Scala: Generic method composing

Question

I have just started learning scala and I am pretty confused with Generics:

Question 1:

def getRemainingItem[X](list : List[X], fun : (X) => Boolean) : List[X] = {
    list.filterNot(fun)
}

What is X here looks like E of Java(This code works fine)

Question 2:

def getRemainingItem(list : List[Any], fun : (Any) => Boolean) : List[Any] = {
    list.filterNot(fun)
}

Why is Any not working. I am thinking Any can accept any type (this isn't compiling)

Question 3:

def getRemainingItem(list : List[_], fun : (_) => Boolean) : List[_] = {
    list.filterNot(fun)
}

Again this is not working and what is the difference between "_" and "Any"

`What is X here looks like E of Java(This code works fine)` What `E`? — pedrofurla, Dec 10 '15 at 19:42
The underscores in the a time parameter position means some unknown time. So, `List[_]` and `(_) => Boolean`, the `_`s are not known,therefore not necessarily the same. — pedrofurla, Dec 10 '15 at 19:47

score 0 · Accepted Answer · edited May 23 '17 at 12:07

Answer 1: X is a generic type parameter. So method getRemainingItem is a generic method which will be called with a concrete type argument (a type instantiation such as X = Int).

Answer 2: that code snippet does compile (tested with Scala 2.11)

Answer 3: a type T[_] is a shortcut for T[X] forSome { type X } (which is an existential type). This is similar to Java wildcards (https://docs.oracle.com/javase/tutorial/extra/generics/wildcards.html). Of course, saying "some unknown type T" is different from saying "Any" (which is a concrete type -- the top type).

For more information: - What is an existential type?

Scala: Generic method composing

1 Answers1