Scala入门笔记

一个构建有理数的scala例子

class Rational(n: Int, d: Int) {  require(d!=0)  private val g = gcd(n.abs, d.abs)  //最大公约数  val numer: Int = n / g  val denom: Int = d / g  def this(n: Int) = this(n, 1)  override def toString = numer + "/" + denom  def add(that: Rational): Rational =    new Rational(numer*that.denom+that.numer*denom,      denom * that.denom)  def +(that: Rational): Rational = add(that)  def mul(that: Rational): Rational =    new Rational(numer*that.numer, denom*that.denom)  def *(that: Rational): Rational = mul(that)  def sub(that: Rational): Rational = add(new Rational(-that.numer, that.denom))  def -(that: Rational): Rational = sub(that)  def div(that: Rational): Rational = mul(new Rational(that.denom, that.numer))  def /(that: Rational): Rational = div(that)  private def gcd(a: Int, b: Int): Int =    if(b == 0) a else gcd(b, a % b)}

调用：

object test {  //下面这个函数是为了实现将整数隐式转换为有理数，在有必要的时候。  implicit def IntToRational(i: Int) = new Rational(i)  def main(args: Array[String]) {    val a = new Rational(1,2)    val b = new Rational(3, 5)    val c = 2 * b    println(c)  }}

简单的流程控制

val filesHere = (new java.io.File("./src")).listFiles()for (f <- filesHere if f.getName.endsWith(".scala"))    println(f)//下面这句话和上面的其实是等价的！filesHere.foreach(f => println(if (f.getName.endsWith(".scala")) f))//也可以对for循环安装多个if过滤for (f <- filesHere if f.getName.endsWith(".scala"); if f.getName.length > 3)    println(f)

object test{  def main(args: Array[String]) {    val filesHere = (new java.io.File("./src")).listFiles()    for (f <- filesHere if f.getName.endsWith(".scala"); if f.getName.length > 3)      println(f)    filesHere.foreach(f => println(if (f.getName.endsWith(".scala") && f.getName.length > 3) f))    def fileLines(file: java.io.File) =      scala.io.Source.fromFile(file).getLines.toList    def grep(pattern: String) =      for {        file <- filesHere  //这个是第一层循环        if file.getName.endsWith(".scala")        line <- fileLines(file = file)  //注意这里是第二层循环        trimmed = line.trim        if trimmed.matches(pattern)      } println(file +": " + trimmed)    grep(".*gcd.*")  }}

trait的混入

在scala中，可以通过混入特质来更新子类的行为，但是又不需要修改父类，这样可以轻松的更新功能，下面就演示一个《scala编程》中的例子：

import scala.collection.mutable.ArrayBuffer/**  * Created  on 2016/10/5.  */abstract class IntQueue{  def get: Int  def put(x: Int): Unit}trait Doubling extends IntQueue{  abstract override def put(x: Int): Unit = {super.put(2*x)}}trait Increment extends IntQueue{  abstract override def put(x: Int): Unit = {super.put(x + 1)}}trait Filter extends IntQueue{  abstract override def put(x: Int): Unit = {if(x > 0) super.put(x)}}class BasicIntQueue extends IntQueue {  private val buf = new ArrayBuffer[Int]  def get = buf.remove(0)  def put(x: Int) { buf += x }}class StandardIntQueue extends BasicIntQueue with Increment with Doubling  with Filter

这里定义了一个抽象类IntQueue，然后定义了一个基类BasicIntQueue，然后定义了三个trait，但是都是extends IntQueue，所以，它们只用用来修饰继承了IntQueue的子类。这里的BasicIntQueue是不能直接用这三个 trait来修饰的。IntQueue是一个抽象类，所以继承类必须实现抽象类的所有方法，如果直接用trait修饰BasicIntQueue，则与BasicIntQueue中的put方法冲突。

object MyInt {  def main(args: Array[String]): Unit = {    val sq = new StandardIntQueue    sq.put(10)    sq.put(2)    println(sq.get)    println(sq.get)  }}

另外需要说明的是，多个trait作用时，是最右边的开始起作用，然后从右到左。