Scala - 集合比较 - 为什么Set(1)== ListSet(1)？

Question

为什么这个比较的输出会输出true？

import scala.collection.immutable.ListSet

Set(1) == ListSet(1) // Expect false

//Output
res0: Boolean = true 

从更一般的意义上讲,实际比较是如何进行的？

Answer 1

由于继承链Set <: GenSet <: GenSetLike有点冗长,可能不会立即明显在哪里查找代码equals,所以我想也许我在这里引用它:

GenSetLike.scala:

  /** Compares this set with another object for equality.
   *
   *  '''Note:''' This operation contains an unchecked cast: if `that`
   *        is a set, it will assume with an unchecked cast
   *        that it has the same element type as this set.
   *        Any subsequent ClassCastException is treated as a `false` result.
   *  @param that the other object
   *  @return     `true` if `that` is a set which contains the same elements
   *              as this set.
   */
  override def equals(that: Any): Boolean = that match {
    case that: GenSet[_] =>
      (this eq that) ||
      (that canEqual this) &&
      (this.size == that.size) &&
      (try this subsetOf that.asInstanceOf[GenSet[A]]
       catch { case ex: ClassCastException => false })
    case _ =>
      false
  }

本质上,它检查另一个对象是否也是a GenSet,如果是,它会尝试执行一些快速失败检查(比如比较size和调用canEqual),如果大小相等,它会检查这个集合是否是另一个集合的子集,大概是通过检查每个元素.

因此,用于在运行时表示集合的确切类是无关紧要的,重要的是比较对象也是a GenSet并且具有相同的元素.