Filtering collection with collection of predicates and conforming to threshold

Question

I'm stuck at one of exercises that I'm doing to get into functional programming a bit, so my main issue is how to count passed predicates on element of a collection and if it conform to given threshold than collect it to the final collection.

I know how to make it without threshold, and here is my solution:

List<String> filter(List<Predicate<String>> predicateList, List<String> stringList) {
    return stringList.stream()
                     .filter(predicateList.stream()
                                          .reduce(x -> !(x.isEmpty()), Predicate::and))
                     .collect(Collectors.toList());
}

But what should I do while there is parameter which determines how many percent of predicates need to be passed to make string be collected, let's say the body of method could look like this:

List<String> filtrr(List<Predicate<String>> predicateList, List<String> stringList, Double threshold) {}

EDIT: For example method should pass unit test like this:

@Test
void filtrrTest() {
    Double threshold = 0.65;

    Predicate<String> predicate = p -> p.contains("lorem");
    Predicate<String> predicate1 = p -> p.length() > 10;
    Predicate<String> predicate2 = p -> p.contains("sed");

    List<Predicate<String>> predicateList = Arrays.asList(predicate, predicate1, predicate2);

    String string = "lorem ipsum dolor sit amet sed";
    String string1 = "consectetur adipiscing elit, sed do eiusmod";
    String string2 = "ipsum";
    String string3 = "adipiscing elit";
    String string4 = "sed do eiusmod";

    List<String> stringList = Arrays.asList(string, string1, string2, string3, string4);
    List<String> expectedList = Arrays.asList(string, string1, string4);

    assertEquals(expectedList, ClassName.filtrr(predicateList, stringList, threshold));
}

Answer 1

I would first create a generic utility method that checks whether a given element matches up to threshold % of the predicates:

public static <T> boolean matchingSome(
        T elem, 
        List<Predicate<T>> predicates, 
        double threshold) {

    long predsToMatch = (long) ((double) predicates.size() * threshold);

    long predsMatched = predicates.stream()
        .filter(p -> p.test(elem))
        .limit(predsToMatch) // this limit is to short-circuit as soon as
        .count();            // predsToMatch predicates evaluate to true

    return predsMatched == predsToMatch;
}

Note that I'm short-circuiting the stream as soon as predsToMatch predicates have evaluated to true . This is to avoid keep on evaluating further predicates.

Now simply collect all the strings for which the above method returns true :

List<String> result = stringList.stream()
    .filter(s -> matchingSome(s, predicateList, threshold))
    .collect(Collectors.toList());