实例化不可变的配对对象 [英] Instantiating immutable paired objects

问题描述

是否可以创建一个对伙伴对象有不变引用的类，还是必须是我在创建后分配的var?

Is it possible to create a class with an immutable reference to a partner object, or does it have to be a var that I assign after creation?

例如

class PairedObject (p: PairedObject, id: String) {
  val partner: PairedObject = p  // but I need ref to this object to create p!
}

或者类似地，我该如何实例化以下一对?

or similarly how could I instantiate the following pair?

class Chicken (e: Egg) { 
  val offspring = e
}

class Egg (c: Chicken) {
  val mother = c
}

推荐答案

以下是解决Chicken/Egg问题的完整解决方案:

Here is a complete solution to the Chicken/Egg problem:

class Chicken (e: =>Egg) { 
  lazy val offspring = e 
}

class Egg (c: =>Chicken) {
  lazy val mother = c
}

lazy val chicken: Chicken = new Chicken(egg)
lazy val egg: Egg         = new Egg(chicken)

请注意，您必须为chicken和egg变量提供显式类型.

Note that you have to provide explicit types to the chicken and egg variables.

对于PairedObject:

And for PairedObject:

class PairedObject (p: => PairedObject, val id: String) {
  lazy val partner: PairedObject = p
}

lazy val p1: PairedObject = new PairedObject(p2, "P1")
lazy val p2: PairedObject = new PairedObject(p1, "P2")

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