[FIXED] Map over generic collection in Java 11

Issue

I've been trying to find a way to write a generic function -- possibly not using generics -- to map over a collection.

Let's say I have a function A to B, I'd like to write a function that takes a Collection<A> and returns a Collection<B>. NOTE that A and B are not generics, just a way to express a general pattern.

What I have so far is

public static Collection<Point> points2dToPoints(Collection<Point2D> points) {
    return points.stream()
            .map(Utils::point2dToPoint)
            .collect(Collectors.toCollection(() -> points));

}

However, I get a type error in the .collect, because obviously I want the new collection to be Collection<Point> but I'm not sure how to get a Supplier for that?

Edit: I would like to be able to use this function in a generic way: if I pass it a Set I get a Set in return, but if I pass it a List I get a list in return. is it even possible to do this?

Solution

The best option is to not overcomplicate, and simply do:

public static Collection<Point> points2dToPoints(Collection<Point2D> points) {
    return points.stream()
            .map(Utils::point2dToPoint)
            .collect(Collectors.toList());
}

returning a concrete implement of interface Collection (e.g., Collectors.toList()) while hiding from the outside the implementation details, (i.e., having Collection in the method signature).

You can, however, make your method more generic by passing to it -- as Supplier -- which concrete implementation of the interface Collection you want it to return, namely

 public static Collection<Point> points2dToPoints(Collection<Point2D> points, Supplier<Collection<Point>> aNew) {
        return points.stream()
                .map(Utils::point2dToPoint)
                .collect(toCollection(aNew));

In this way you can pass the concrete Collection implementation that will be returned, for example:

points2dToPoints(.., ArrayList::new);
points2dToPoints(.., TreeSet::new);

Answered By - dreamcrash
Answer Checked By - Timothy Miller (JavaFixing Admin)