摘要：
此外，matchcase中，只要一个case分支满足并处理了，就不会继续判断下一个case分支了。对变量值进行匹配//案例：成绩评价defjudgeGrade{gradematch{case"A"=˃printlncase"B"=˃printlncase"C"=˃printlncase_=˃println}}judgeGrade:Unitscala˃judgeGrade("E")youneedworkharderscala˃judgeGrade("B")Good在模式匹配中使用if守卫//案例：成绩评价（升级版）defjudgeGrade{gradematch{case"A"=˃printlncase"B"=˃printlncase"C"=˃printlncase_ifname=="leo"=˃printlncase_=˃println}}judgeGrade:Unitscala˃judgeGradeleo,youareagoodboy,comeonscala˃judgeGradeyouneedtoworkharder在模式匹配中进行变量赋值模式匹配中可以将默认情况即下划线替换为一个变量名，这样模式匹配语法就会将要匹配的值赋值给这个变量，从而可以在后面的处理语句中使用要匹配的值。
引言
模式匹配是Scala中非常有特色，非常强大的一种功能。
类似于Java中的switch case语法，但是模式匹配的功能要比它强大得多，switch只能对值进行匹配，但是Scala的模式匹配除了可以对值进行匹配之外，还可以对类型进行匹配、对Array和List的元素情况进行匹配、对case class进行匹配甚至对有值或没值(Option)进行匹配。
而且在Spark源码中也大量地使用了模式匹配功能。因此为了更好地编写Scala程序，并且更加通畅地看懂Spark源码，学好模式匹配是非常重要的。
基础语法
match case的语法如下：变量 match { case 值 => 代码}。如果值为下划线，则代表不满足以上所有情况下的默认情况如何处理。此外，match case中，只要一个case分支满足并处理了，就不会继续判断下一个case分支了。（这一点与java不同，java的switch case需要用break阻止）
对变量值进行匹配
// 案例：成绩评价
def judgeGrade(grade: String) {
grade match {
case "A" => println("Excellent")
case "B" => println("Good")
case "C" => println("Just so so")
case _ => println("you need work harder")
}
}
judgeGrade: (grade: String)Unit
scala> judgeGrade("E")
you need work harder
scala> judgeGrade("B")
Good
在模式匹配中使用if守卫
// 案例：成绩评价（升级版）
def judgeGrade(name: String, grade: String){
grade match {
case "A" => println("Excellent")
case "B" => println("Good")
case "C" => println("Just so so")
case _ if name == "leo" => println(name + ", you are a good boy, come on")
case _ => println("you need to work harder")
}
}
judgeGrade: (name: String, grade: String)Unit
scala> judgeGrade("leo", "E")
leo, you are a good boy, come on
scala> judgeGrade("sparks", "E")
you need to work harder
在模式匹配中进行变量赋值
模式匹配中可以将默认情况即下划线替换为一个变量名，这样模式匹配语法就会将要匹配的值赋值给这个变量，从而可以在后面的处理语句中使用要匹配的值。
// 案例：成绩评价（升级版）
def judgeGrade(grade: String){
grade match {
case "A" => println("you got A grade, excellent!")
case "B" => println("you got B grade, good")
case "C" => println("you got C grade, so so")
case badGrade => println("you got " + badGrade + "grade, i hope that you can get C next time")
}
}
judgeGrade: (grade: String)Unit
scala> judgeGrade("E")
you got Egrade, i hope that you can get C next time
scala> judgeGrade("F")
you got Fgrade, i hope that you can get C next time
类型匹配
Scala的模式匹配强大之处就在于可以直接匹配类型而不仅仅是值，这也是Java中switch case绝对做不到的。
匹配类型的语法为case 变量 : 类型 => 代码
// 案例：异常处理
import java.io._
def processException(e: Exception){
e match {
case e1:IllegalArgumentException => println("you passed illegal argument, exception is : " +e1)
case e2:IOException => println("you got an error while doing IO operation!" + e2)
case _:Exception => println("cannot know which exception you have")
}
}
processException: (e: Exception)Unit
// 测试能否匹配类型
scala> processException(new IllegalArgumentException("expect two arguments, but found only one"))
you passed illegal argument, exception is : java.lang.IllegalArgumentException: expect two arguments
, but found only one
scala> processException(new ArrayIndexOutOfBoundsException("array is null."))
cannot know which exception you have
对Array和List的元素进行匹配
对Array进行模式匹配，分别可以匹配带有指定元素的数组、带有指定个数的数组、以某元素打头的数组。
def greeting(arr: Array[String]) {
arr match {
case Array("Leo") => println("Hi, Leo!")
case Array(girl1,girl2,girl3) => println("Hi, girls, may I know you name? "+girl1+" "+girl2+" "+
girl3)
case Array("Leo", _*) => println("Hi, leo, please introduce your friends to me.")
case _ => println("hey, who are you?")
