scala - scala.Equals trait 中的 canEqual()

Question

来自源码scala/Equals.scala ( here ):

package scala
trait Equals extends scala.Any {
  def canEqual(that: scala.Any): scala.Boolean
  def equals(that: scala.Any): scala.Boolean
}

在文档中，它说:

A method that should be called from every well-designed equals method that is open to be overridden in a subclass.

我随机选择了一个扩展 scala.Equals 的类这很容易理解。我选了scala.Tuple2[+T1, +T2] ，它扩展了特征 scala.Product[T1, T2] ，这反过来又扩展了特征 scala.Product ，这反过来又扩展了特征 scala.Equals .

不幸的是，似乎是因为scala.Tuple2是一个案例类，canEqual()和 equals()方法是自动生成的，因此在源代码中找不到 scala/Tuple2.scala ( here )。

我的问题是:

什么时候是扩展特征的好时机 scala.Equals ?

应该怎么做canEqual()被执行？

使用的最佳实践(或样板)是什么 canEqual()在 equals() ?

提前致谢!

PS:如果有关系，我使用的是 Scala 2.11.7。

Answer 1

canEquals方法用于覆盖期望equals应该是对称的——也就是说，当(且仅当)a.equals(b)为真，则 b.equals(a)也应该是真的。当将类的实例与子类的实例进行比较时，可能会出现与此有关的问题。例如。

class Animal(numLegs: Int, isCarnivore: Boolean) {
  def equals(other: Any) = other match {
    case that: Animal => 
      this.numLegs == that.numLegs && 
      this.isCarnivore == that.isCarnivore
    case _ => false
  }
}

class Dog(numLegs: Int, isCarnivore: Boolean, breed: String) extends Animal(numLegs, isCarnivore) {
  def equals(other: Any) = other match {
    case that: Dog => 
      this.numLegs == that.numLegs && 
      this.isCarnivore == that.isCarnivore &&
      this.breed == that.breed
    case _ => false
  }
}

val cecil = new Animal(4, true)
val bruce = new Dog(4, true, "Boxer")
cecil.equals(bruce) // true
bruce.equals(cecil) // false - cecil isn't a Dog!

要解决此问题，请使用 canEqual 确保两个实体属于相同(子)类型。在equals的定义中:

class Animal(numLegs: Int, isCarnivore: Boolean) {
  def canEqual(other: Any) = other.isInstanceOf[Animal]
  def equals(other: Any) = other match {
    case that: Animal => 
      that.canEqual(this) &&
      this.numLegs == that.numLegs && 
      this.isCarnivore == that.isCarnivore
    case _ => false
  }
}

class Dog(numLegs: Int, isCarnivore: Boolean, breed: String) extends Animal(numLegs, isCarnivore) {
  def canEqual(other: Any) = other.isInstanceOf[Dog]
  def equals(other: Any) = other match {
    case that: Dog => 
      that.canEqual(this) &&
      this.numLegs == that.numLegs && 
      this.isCarnivore == that.isCarnivore &&
      this.breed == that.breed
    case _ => false
  }
}

val cecil = new Animal(4, true)
val bruce = new Dog(4, true, "Boxer")
cecil.equals(bruce) // false - call to bruce.canEqual(cecil) returns false
bruce.equals(cecil) // false