From the source code 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
}
In the documentation, it says:
A method that should be called from every well-designed equals method that is open to be overridden in a subclass.
I randomly picked a class which extends scala.Equals
and which is simple enough to understand. I picked scala.Tuple2[+T1, +T2]
, which extends the trait scala.Product[T1, T2]
, which in turn extends the trait scala.Product
, which in turn extends the trait scala.Equals
.
Unfortunately, it seems that because scala.Tuple2
is a case class, the canEqual()
and equals()
methods are automatically generated and therefore could not be found in the source code scala/Tuple2.scala
(here).
My questions are:
scala.Equals
?canEqual()
be implemented?canEqual()
in equals()
?Thanks in advance!
PS: In case if it matters, I'm using Scala 2.11.7.
The canEquals
method is used to cover the expectation that equals
should be symmetric - that is, if (and only if) a.equals(b)
is true, then b.equals(a)
should also be true. Problems with this can arise when comparing an instance of a class with an instance of a sub-class. Eg.
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!
To fix this, ensure the two entities are of the same (sub-)type using canEqual
in the definition of equals
:
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
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