}
}
greeting: (arr: Array[String])Unit
// 测试
scala> greeting(Array("Leo"))
Hi, Leo!
scala> greeting(Array("jen", "Alice", "lory"))
Hi, girls, may I know you name? jen Alice lory
scala> greeting(Array("sparks"))
hey, who are you?
对List进行模式匹配，与Array类似，但是需要使用List特有的::操作符。
def greeting(list: List[String]){
list match {
case "Leo" :: Nil => println("Hi, Leo!")
case girl1 :: girl2 :: girl3 ::Nil => println("Hi, girls, nice to meet you." + girl1 + " " + gir
l2 + " " + girl3)
case "Leo" :: tail => println("Hi, Leo, please introduce your friends to me.")
case _ => println("hey, who are you?")
}
}
greeting: (list: List[String])Unit
// 测试
scala> greeting(List("Leo"))
Hi, Leo!
scala> greeting(List("Marry", "Alice", "lory"))
Hi, girls, nice to meet you.Marry Alice lory
scala> greeting(List("Sparks"))
hey, who are you?
case class匹配
Scala中提供了一种特殊的类，用case class进行声明，中文也可以称作样例类。case class其实有点类似于Java中的JavaBean的概念。即只定义field，并且由Scala编译时自动提供getter和setter方法，但是没有method。
case class的主构造函数接收的参数通常不需要使用val或var修饰，Scala自动就会使用val修饰（但是如果你自己使用var修饰，那么还是会按照var来定义）
Scala自动为case class定义了伴生对象，也就是object，并且定义了apply()方法，该方法接收主构造函数中相同的参数，并返回case class对象。
// 定义case class
class Person
case class Teacher(name: String, subject: String) extends Person
case class Student(name: String, classroom: String) extends Person
case class Worker(name: String) extends Person
// 案例：学校门禁
def judgeIdentify(p: Person) {
p match {
case Teacher(name, subject) => println("Teacher, name is " +name+ ",subject is " + subject)
case Student(name, classroom) => println("Student, name is" + name + ", classroom is " + classro
om)
case _ => println("Illegal access, please go out of the school")
}
}
defined class Person
defined class Teacher
defined class Student
defined class Worker
judgeIdentify: (p: Person)Unit
// 测试匹配效果
scala> val leo: Person = Student("leo", "class1")
leo: Person = Student(leo,class1)
scala> val tom: Person = Teacher("tom", "Math")
tom: Person = Teacher(tom,Math)
scala> val jack: Person = Worker("jack")
jack: Person = Worker(jack)
scala> judgeIdentify(leo)
Student, name isleo, classroom is class1
scala> judgeIdentify(tom)
Teacher, name is tom,subject is Math
scala> judgeIdentify(jack)
Illegal access, please go out of the school
Option匹配
Scala中有一种特殊的类型，叫做Option。Option有两种值，一种是Some，表示有值，一种是None，表示没有值。
Option通常会用于模式匹配中，用于判断某个变量是有值还是没有值，这比null来的更加简洁明了。
Spark源码中大量地使用了Option，比如Some(a)、None这种语法。
// 案例：成绩查询
val grades = Map("Leo" -> "A", "Jack" -> "B", "Sparks" -> "C")
def getGrade(name: String) {
val grade = grades.get(name)
grade match {
case Some(grade) => println("your grade is " + grade)
case None => println("Sorry, your grade information is not in the system")
}
}、
grades: scala.collection.immutable.Map[String,String] = Map(Leo -> A, Jack -> B, Sparks -> C)
getGrade: (name: String)Unit
// 测试
scala> getGrade("Leo")
your grade is A
scala> getGrade("Sparks")
your grade is C
scala> getGrade("haha")
Sorry, your grade information is not in the system