Scala List.contains(x) return false, but exists(_.== x) returns true

Question

I'm working with some simple data structures and collections in Scala and I've noticed what I think is strange behavior. Here's the object:

class State (protected val trackmap: Map[Int, List[String]]) {

  override def clone : State = {
    new State(Map() ++ trackmap)
  }

  override def toString = { "State: " + trackmap.toString }

  def equals (other: State) : Boolean = {
    //println("Comparing " + trackmap + " to " + other.trackmap)
    trackmap == other.trackmap  

  }

  def == (other: State) : Boolean = {
    this equals other
  }
}

And my related tests:

  test("state equality") {
    val state = new State( Map(1 -> List("engine"), 2 -> List("a"), 3 -> List("b")) )

    expect(true) { state equals state.clone }
    expect(true) { state == state.clone }
    expect(false) { state == new State(Map(1 -> List("a"))) }
    expect(false) { state equals new State(Map(1 -> List("a"))) }

    expect(true) { List(state).exists( _.equals (state.clone) )}
    expect(true) { List(state).exists( _.== (state.clone) )}
    expect(true) { List(state).contains( state.clone )}
  }

All of them pass except for the last one, which I expect should pass. I haven't looked at the Scala source, but I assume contains would be implemented basically as the second exists call.

Answer 1

You are not overriding the actual equals method of Scala, that is why it behaves strangely. Re-write your equals method as this, and things should work:

override def equals (other: Any) : Boolean = {
    other match{
      case that: State =>
        //println("Comparing " + trackmap + " to " + other.trackmap)
        trackmap == that.trackmap
      case _ => false
    }
}

See, equals method in Scala takes a parameter of type Any not State, and you need to add the override keyword for it.

BTW, you don't even need the == method, as Scala automatically remaps it to equals method!