Scala：在构造函数外部的类中初始化val

Question

In Java, I know it is bad to define constructors or methods which have more than 4 or 5 parameters.

In Scala, using val over var is preferred.

I have classes which have a lot of fields (like 7 fields or 10 fields). Most of the fields are not changed after the instantiation. So I'm trying to use val s for those fields. However, the only way to initialize val s in a class that I can think up is passing them to the primary constructor.

class Person(
    val name: String,
    val gender: String,
    val age: Int,
    val height: Double,
    val weight: Double,
    val birthday: Date,
    val address: String) {
  def printInfo(): Unit = {
  ...
  }
  ...
}

But in this way, the primary constructor looks ugly. Not only its definition, but also calling the constructor is.

val person = new Person(
  parser.getName(), parser.getGender(), parser.getAge(), ..., parser.getAddress())

This does not look good.

If they were var s, I could use other methods to set them. However, now they are val s, so they cannot be changed later.

Is there any better way other than using the primary constructor? Or should I use var s in this case?

Answer 1

Sometimes, formatting (and maybe a bit of aliasing) can make a world of difference. Add a sprinkling of named parameters then bake on medium for 20 minutes...

The world can be your oyster:

case class Person(
  name     : String,
  gender   : String,
  age      : Int,
  height   : Double,
  weight   : Double,
  birthday : Date,
  address  : String
) {
  ...
}

val p = parser

val person = Person(
  name    = p.getName(),
  gender  = p.getGender(),
  age     = p.getAge(),
  ...
  address = p.getAddress()
)

Answer 2

Use the builder pattern. A builder can use var to keep track of the data that is going to be used to initialize, and then the class can be initialized at once. It will still have all these parameters, but you hide their usage from view by the builder.

You can even go further and have type safe builders, but that's an advanced technique and its added complexity is usually not worth the gain.

Answer 3

您可以让Person类的构造函数获取一个解析器，并在类内部执行“获取”逻辑