Scala基础应用（4）- 样本类和模式匹配

Scala基础应用（4）- 样本类和模式匹配

样本类

就是在普通类前加了case

case class Test(param1: Int, param2: String ...) {
}

样本类带来的好处

1. 省略掉new, 样本类实例化不需要在前面加new, 直接产生对象。
2. 参数自动加val, 即样本类的参数缺省是不可修改的
3. 自动加上toString, equals, hashcode三个方法
4. 之所以可以省略new, 是因为Scala为样例类自动创建一个包含了apply和unapply的伴生类

模式匹配

1. 模式在变量定义中

   val myTuple = (123, "abc")
   val (aNumber, aString) = myTuple
   
   声明两个变量及其值：
    aNumber = 133
    aString = "abc"

2. for表达式里的模式

   例1：
   var capitals = Map[String, String]("China" -> "BeiJing", "France" -> "Paris")
   for ((country, city) <- capitals)
       println(s"The capital of ${country} is ${city}")
   
   例2:
   val results = List(Some("apple"), None, Some("orange"))
   for (Some(fruit) <- results) print(fruit)
   
   结果为
   apple
   orange
   
   正如你看到的，for表达式模式还可以过滤掉不匹配的值。

3. 偏函数模式

   val withDefault: Option[Int] => Int = {
   case Some(x) => x
   case None => 0
   }

4. match

   4.1 通配模式

   • 样式

   变量 match {
   case 指定匹配值 =>
   case _ =>   // 通配模式
   }

   4.2 简单模式种类

   • 常量

   val x = 5
   val result = x match {
   case 5 => "five"
   }
   
   val x = 5
   val y = 5
   val result = x match {
   case `y` => "five"
   }

   • 变量

   val x = 5
   val y = 5
   val result = x match {
   case y => "five"
   }

   • 构造器

   case class Test(x: Int)
   
   val test1 = Test(5)
   val result = test1 match {
   case Test(5) => "five"
   case _ => 
   }
   

   • 序列模式

   例1:
   val lst1 = List(0, 1, 2)
   val result = lst1 match {
   case List(0, _, _) => "3 values"
   case _ =>
   }
   
   例2:
   val lst1 = List(0, 1, 2)
   val result = lst1 match {
   case List(0, _*) => "3 values"
   case _ =>
   }

   • 元组模式

   val tuple1 = (1, 2, 3)
   val result = tuple1 match {
   case (a, b, c) => "threee values"
   case _ =>
   }

   • 类型模式

   val str = "this is a string"
   val result = str match {
   case s: String => "match"
   case _ =>
   }

   • 绑定变量

   val x = 5
   val result = x match {
   case y @ 5 => y + "five"
   }

   • 模式守卫

   val str = "this is a string"
   val result = str match {
   case s: String if s(0) == 't' => "match"
   case _ =>
   }

   • 模式重叠

   val str = "this is a string"
   val result = str match {
   case s: String if s(0) == 't' => "match 0"
   case s: String if s(1) == 'h' => "match 1"
   case _ =>
   }

4.3 封闭类

• 声明封闭类

sealed abstract class Expr
case class Var(name: String) extends Expr
case class Number(num: Double) extends Expr
case class UnOp(operator: String, arg: Expr) extends Expr
case class BinOp(operator: String, left: Expr, right: Expr) extends Expr

• 模式匹配

def describe(e: Expr) : String = e match {
    case Number(_) => "a number"
    case Var(_) => "a variable"
}

上面声明在编译时会出现警告，是的，编译器将检查封闭类所有可能的值是否都做了匹配