比较Scala中哈希映射的关键

Question

I need to read some Scala code (starting from a few hours ago), I also need to make sure how hash maps in Scala compares keys. After reading some posts, I know that the == is for value comparison, but it is not clear that hash maps (eg scala.collection.immutable.Map ) keys are compared using value comparison ( == ) or reference comparison?

Answer 1

The HashMap uses == and the hashing method ## to compare keys. It couldn't use the reference because then something like Map(List(1) -> 'a, List(1) -> 'b) would contain two entries, which would be wrong.

Answer 2

See source code (it is on web site):

http://www.scala-lang.org/api/current/index.html#scala.collection.immutable.HashMap

I wrote simple test case (scala 2.9.1 on eclipse). You can debug it on eclipse - the lines of source code are the same like on api site.

 class EqualsTest {

      case class Key(private val value: Int) {

        override def hashCode() = value

        override def equals(other: Any) = other match {
          case that: Key => that.value == value
          case _ => false
        }
      }

      @Test
      def test() {

        val map = Map(Key(1)->1,Key(2)->3,Key(4)->5,Key(5)->2,Key(9)->9)
        val key = Key(1)
        val value = map.get(key) //add breakpoint here

      }
    }