In [4]:
// 合理考虑是否使用括号有助于构建更具有表现力的方法调用链
def isEven(n: Int) = (n % 2) == 0
List(1, 2, 3, 4).filter((i: Int) => isEven(i)).foreach((i: Int) => print(i))
println
List(1, 2, 3, 4).filter(i => isEven(i)).foreach(i => print(i))
println
List(1, 2, 3, 4).filter(isEven).foreach(print)
println
List(1, 2, 3, 4) filter isEven foreach print
In [5]:
val dogBreeds = List("Doberman", "Yorkshire Terrier", "Dachshund",
"Scottish Terrier", "Great Dane", "Portuguese Water Dog")
// 使用if守护式（guard）
for(breed <- dogBreeds
if breed.contains("Terrier"))
println(breed)
In [6]:
for(breed <- dogBreeds
if breed.contains("Terrier")
if !breed.startsWith("Yorkshire"))
println(breed)
In [7]:
// 使用yield关键字能在for表达式中生成新的集合
val filteredBreeds = for {
breed <- dogBreeds
if breed.contains("Terrier") && !breed.startsWith("Yorkshire")
} yield breed
println(filteredBreeds)
In [8]:
val dogBreeds2 = List(Some("Doberman"), None, Some("Yorkshire Terrier"), Some("Dachshund"), None,
Some("Scottish Terrier"), Some("Great Dane"), None, Some("Portuguese Water Dog"))
In [9]:
for {
Some(breed) <- dogBreeds2
upcasedBreed = breed.toUpperCase()
} println(upcasedBreed)
上式中，在for表达式中定义upcasedBreed，然后在后面的表达式中使用该值。
由于dogBreeds2列表中包含都是Option对象，在for推导式中使用模式匹配提取出Some类型，而不处理None元素。
在大部分语言中，可以使用break或continue跳出循环。而Scala并未提供该功能。编写地道的Scala代码，可以使用条件表达式或者使用递归判断循环是否应该继续。在一开始对集合进行过滤以消除循环中的复杂条件就更好了，不过为了满足break的需求，Scala提供scala.util.control.Breaks对象来实现break功能。
In [10]:
// break/continue的例子
import util.control.Breaks._
In [11]:
println("==== BREAK EXAMPLE ====")
breakable {
for (i <- 1 to 10) {
println(i)
if (i > 4) break // break out of the for loop
}
}
println("==== CONTINUE EXAMPLE ====")
val searchMe = "peter piper picked a peck of pickled peppers"
var numPs = 0
for (i <- 0 until searchMe.length) {
breakable {
if (searchMe.charAt(i) != 'p') {
break // break out of the 'breakable', continue the outside loop
} else {
numPs += 1
}
}
}
println("Found " + numPs + " p's in the string.")
In [12]:
// 资源管理场景中处理异常
// 统计文件行数
import scala.io.Source
import scala.util.control.NonFatal
def countLines(fileName: String) = {
var source: Option[Source] = None
try {
source = Some(Source.fromFile(fileName))
val size = source.get.getLines.size
println(s"file $fileName has $size lines")
} catch {
case NonFatal(ex) => println(s"Non fatal exception! $ex")
} finally {
for (s <- source) {
println(s"Closing $fileName ...")
s.close
}
}
}
In [13]:
val files = Array("fileBulkReaderTestFile.txt", "nonexistentFile.txt")
files foreach (fileName => countLines(fileName))
In [14]:
object ExpensiveResource {
lazy val resource: Int = init()
def init(): Int = {
Thread.sleep(3000)
0
}
}
In [15]:
ExpensiveResource.resource
惰性求值与方法调用的区别：
In [16]:
object Breed extends Enumeration {
type Breed = Value
val doberman = Value("Doberman Pinscher")
val yorkie = Value("Yorkshire Terrier")
val scottie = Value("Scottish Terrier")
val dane = Value("Great Dane")
val portie = Value("Portuguese Water Dog")
}
In [17]:
import Breed._
println("ID\tBreed")
for (breed <- Breed.values) println(s"${breed.id}\t$breed")
println("\nJust Terriers:")
Breed.values filter (_.toString.endsWith("Terrier")) foreach println
def isTerrier(b: Breed) = b.toString.endsWith("Terrier")
println("\nTerriers Again:")
Breed.values filter isTerrier foreach println
Scala提供了很多个Value的重载方法：
In [18]:
object WeekDay extends Enumeration {
type WeekDay = Value
val Mon, Tue, Wed, Thu, Fri, Sat, Sun = Value
}
In [19]:
import WeekDay._
def isWorkingDay(d: WeekDay) = ! (d == Sat || d == Sun)
WeekDay.values filter isWorkingDay foreach println
In [20]:
// 在应用中混入日志
class ServiceImportance(name: String) {
def work(i: Int): Int = {
println(s"ServiceImportance: Doing important work! $i")
i + 1
}
}
val service1 = new ServiceImportance("uno")
(1 to 3) foreach (i => println(s"Result: ${service1.work(i)}"))
In [21]:
// 在服务中混入标准日志库，使用println输出日志
// 定义日志抽象
trait Logging {
def info(message: String): Unit
def warning(message: String): Unit
def error(message: String): Unit
}
// 定义日志信息输出到标准输出
trait StdoutLogging extends Logging {
def info(message: String) = println(s"INFO: $message")
def warning(message: String) = println(s"WARNING: $message")
def error(message: String) = println(s"ERROR: $message")
}
In [22]:
val service2 = new ServiceImportance("dos") with StdoutLogging {
override def work(i: Int): Int = {
info(s"Starting work: i = $i")
val result = super.work(i)
info(s"Ending work: i = $i, result = $result")
result
}
}
In [23]:
(1 to 3) foreach (i => println(s"Result: ${service2.work(i)}"))