How to Get All Maximum Values from a Java Stream?

How to Return Multiple Maximum Values in a Java Stream

In Java Stream API, the max function typically returns one of the maximum elements from a collection when provided with a comparator. However, what if we desire all maximum values, especially when several elements have the same maximum value?

Trick for Returning All Maximum Values

Unfortunately, the Java API does not provide a direct way to retrieve all maximum values. However, a workaround involves storing partial results using a Collector. Here's a helper function that generates a Collector based on a comparator:

static <T> Collector<T, ?, List<T>> maxList(Comparator<? super T> comp) {
    return Collector.of(
        ArrayList::new,
        (list, t) -> {
            int c;
            if (list.isEmpty() || (c = comp.compare(t, list.get(0))) == 0) {
                list.add(t);
            } else if (c > 0) {
                list.clear();
                list.add(t);
            }
        },
        (list1, list2) -> {
            if (list1.isEmpty()) {
                return list2;
            } 
            if (list2.isEmpty()) {
                return list1;
            }
            int r = comp.compare(list1.get(0), list2.get(0));
            if (r < 0) {
                return list2;
            } else if (r > 0) {
                return list1;
            } else {
                list1.addAll(list2);
                return list1;
            }
        });
}

This Collector maintains a list of elements that are equivalent in terms of the Comparator. When merging results, the Collector returns the list with the greater elements. If the elements are equal, it appends the lists.

To use this Collector on a Stream, simply apply it as follows:

Stream<String> input = ... ;

List<String> result = input.collect(maxList(comparing(String::length)));

Example Usage

For instance, let's find all strings with the maximum length in a list.

List<String> strings = Arrays.asList("apple", "banana", "pear", "orange", "grapes", "kiwi");

Stream<String> stringStream = strings.stream();

List<String> maxStrings = stringStream.collect(maxList(Comparator.comparingInt(String::length)));

System.out.println(maxStrings); // [apple, banana, grapes, orange]

In this example, the maxList Collector will return a list containing all strings that have the maximum length.

The above is the detailed content of How to Get All Maximum Values from a Java Stream?. For more information, please follow other related articles on the PHP Chinese website!