Conditionally remove elements from a List in Java 8

Question

As far as I know Java 8 introduces a new method available for Collection types: removeif(). It accepts a predicate which defines the condition on which the elements should be removed. It returns a boolean where a true response means that at least one item has been removed and false otherwise: I have this class:

HotelPriceSummary {
     Hotel hotel;
     float price
}

a List<HotelPriceSummary> allHotels;

and Iterable<Hotel> discardedHotels

I would like to do something like (obviously existsIn is a functions that does not exist, is to express what I would like to do but I didn't find the way)

allHotels.removeIf(h -> h.getHotel().existsIn (discardedHotels))

You can produce a `HashSet` from the `discardedHotels` Iterable (assuming `Hotel` overrides equals and hashCode) and use `set.contains()` — Eran, Feb 28 '18 at 08:00
Could you get by using a `Collection` instead of an `Iterable`? It would make this much simpler. — Dawood ibn Kareem, Feb 28 '18 at 08:01
How about `discardedHotels.forEach(h -> allHotels.removeIf(hps -> hps.getHotel().equals(h)))`? Not very efficient, I know. — shmosel, Feb 28 '18 at 08:03
@Harshit There's no `ArrayList`, no `Collection`, and they're not even the same element type. — shmosel, Feb 28 '18 at 08:04
@Harshit it won't work objects are of different types. How Hotel will be compared to HotelPriceSummary? — Vinay Prajapati, Feb 28 '18 at 08:07

score 8 · Accepted Answer · answered Feb 28 '18 at 08:18

8

You can't efficiently locate an item in an Iterable. I would suggest copying it to a temporary set (unless it already is a set) and then calling contains():

Set<Hotel> discardedSet = new HashSet<>();
discardedHotels.forEach(discardedSet::add);
allHotels.removeIf(h -> discardedSet.contains(h.getHotel()));

If you don't mind the O(n*m) complexity, you can call List.removeIf() within Iterable.forEach():

discardedHotels.forEach(h -> allHotels.removeIf(hps -> hps.getHotel().equals(h)))

answered Feb 28 '18 at 08:18

shmosel

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Set creation can be simplified by `Set discardedSet = StreamSupport.stream(discardedHotels.spliterator(), false).collect(Collectors.toSet());` – Flown Feb 28 '18 at 08:39
3

@Flown Thanks for the suggestion, but I don't see that as a simplification. – shmosel Feb 28 '18 at 08:41

Conditionally remove elements from a List in Java 8

1 Answers1