Scala class inheritance

Question

Tagged as homework. I'm having trouble in the object oriented world while trying to implement a class.

I'm implenting various functions to take action on lists, that I'm using to mock a set. I'm not too worried about my logic on how to find union, for example, but really just the structure.

For eg:

abstract class parentSet[T] protected () {

    def union(other:parentSet[T]):parentSet[T]

}

Now I want a new class extending parentSet:

class childSet[T] private (l: List[T]) extends parentSet[T] {
    def this() = this(List())
    private val elems = l
    val toList = List[T] => new List(l)

    def union(other:parentSet[T]):childSet[T] = {
        for (i <- this.toList) {
            if (other contains i) {}
            else {l :: i}
        }
        return l
    }
}

Upon compiling, I receive errors such that type childSet isn't found in def union, nor is type T to keep it parametric. Also, I assume my toList isn't correct as it complains that it isn't a member of the object; to name a few.

Where in my syntax am I wrong?

EDIT

Now I've got that figured out:

  def U(other:parentSet[T]):childSet[T] = {
    var w = other.toList
    for (i <- this.toList) {
        if (!(other contains i)) {w = i::w}
    }
    return new childSet(w)

}

Now, I'm trying to do the same operations with map, and this is what I'm working on/with:

def U(other:parentSet[T]):MapSet[T] =  {
    var a = Map[T,Unit]
    for (i <- this.toList) {
        if (!(other contains i)) {a = a + (i->())}
    }
    return new MapSet(elems + (a->()))
  }

I still want to use toList to make it easily traversable, but I'm still getting type errors while messing with maps..

Dan Simon · Accepted Answer · 2012-10-02 18:46:43Z

This code has a few problems:

It seems that you are not realizing that List[T] is an immutable type, meaning you cannot change its value once created. So if you have a List[T] and you call the :: method to prepend a value, the function returns a new list and leaves your existing one unchanged. Scala has mutable collections such as ListBuffer which may behave more like you expect. So when you return l, you're actually returning the original list.

Also, you have the order wrong in using ::. It should go i :: l, since :: is a right-binding function (because it ends with a :).

Lastly, in your union method you are doing (other contains i). Maybe it's just the Scala syntax that's confusing you, but this is the same as doing (other.contains(i)) and clearly contains is not a defined method of parentSet. It is a method on the List[T] type, but you're not calling contains on a list.

You tagged this as homework so I'm not going to fix your code, but I think you should

Look at some examples of correct Scala code involving lists, try here for starters
Play around in the Scala REPL and try creating and working with some lists, so you get a feel for how immutable collections work.

Eyal · Accepted Answer · 2012-10-02 21:14:53Z

1

To answer your direct question, even though childSet is inheriting parentSet the original method specify parentSet as the return type and not childSet. You can either only use parentSet as the type OR you could specify the return type to be anything that inherits parentSet.

answered Oct 2, 2012 at 21:14

Eyal

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