match模式匹配和try/catch/finally表达式

Scala有一个 match 表达式的概念。在大多数简单情况下，你可以像Java的switch语句那样使用match表达式：

// i is an integer
i match {
    case 1  => println("January")
    case 2  => println("February")
    case 3  => println("March")
    case 4  => println("April")
    case 5  => println("May")
    case 6  => println("June")
    case 7  => println("July")
    case 8  => println("August")
    case 9  => println("September")
    case 10 => println("October")
    case 11 => println("November")
    case 12 => println("December")
    // catch the default with a variable so you can print it
    case _  => println("Invalid month")
}

match 表达式是很好的，因为它还会返回值。所有，你还可以这样写：

val monthName = i match {
    case 1  => "January"
    case 2  => "February"
    case 3  => "March"
    case 4  => "April"
    case 5  => "May"
    case 6  => "June"
    case 7  => "July"
    case 8  => "August"
    case 9  => "September"
    case 10 => "October"
    case 11 => "November"
    case 12 => "December"
    case _  => "Invalid month"
}

快速大概了解一下scala中的方法

下面是方法的一般语法：

def convertBooleanToStringMessage(bool: Boolean): String = {
    if (bool) "true" else "false"        
}

scala> val answer = convertBooleanToStringMessage(true)
answer: String = true

scala> val answer = convertBooleanToStringMessage(false)
answer: String = false

使用match表达式作为方法体

def convertBooleanToStringMessage(bool: Boolean): String = bool match {
    case true => "you said true"
    case false => "you said false"
}

val result = convertBooleanToStringMessage(true)
println(result)

在单个case中匹配多个可能的值

def isTrue(a: Any) = a match {
    case 0 | "" => false
    case _ => true
}

scala> isTrue(0)
res0: Boolean = false

scala> isTrue("")
res1: Boolean = false

scala> isTrue(1.1F)
res2: Boolean = true

scala> isTrue(new java.io.File("/etc/passwd"))
res3: Boolean = true

还可以这样：

val evenOrOdd = i match {
    case 1 | 3 | 5 | 7 | 9 => println("odd")
    case 2 | 4 | 6 | 8 | 10 => println("even")
    case _ => println("some other number")
}

cmd match {
    case "start" | "go" => println("starting")
    case "stop" | "quit" | "exit" => println("stopping")
    case _ => println("doing nothing")
}

在case语句中用if表达式

count match {
    case 1 => println("one, a lonely number")
    case x if x == 2 || x == 3 => println("two's company, three's a crowd")
    case x if x > 3 => println("4+, that's a party")
    case _ => println("i'm guessing your number is zero or less")
}

也可以在 if 语句后加 （） ：

count match {
    case 1 => println("one, a lonely number")
    case x if (x == 2 || x == 3) => println("two's company, three's a crowd")
    case x if (x > 3) => println("4+, that's a party")
    case _ => println("i'm guessing your number is zero or less")
}

在 => 的右边可以写多行代码的：

count match {
    case 1 => 
        println("one, a lonely number")
    case x if x == 2 || x == 3 => 
        println("two's company, three's a crowd")
    case x if x > 3 => 
        println("4+, that's a party")
    case _ => 
        println("i'm guessing your number is zero or less")
}

也可以用 { } 扩起来：

count match {
    case 1 => {
        println("one, a lonely number")
    }
    case x if x == 2 || x == 3 => {
        println("two's company, three's a crowd")
    }
    case x if x > 3 => {
        println("4+, that's a party")
    }
    case _ => {
        println("i'm guessing your number is zero or less")
    }
}

一些例子：

i match {
  case a if 0 to 9 contains a => println("0-9 range: " + a)
  case b if 10 to 19 contains b => println("10-19 range: " + b)
  case c if 20 to 29 contains c => println("20-29 range: " + c)
  case _ => println("Hmmm...")
}

stock match {
  case x if (x.symbol == "XYZ" && x.price < 20) => buy(x)
  case x if (x.symbol == "XYZ" && x.price > 50) => sell(x)
  case x => doNothing(x)
}

catch 表达式 是很强大的，还有很多用法会在后面的进行介绍。

try/catch/finally

scala中的try/catch语法：

var text = ""
try {
    text = openAndReadAFile(filename)
} catch {
    case e: FileNotFoundException => println("Couldn't find that file.")
    case e: IOException => println("Had an IOException trying to read that file")
}

finally中的内容会在最后执行：

try {
    // your scala code here
} 
catch {
    case foo: FooException => handleFooException(foo)
    case bar: BarException => handleBarException(bar)
    case _: Throwable => println("Got some other kind of Throwable exception")
} finally {
    // your scala code here, such as closing a database connection
    // or file handle
}