scala学习笔记：match与unapply()

编写如下代码：

object MatchTest {    def foo(a : Any) : String =        {            a match {                case 1 => "int:1"                case x : Int if (x > 50) => "int(>50):" + x                case x : Int => "int:" + x                case (x, y) => "tuple:" + x + "," + y                case h :: t => "list:" + h + "," + t                case Array(90) => "array:90"                case Array(90, x) => "array:90," + x                case _ => ":("            }        }    def main(args : Array[String]) {        println(foo(1))        println(foo(51))        println(foo(49))        println(foo((1, 2)))        println(foo((1, 2, 3)))        println(foo((3 -> 4)))        println(foo(List(80, 81)))        println(foo(Array(90, 91)))    }}

通过class代码反编译，看到java源代码如下：

  public String foo(Object a)  {    int i = 0; int j = 0;    Object localObject1 = a;    String str;    if (BoxesRunTime.equals(BoxesRunTime.boxToInteger(1), localObject1)) { str = "int:1"; } else {      if ((localObject1 instanceof Integer)) { i = 1; j = BoxesRunTime.unboxToInt(localObject1); if (j > 50) { str = new StringBuilder().append("int(>50):").append(BoxesRunTime.boxToInteger(j)).toString(); break label464;        }      }      if (i != 0)      {        str = new StringBuilder().append("int:").append(BoxesRunTime.boxToInteger(j)).toString();      } else if ((localObject1 instanceof Tuple2)) { Tuple2 localTuple2 = (Tuple2)localObject1; Object x = localTuple2._1(); Object y = localTuple2._2(); str = new StringBuilder().append("tuple:").append(x).append(",").append(y).toString();      } else if ((localObject1 instanceof .colon.colon)) { .colon.colon localcolon = (.colon.colon)localObject1; Object h = localcolon.head(); List t = localcolon.tl$1(); str = new StringBuilder().append("list:").append(h).append(",").append(t).toString(); } else {        if (ScalaRunTime..MODULE$.isArray(localObject1, 1)) { Object localObject2 = localObject1; Option localOption1 = Array..MODULE$.unapplySeq(localObject2); if ((!localOption1.isEmpty()) && (localOption1.get() != null) && (((SeqLike)localOption1.get()).lengthCompare(1) == 0)) { Object localObject3 = ((SeqLike)localOption1.get()).apply(0); if (BoxesRunTime.equals(BoxesRunTime.boxToInteger(90), localObject3)) { str = "array:90"; break label464; } } }        if (ScalaRunTime..MODULE$.isArray(localObject1, 1)) { Object localObject4 = localObject1; Option localOption2 = Array..MODULE$.unapplySeq(localObject4); if ((!localOption2.isEmpty()) && (localOption2.get() != null) && (((SeqLike)localOption2.get()).lengthCompare(2) == 0)) { Object localObject5 = ((SeqLike)localOption2.get()).apply(0); Object x = ((SeqLike)localOption2.get()).apply(1); if (BoxesRunTime.equals(BoxesRunTime.boxToInteger(90), localObject5)) { str = new StringBuilder().append("array:90,").append(x).toString(); break label464; } } }        str = ":(";      }    }    label464: return str;  }

对于h::t的写法，其解释是：

• 它其实是个中置表示法，等价于::(h,t)
• ::是一个样例类！（java里该写的类名叫.colon.colon）

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再试一个简单的自定义object的例子：

object Email {    def unapply(str : String) : Option[(String, String)] = {        val parts = str split "@"        if (parts.length == 2) Some(parts(0), parts(1)) else None    }    def main(args : Array[String]) {        val str = "[e-mail]"        str match {            case Email(username, host) => println(s"username: $username  host: $host");            case _ => println("this is not an email address ");        }    }}

反编译代码：

  public void main(String[] args) {    String str = "[e-mail]";    String str1 = str;    Option localOption = unapply(str1);    BoxedUnit localBoxedUnit;    if (localOption.isEmpty()) {      Predef..MODULE$.println("this is not an email address "); localBoxedUnit = BoxedUnit.UNIT;    }    else    {      String username = (String)((Tuple2)localOption        .get())._1(); String host = (String)((Tuple2)localOption        .get())._2(); Predef..MODULE$.println(new StringContext(Predef..MODULE$.wrapRefArray((Object[])new String[] { "username: ", "  host: ", "" })).s(Predef..MODULE$.genericWrapArray(new Object[] { username, host }))); localBoxedUnit = BoxedUnit.UNIT;    }  }

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如下内容摘录于http://cuipengfei.me/blog/2014/06/14/scala-unapply-method/：

unapplySeq和unapply的作用很是类似，例如这样：

object Names {  def unapplySeq(str: String): Option[Seq[String]] = {    if (str.contains(",")) Some(str.split(","))    else None  }}

这里定义了一个unapplySeq方法，用逗号作为分隔符来把字符串拆开。然后我们可以这样应用它：

val namesString = "xiao ming,xiao hong,tom"namesString match {  case Names(first, second, third) => {    println("the string contains three people's names")    println(s"$first $second $third")  }  case _ => println("nothing matched")}

与上面的例子很是类似，不过编译器在这里替我们做的事情更多了：

• 调用unapplySeq，传入namesString
• 接收返回值并判断返回值是None，还是Some
• 如果是Some，则将其解开
• 判断解开之后得到的sequence中的元素的个数是否是三个
• 如果是三个，则把三个元素分别取出，赋值给first，second和third