在参数中给定多个条件时过滤 stream

Question

I am trying to create a function that takes multiple predicate conditions and filters a stream according to these conditions. I cannot find anything but my retList just seems to consist of the input rather than a filtered version of the input.

Here are the instructions I was given:

//TODO Make a Stream of Integer and set it equal to input for each predicate p on conditions, set this stream variable equals to the result of applying filter(p) to the stream. after completing the for each loop, collect the resulting stream into a List as we have done in previous problems, return the resulting list

This is what I have currently:

public static List<Integer> matchAll(Stream<Integer> input,  Predicate<Integer>... conditions) { 
    Stream<Integer> stream = input;
    Stream.of(conditions).peek(p -> stream.filter(p));

    List<Integer> retList = stream.collect(Collectors.toList());

    return retList;
}

and this is how it is tested:

public static void main(String[] args) {
    Stream<Integer> stream = Stream.of(5,7,9,11,13,14,21,28,35,42,49,56,63,70,71);
    Predicate<Integer> p0 = n -> n > 10;
    Predicate<Integer> p1 = n -> n % 2 != 0;
    Predicate<Integer> p2 = n -> isPrime(n);
    System.out.println(Matcher.matchAll(stream, p0, p1, p2));
    // should get [11, 13, 71] 
}

but what I actually get is [5,7,9,11,13,14,21,28,35,42,49,56,63,70,71], the input.

Answer 1

Try

public static List<Integer> matchAll(List<Integer> input, Predicate<Integer>... conditions) {
    Stream<Integer> streams = input.stream();
    for (Predicate<Integer> predicate : conditions) {
        streams = streams.filter(predicate);
    }
    return streams.collect(Collectors.toList());
}

public static void main(String[] args) {
    List<Integer> list = Arrays.asList(5, 7, 9, 11, 13, 14, 21, 28, 35, 42, 49, 56, 63, 70, 71);
    Predicate<Integer> p0 = n -> n > 10;
    Predicate<Integer> p1 = n -> n % 2 != 0;
    Predicate<Integer> p2 = n -> isPrime(n);
    System.out.println(matchAll(list, p0, p1, p2)); // [11, 13, 71]
}

I see here: https://stackoverflow.com/a/51187234/10910098

Answer 2

You could simply create a composite Predicate in your matchAll method and use it to filter the input stream as:

public static List<Integer> matchAll(Stream<Integer> input, Predicate<Integer>... conditions) {
    Predicate<Integer> compositePredicate =
        Arrays.stream(conditions).reduce(Predicate::and).orElse(p -> true); // or else filter in all
    return input.filter(compositePredicate).collect(Collectors.toList());
}

As suggested by Holger , alternatively you can use:

return Arrays.stream(conditions)
             .reduce(Predicate::and)
             .map(input::filter) // Optional.map here
             .orElse(input)
             .collect(Collectors.toList()); // terminal operation

Answer 3

In your current implementation, you are not storing the "results" of the filtering anywhere. The filter method does not modify the original stream, which is immutable, but returns a new stream that only contains the matching values from the original stream.

One way to achieve this is to loop through your conditions, replacing the previous stream with a new one that includes the applied filtering:

public static List<Integer> matchAll(Stream<Integer> input,  Predicate<Integer>... conditions) { 
    Stream<Integer> stream = input;
    for (Predicate<Integer> condition : conditions) {
       stream = stream.filter(condition);
    }
    return stream.collect(Collectors.toList());
}

Answer 4

In your matchAll function, you may consider to change following 2 lines to 1 line. Check if it works for multiple predicate filter of a data input stream.

Change from

Stream.of(conditions).peek(p -> stream.filter(p));

List<Integer> retList = stream.collect(Collectors.toList());

Change to

List<Integer> retList = stream.filter(t -> Arrays.stream(conditions).allMatch(f -> f.test(t))).collect(Collectors.toList());

I assume it should give expected output. Hope it helps!