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如何使用scala类成员名称作为变量

var class scala

xuhai · 技术社区 · 7 年前

在scala中,有没有方法访问类成员,但成员名称是var?

case class Features(
  // star about
  var one_star: String = "0",
  var two_star: String = "0",
  var three_star: String = "0",
  var four_star: String = "0",
  var five_star: String = "0"

  // other about
)

object Features {
  def apply(): Features = {
    val features = new Features()
    var testVar = "one_star"
    features.${testVar} = "1"
    features
  }
}

3 回复 | 直到 7 年前

Ra Ka 7 年前

i、 e.提供类变量名作为值,查找与给定变量名匹配的字段,最后更改该字段的值 ,有几种方法:简单的方法是使用模式匹配来检查字段值,自己更改实例值并返回实例。然而,由于需要处理类中定义的每个字段,所以它可能会非常混乱,如果有许多字段,那么它可能会非常麻烦。因此,您需要一些通用的方法来解决这个问题。

另一种方法是使用 scala反射

  case class Features(
                       // star about
                       var one_star: String = "0",
                       var two_star: String = "0",
                       var three_star: String = "0",
                       var four_star: String = "0"

                       // other about
                     ) {
    def copyInstance(variableName: String, value: String): Features = {
      // 1. Initialize Features instance
      val instance = Features()

      // 2. Import scala reflection api.
      import scala.reflect.runtime.{universe => ru}

      // 3. Get the mirror of instance of Features class.
      // Mirror will reflect to instance of Features case class, and we will use this instance mirror to modify its fields value.
      val instanceMirror = ru.runtimeMirror(instance.getClass.getClassLoader).reflect(instance)

      // 4. Now, Get the field whose value need to be changed - i.e. name you provide - variableName.
      val field = ru.typeOf[Features].decl(ru.TermName(variableName)).asTerm

      // 5. Get the mirror for that field so that we can read and write to this field.
      val fieldMirror = instanceMirror.reflectField(field)

      // 6. Finally, set the value to this field.
      fieldMirror.set(value)

      // 7. Return the changed instance.
      instance
    }
  }

  val features = Features()
  val changedFeatures = features.copyInstance("one_star", "changed")

  println(features)
  println(changedFeatures)

//Result
Features(0,0,0,0)
Features(changed,0,0,0)

Exception 对于提供无效变量名和值的情况。此外,如果您的案例类包含>22个字段参数,案例类的某些功能不可用。

Ruslan K. 7 年前

class Features (
  var one_star: String = "0",
  var two_star: String = "0",
  var three_star: String = "0",
  var four_star: String = "0",
  var five_star: String = "0") {

  def setField(field: String, value: String): Unit = {
    field match {
      case "one_star" => one_star = value
      case "two_star" => two_star = value
      case "three_star" => three_star = value
      case "four_star" => four_star = value
      case "five_star" => five_star = value
    }
  }
}

ephemient 7 年前

这可以使用 scala-reflect ,尽管在大多数情况下,我不建议这样做。

import scala.reflect.runtime.universe._

val field = typeOf[Features].decl(TermName(testVar)).asTerm.accessed.asTerm
val mirror = runtimeMirror(classOf[Features].getClassLoader)
mirror.reflect(features).reflectField(field).set("1")

Map[String, String] 为了你们班?如此多的属性并不典型。