Scala 默认构造函数参数中缺少成员

下面的类有一个辅助构造函数以不变的方式更改一个属性

class AccUnit(size: Long, start: Date, direction:Direction, protocol:String) {
    def this(size:Long, that:AccUnit) {this(size, that.start, that.direction, that.protocol)}
}

编译器返回一个错误：

AccUnit.scala:26: error: value start is not a member of trafacct.AccUnit
        def this(size:Long, that:AccUnit) {this(size, that.start, that.direction, that.protocol)}
                                                           ^
AccUnit.scala:26: error: value direction is not a member of trafacct.AccUnit
        def this(size:Long, that:AccUnit) {this(size, that.start, that.direction, that.protocol)}
                                                                       ^
AccUnit.scala:26: error: value protocol is not a member of trafacct.AccUnit
        def this(size:Long, that:AccUnit) {this(size, that.start, that.direction, that.protocol)}

为什么它认为没有这样的成员呢？

因为它应该是这样的

class AccUnit(val size: Long, val start: Date, val direction:Direction, val protocol:String) {...}

或

在您的版本中，

size

和其他参数只是构造函数参数，而不是成员

更新：您可以自己检查：

// Main.scala
class AccUnit(size: Long, protocol: String)

F:\MyProgramming\raw>scalac Main.scala

F:\MyProgramming\raw>javap -private AccUnit
Compiled from "Main.scala"
public class AccUnit extends java.lang.Object implements scala.ScalaObject{
    public AccUnit(long, java.lang.String);
}

---

如果您使用的是Scala 2.8，那么更好的解决方案是使用在case类上定义的copy方法，它利用了命名/默认参数功能：

case class AccUnit(size: Long, start: Date, direction:Direction, protocol:String)

val first = AccUnit(...)
val second = first.copy(size = 27L)

---

最后一句我想是不正确的，他们是会员，但他们是私人的。scala>class AccUnit（size:Long，start:Date，direction:direction，protocol:String）{override def toString=“size:”+size+”，start:“+start}scala>new AccUnit（1，new Date（），new direction（），“test”）res2:AccUnit=size:1，start:Tue Oct 19 20:37:44 CEST 2010如果它们在类的主体中使用，它们将自动成为成员：）嗯，WFT。。。啊哈。。。你有任何关于doc的参考资料吗？“Scala中的编程”中的第6.5章查看了Scala参考，但我没有看到一个清晰的语句。或许可以在scala用户邮件列表中询问？

case class AccUnit(size: Long, start: Date, direction:Direction, protocol:String)

val first = AccUnit(...)
val second = first.copy(size = 27L)