scala – Missing members from default constructor arguments – Education Career Blog

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The following class has an auxillary constructor to change one property in an immutable way.

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

Compiler returns an errors:

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)}

Why does it think, there is no such members?

,

Because it should be

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

or 

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

In your version, size and others are only constructor arguments, but not members.

UPDATE: You can check it yourself:

// 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);
}

,

If you’re using Scala 2.8, then a better solution is to use the copy method defined on case classes, which takes advantage of the named/default parameters feature:

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

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

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