Scala教程之:Enumeration

Enumeration应该算是程序语言里面比较通用的一个类型，在scala中也存在这样的类型，我们看下Enumeration的定义：

abstract class Enumeration (initial: Int) extends Serializable

Enumeration是一个抽象类，它定义四个value方法，来设置内部的值，四个value方法如下定义：

  /** Creates a fresh value, part of this enumeration. */

  protected final def Value: Value = Value(nextId)

  /** Creates a fresh value, part of this enumeration, identified by the

   *  integer `i`.

   *

   *  @param i An integer that identifies this value at run-time. It must be

   *           unique amongst all values of the enumeration.

   *  @return  Fresh value identified by `i`.

   */

  protected final def Value(i: Int): Value = Value(i, nextNameOrNull)

  /** Creates a fresh value, part of this enumeration, called `name`.

   *

   *  @param name A human-readable name for that value.

   *  @return  Fresh value called `name`.

   */

  protected final def Value(name: String): Value = Value(nextId, name)

  /** Creates a fresh value, part of this enumeration, called `name`

   *  and identified by the integer `i`.

   *

   * @param i    An integer that identifies this value at run-time. It must be

   *             unique amongst all values of the enumeration.

   * @param name A human-readable name for that value.

   * @return     Fresh value with the provided identifier `i` and name `name`.

   */

  protected final def Value(i: Int, name: String): Value = new Val(i, name)

知道如何设置Enum的值后，我们就可以尝试创建一个Enum了。

println("Step 1: How to create an enumeration")

object Donut extends Enumeration {

type Donut = Value

val Glazed      = Value("Glazed")

val Strawberry  = Value("Strawberry")

val Plain       = Value("Plain")

val Vanilla     = Value("Vanilla")

}

上面的例子中，我们创建了一个Enum，并且设置了几个值。

下面我们看下怎么取到Enum的值：

println("\nStep 2: How to print the String value of the enumeration")

println(s"Vanilla Donut string value = ${Donut.Vanilla}")

你可以看到如下的输出：



Step 2: How to print the String value of the enumeration

Vanilla Donut string value = Vanilla

下面是怎么输出Enum的id：

println("\nStep 3: How to print the id of the enumeration")

println(s"Vanilla Donut's id = ${Donut.Vanilla.id}")

结果如下：

Step 3: How to print the id of the enumeration

Vanilla Donut's id = 3

怎么输出所有的Enum项呢？

println("\nStep 4: How to print all the values listed in Enumeration")

println(s"Donut types = ${Donut.values}")

输出结果如下：

Step 4: How to print all the values listed in Enumeration

Donut types = Donut.ValueSet(Glazed, Strawberry, Plain, Vanilla)

接下来，我们看下怎么打印出所有的Enum：

println("\nStep 5: How to pattern match on enumeration values")

Donut.values.foreach {

  case d if (d == Donut.Strawberry || d == Donut.Glazed) => println(s"Found favourite donut = $d")

  case _ => None

}

输出如下：



Step 5: How to pattern match on enumeration values

Found favourite donut = Glazed

Found favourite donut = Strawberry

最后，我们看下怎么改变Enum值的顺序：

println("\nStep 6: How to change the default ordering of enumeration values")

object DonutTaste extends Enumeration{

  type DonutTaste = Value

  val Tasty       = Value(0, "Tasty")

  val VeryTasty   = Value(1, "Very Tasty")

  val Ok          = Value(-1, "Ok")

}

println(s"Donut taste values = ${DonutTaste.values}")

println(s"Donut taste of OK id = ${DonutTaste.Ok.id}")

输出结果如下：



Step 6: How to change the default ordering of enumeration values

Donut taste values = DonutTaste.ValueSet(Ok, Tasty, Very Tasty)

Donut taste of OK id = -1

更多教程请参考 flydean的博客

巴特西

Scala教程之:Enumeration

最新文章

热门文章