《快学scala》习题解答-第十章-特质

《快学Scala》(英文版:《Scala for the Impatient》),代码已传github:

https://github.com/vernonzheng/scala-for-the-Impatient

书为第一版。scala为2.11.4,jdk1.7.45,操作系统Mac OS X Yosemite 10.10.1。

第十章 特质

10.1
java.awt.Rectangle类有两个很有用的方法translate和grow,但可惜的是像java.awt.geom.Ellipse2D这样的类没有。在Scala中,你可以解决掉这个问题。定义一个RenctangleLike特质,加入具体的translate和grow方法。提供任何你需要用来实现的抽象方法,以便你可以像如下代码这样混入该特质:
val egg = new java.awt.geom.Ellipse2D.Double(5,10,20,30) with RectangleLike
egg.translate(10,-10)
egg.grow(10,20)

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import java.awt.geom.Ellipse2D
trait RectangleLike{
this:Ellipse2D.Double=>
def translate(x:Double,y:Double){
this.x = x
this.y = y
}
def grow(x:Double,y:Double){
this.x += x
this.y += y
}
}
object Test extends App{
val egg = new Ellipse2D.Double(5,10,20,30) with RectangleLike
println("x = " + egg.getX + " y = " + egg.getY)
egg.translate(10,-10)
println("x = " + egg.getX + " y = " + egg.getY)
egg.grow(10,20)
println("x = " + egg.getX + " y = " + egg.getY)
}

10.2
通过把scala.math.Ordered[Point]混入java.awt.Point的方式,定义OrderedPoint类。按辞典编辑方式排序,也就是说,如果x<x’或者x=x’且y<y’则(x,y)<(x’,y’)

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import java.awt.Point
class OrderedPoint extends Point with Ordered[Point]{
override def compare(that: Point): Int = {
if (this.x <= that.x && this.y < that.y) -1
else if (this.x == that.x && this.y == that.y) 0
else 1
}
}

10.3
查看BitSet类,将它的所有超类和特质绘制成一张图。忽略类型参数([…]中的所有内容)。然后给出该特质的线性化规格说明

10.4
提供一个CryptoLogger类,将日志消息以凯撒密码加密。缺省情况下密匙为3,不过使用者也可以重写它。提供缺省密匙和-3作为密匙是的使用示例

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trait Logger{
def log(str:String,key:Int = 3):String
}
class CryptoLogger extends Logger{
def log(str: String, key:Int): String = {
for ( i <- str) yield if (key >= 0) (97 + ((i - 97 + key)%26)).toChar else (97 + ((i - 97 + 26 + key)%26)).toChar
}
}
object Test extends App{
val plain = "chenzhen";
println("明文为:" + plain);
println("加密后为:" + new CryptoLogger().log(plain));
println("加密后为:" + new CryptoLogger().log(plain,-3));
}

10.5
JavaBean规范里有一种提法叫做属性变更监听器(property change listener),这是bean用来通知其属性变更的标准方式。PropertyChangeSupport类对于任何想要支持属性变更通知其属性变更监听器的bean而言是个便捷的超类。但可惜已有其他超类的类—比如JComponent—必须重新实现相应的方法。将PropertyChangeSupport重新实现为一个特质,然后将它混入到java.awt.Point类中

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import java.awt.Point
import java.beans.PropertyChangeSupport
trait PropertyChange extends PropertyChangeSupport
val p = new Point() with PropertyChange

10.6
在Java AWT类库中,我们有一个Container类,一个可以用于各种组件的Component子类。举例来说,Button是一个Component,但Panel是Container。这是一个运转中的组合模式。Swing有JComponent和JContainer,但如果你仔细看的话,你会发现一些奇怪的细节。尽管把其他组件添加到比如JButton中毫无意义,JComponent依然扩展自Container。Swing的设计者们理想情况下应该会更倾向于图10-4中的设计。但在Java中那是不可能的。请解释这是为什么?Scala中如何用特质来设计出这样的效果?

Java只能单继承。

10.7
市面上有不下数十种关于Scala特质的教程,用的都是些”在叫的狗”啦,”讲哲学的青蛙”啦之类的傻乎乎的例子。阅读和理解这些机巧的继承层级很乏味且对于理解问题没什么帮助,但自己设计一套继承层级就不同了,会很有启发。做一个你自己的关于特质的继承层级,要求体现出叠加在一起的特质,具体的和抽象的方法,以及具体的和抽象的字段

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trait Fly{
def fly(){
println("flying")
}
def flywithnowing()
}
trait Walk{
def walk(){
println("walk")
}
}
class Bird{
var name:String = _
}
class BlueBird extends Bird with Fly with Walk{
def flywithnowing() {
println("BlueBird flywithnowing")
}
}
object Test extends App{
val b = new BlueBird()
b.walk()
b.flywithnowing()
b.fly()
}

10.8
在java.io类库中,你可以通过BufferedInputStream修饰器来给输入流增加缓冲机制。用特质来重新实现缓冲。简单起见,重写read方法

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import java.io.{InputStream, FileInputStream}
trait Buffering {
this: InputStream =>
val BUF_SIZE: Int = 5
private val buf = new Array[Byte](BUF_SIZE)
private var bufsize: Int = 0
private var pos: Int = 0
override def read(): Int = {
if (pos >= bufsize) {
bufsize = this.read(buf, 0, BUF_SIZE)
if (bufsize > 0) -1
pos = 0
}
pos += 1
buf(pos-1)
}
}
val f = new FileInputStream("08.txt") with Buffering
for(i <- 1 to 10) println(f.read())

10.9
使用本章的日志生成器特质,给前一个练习中的方案增加日志功能,要求体现缓冲的效果

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import java.io.{InputStream, FileInputStream}
trait Logger {
def log(msg: String)
}
trait NoneLogger extends Logger {
def log(msg: String) = {}
}
trait PrintLogger extends Logger {
def log(msg: String) = println(msg)
}
trait Buffering {
this: InputStream with Logger =>
val BUF_SIZE: Int = 5
private val buf = new Array[Byte](BUF_SIZE)
private var bufsize: Int = 0
private var pos: Int = 0
override def read(): Int = {
if (pos >= bufsize) {
bufsize = this.read(buf, 0, BUF_SIZE)
log("buffered %d bytes: %s".format(bufsize, buf.mkString(", ")))
if (bufsize > 0) -1
pos = 0
}
pos += 1
buf(pos-1)
}
}
val f = new FileInputStream("exercise08.txt") with Buffering with PrintLogger
for(i <- 1 to 10) println(f.read())

10.10
实现一个IterableInputStream类,扩展java.io.InputStream并混入Iterable[Byte]特质

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class IterableInputStream extends java.io.InputStream with Iterable[Byte]{
class InputStreamIterator(outer: IterableInputStream) extends Iterator[Byte] {
def hasNext: Boolean = outer.available() > 0
def next: Byte = outer.read().toByte
}
override def iterator: Iterator[Byte] = new InputStreamIterator(this)
override def read(): Int = 0
}

参考:
《快学Scala》:http://book.douban.com/subject/19971952/

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