在Scala中重新分配给val

Question

我正在Scala中进行训练并获得此val重新分配错误.我没有看到我将一个新值重新分配给val

class personTest
{
  val alf = Person("Alf", 30, List(EmailAddress("alf.kristian@gmail.com")))
  val fredrik = Person("Fredrik", 33, List(EmailAddress("fredrik@vraalsen.no"), EmailAddress("fvr@knowit.no")))
  val johannes = Person("Johannes", 0, Nil)

  val persons = List(alf, fredrik, johannes)

   @Test
  def testNameToEmailAddress
  {
    // Create a map from each persons name to their e-mail addresses,
    // filtering out persons without e-mail addresses
    // Hint: First filter list, then use foldLeft to accumulate...
    val emptyMap: Map[String, List[EmailAddress]] = Map()

    val nameToEmail = persons.filter(_.emailAddresses.length>0).foldLeft(emptyMap)((b,p)=> b+=p.name->p.emailAddresses)

    assertEquals(Map(alf.name -> alf.emailAddresses, fredrik.name -> fredrik.emailAddresses), nameToEmail)
  }

}

我收到了这个错误

error: reassignment to val
val nameToEmail = persons.filter(_.emailAddresses.length>0).foldLeft(emptyMap)((b,p)=> b+=p.name->p.emailAddresses)

Answer 1

b这是闭包的参数名称本身是a val,不能重新分配.

foldLeft通过将闭包的一个调用的返回值作为参数b传递给下一个来工作,所以你需要做的就是返回b + (p.name->p.emailAddresses).(不要忘记括号的优先权.)