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）

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 = "[email protected]"
        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 = "[email protected]";
    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;
    }
  }

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