Lazy view of java.util.Collection in Vavr

Question

I have an existing api which uses java.util.Collection when returning values. I would like to use those values in later parts of my program with Vavr but I don't want to use the eager methods like List.ofAll (because I do not want to traverse those Collection objects twice). My use case is something like this:

List<Product> filter(java.util.Collection products) {
    return List.lazyOf(products).filter(pred1);
}

Is it possible?

Answer 1

Since the input collection to the method is a java Collection, you cannot rely on immutability, so you need to process the values contained in the collection right away. You cannot defer this to a later point in time as there is no guarantee the passed collection remains unchanged.

You can minimize the number of vavr Lists built by doing the filtering on an iteration of the passed collection, then collecting the result to a List.

import io.vavr.collection.Iterator;
import io.vavr.collection.List;
...

List<Product> filter(Collection<Product> products) {
    return Iterator.ofAll(products)
        .filter(pred1)
        .collect(List.collector());
}