Scala入门系列（十一）：模式匹配

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