如何在 Scala 中对嵌套类进行模式匹配？

Question

我尝试过下面的代码（相同的方法是在 Programming in Scala book 之后编写的

class Person() {
    class Room(r: Int, c: Int) {
        val row = r
        val col = c

        override def hashCode: Int =
            41 * (
                41 + row.hashCode
            ) + col.hashCode

        override def equals(other: Any) =
            other match {
                case that: Room =>
                   (that canEqual this) &&
                   this.row == that.row &&
                   this.col == that.col
               case _ => false
            }

        def canEqual(other: Any) =
            other.isInstanceOf[Room]
    }

    val room = new Room(2,1)
}

val p1 = new Person()
val p2 = new Person()

println(p1.room == p2.room)
>>> false

）分析后我发现 Scala 为 Person 的每个实例重新定义了类 Room，这就是两个房间不相等的原因。

解决该问题的一种可能性是将类放在 Person 类之外，但这并不总是最简单的。 （例如，如果类必须访问 Person 的某些参数。）

有哪些替代方法可以编写 equal 方法？

Answer 1

问题是您的两个房间是路径相关类型的实例：它们的类型是 p1.Room 和 p2.Room：

scala> :type p1.room
p1.Room

完成这项工作的一种方法是引用使用类型选择到Room，即Person#Room。

class Person() {
    class Room(r: Int, c: Int) {
        val row = r
        val col = c

        override def hashCode: Int = // omitted for brevity

        override def equals(other: Any) =
            other match {
                case that: Person#Room =>
                   (that canEqual this) &&
                   this.row == that.row &&
                   this.col == that.col
               case _ => false
            }

        def canEqual(other: Any) =
            other.isInstanceOf[Person#Room]
    }

    val room: Room = new Room(2,1)
}

val p1 = new Person()
val p2 = new Person()

scala> p1.room == p2.room
res1: Boolean = true