result type of a method that does not otherwise return an interesting result. Unit has a single instance value, which is written (), as discussed in Section 7.2.

As explained in Chapter 5, the value classes support the usual arithmetic and boolean operators as methods. For instance, Int has methods named + and *, and Boolean has methods named || and &&. Value classes also inherit all methods from class Any. You can test this in the interpreter:

1The only cases where == is does not directly call equals is for Java’s boxed numeric classes such as Integer or Long. In Java, a new Integer(1) does not equal a new Long(1) even though for primitive values 1 == 1L. Since Scala is a more regular language than Java it was necessary correct this discrepancy by special-casing the == method for these classes. Likewise, the ## method provides a Scala version of hashing that is the same as Java’s hashCode, except for boxed numeric types, where it works consistently with ==. For instance new Integer(1) and new Long(1) hash the same with ## even though their Java hashCodes are different.

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