如何在具有额外字段的 scala 中创建枚举

Question

在 Java 中我有这样的东西

public enum FlatFileHeaderMapping {

   HEADER_EL(1),
   HEADER_RESERVED1(5),
   HEADER_RESERVED2(2),
   HEADER_MESSAGE_TYPE(4)

   public final int fieldSize;

    private FlatFileHeaderMapping(int fieldSize) {
        this.fieldSize = fieldSize;
   }

}

然后我可以使用它将每一行放入地图中，然后通过此枚举访问地图中的键（如符号）

就我所见，枚举不具备这种品质，并且案例类不像枚举声明那样排序 - 因此不能用于匹配如上所示的记录布局。至少不是没有有序集合的支持。

我可能遗漏了一些明显的东西，因此问题！

谢谢

雷

Answer 1

object Direction extends Enumeration {
  val North = Value("North")
  val East = Value("East")
  val South = Value("South")
  val West = Value("West")
}

scala> import Direction._
scala> values foreach println
scala> val map = HashMap(North -> 1, South -> 2)

Answer 2

你可以试试case objects:

sealed trait FlatFileHeaderMapping { val fieldSize: Int }                                                                                                                                                                          
case object HEADER_EL extends FlatFileHeaderMapping { val fieldSize = 1 }                                                                                                  
case object HEADER_RESERVED1 extends FlatFileHeaderMapping { val fieldSize = 5 }                                                                                           
case object HEADER_RESERVED2 extends FlatFileHeaderMapping { val fieldSize = 2 }                                                                                           
case object HEADER_MESSAGE_TYPE extends FlatFileHeaderMapping { val fieldSize = 4 } 

然后你可以像这样使用枚举：

object Test {                                                                                                                                                              
  def foo(x: FlatFileHeaderMapping) {                                                                                                                                      
    val result =                                                                                                                                                           
      x match {
        case HEADER_EL => "it's a HEADER_EL!"                                                                                                                              
        case other => "its field size is: " + other.fieldSize                                                                                                             
      }                                                                                                                                                                    
    println(result)                                                                                                                                                        
  }                                                                                                                                                                        

  def main(args: Array[String]) {                                                                                                                                          
    foo(HEADER_EL)                                                                                                                                                         
    foo(HEADER_MESSAGE_TYPE)                                                                                                                                               
  }                                                                                                                                                                        
}

您在这里获得的主要好处是编译时检查所有枚举值是否已处理。即在上面的x match { ... } 代码中，如果你没有'case other => ...` 子句，你会得到一个编译错误。

我只是重申this answer，它列出了这种方法的优缺点。

Answer 3

过度思考是对的，但声明案例对象的方式不那么冗长：

sealed abstract class FlatFileHeaderMapping(val fieldSize: Int)
case object HEADER_EL extends FlatFileHeaderMapping(1)
case object HEADER_RESERVED1 extends FlatFileHeaderMapping(5)
case object HEADER_RESERVED2 extends FlatFileHeaderMapping(2)
case object HEADER_MESSAGE_TYPE extends FlatFileHeaderMapping(4)

Answer 4

Enumeration with constructor and lookup table回答了这个问题

整数值存在一个更简单的解决方案：

object FlatFileHeaderMapping extends Enumeration  {
   type FlatFileHeaderMapping = Value 
   val HEADER_EL = Value(1, "HEADER_EL")
   val HEADER_RESERVED1 = Value(5, "HEADER_RESERVED1")
   val HEADER_RESERVED2 = Value(2, "HEADER_RESERVED2")
   val HEADER_MESSAGE_TYPE = Value(4, "HEADER_MESSAGE_TYPE")
}

Answer 5

复制已接受答案的内容，因为它隐藏在损坏的 Tumblr 链接（我通过 Archive.org 访问）后面，而该链接又指向 this page。

trait Enum { //DIY enum type
  import java.util.concurrent.atomic.AtomicReference //Concurrency paranoia

  type EnumVal <: Value //This is a type that needs to be found in the implementing class

  private val _values = new AtomicReference(Vector[EnumVal]()) //Stores our enum values

  //Adds an EnumVal to our storage, uses CCAS to make sure it's thread safe, returns the ordinal
  private final def addEnumVal(newVal: EnumVal): Int = { import _values.{get, compareAndSet => CAS}
    val oldVec = get
    val newVec = oldVec :+ newVal
    if((get eq oldVec) && CAS(oldVec, newVec)) newVec.indexWhere(_ eq newVal) else addEnumVal(newVal)
  }

  def values: Vector[EnumVal] = _values.get //Here you can get all the enums that exist for this type

  //This is the trait that we need to extend our EnumVal type with, it does the book-keeping for us
  protected trait Value { self: EnumVal => //Enforce that no one mixes in Value in a non-EnumVal type
    final val ordinal = addEnumVal(this) //Adds the EnumVal and returns the ordinal

    def name: String //All enum values should have a name

    override def toString = name //And that name is used for the toString operation
    override def equals(other: Any) = this eq other.asInstanceOf[AnyRef]
    override def hashCode = 31 * (this.getClass.## + name.## + ordinal)
  }
}

//And here's how to use it, if you want compiler exhaustiveness checking
object Foos extends Enum {
  sealed trait EnumVal extends Value /*{ you can define your own methods etc here }*/

  val F = new EnumVal { val name = "F" }
  val X = new EnumVal { val name = "X" }
}

/**
scala> Foos.values.find(_.name == "F")
res3: Option[Foos.EnumVal] = Some(F)

scala> Foos.X.ordinal
res4: Int = 1

scala> def doSmth(foo: Foos.EnumVal) = foo match {
  case Foos.X => println("pigdog")
}

<console>:10: warning: match is not exhaustive!
missing combination        $anon$1
missing combination        $anon$2

scala> def doSmth(foo: Foos.EnumVal) = foo match {
         case Foos.X => println("pigdog")
         case Foos.F => println("dogpig")
       }
doSmth: (foo: Foos.EnumVal)Unit
**/

//But if you don't care about getting exhaustiveness warnings, you can do:

object Foos extends Enum {
  case class EnumVal private[Foos](name: String) extends Value /* { you can define your own methods and stuff here } */

  val F = EnumVal("F")
  val X = EnumVal("X")
}

/**
Which is a bit less boilerplatey.


Cheers,
√
**/