Concise way to get both min and max value of Java 8 stream

Question

Is there a concise way to extract both the min and max value of a stream (based on some comparator) in one pass?

There appear to be many ways to get the min and max values individually, or I can sort the stream into a temporary object, for example:

List<T> sorted = Stream.of(...).sorted().collect(Collectors.toList());
T min = sorted.get(0);
T max = sorted.get(sorted.size() - 1);

But this isn't concise and requires allocating a temporary object. I'd rather not allocate a temporary object or make two passes through the stream. Is there an alternative?

Pair<T> extent = Stream.of(...).???

Answer 1

The summarizingInt collector works well if you have a Stream of Integers.

IntSummaryStatistics stats = Stream.of(2,4,3,2)
      .collect(Collectors.summarizingInt(Integer::intValue));

int min = stats.getMin();
int max = stats.getMax();

If you have doubles you can use the summarizingDouble collector.

DoubleSummaryStatistics stats2 = Stream.of(2.4, 4.3, 3.3, 2.5)
  .collect(Collectors.summarizingDouble((Double::doubleValue)));

Answer 2

If this is a frequently needed feature, we better make a Collector to do the job. We'll need a Stats class to hold count, min, max, and factory methods to creat stats collector.

Stats<String> stats = stringStream.collect(Stats.collector())

fooStream.collect(Stats.collector(fooComparator))

(Maybe a better convenience method would be Stats.collect(stream))

I made an example Stats class -

https://gist.github.com/zhong-j-yu/ac5028573c986f7820b25ea2e74ed672

public class Stats<T>
{
    int count;

    final Comparator<? super T> comparator;
    T min;
    T max;

    public Stats(Comparator<? super T> comparator)
    {
        this.comparator = comparator;
    }

    public int count(){ return count; }

    public T min(){ return min; }
    public T max(){ return max; }

    public void accept(T val)
    {
        if(count==0)
            min = max = val;
        else if(comparator.compare(val, min)<0)
            min = val;
        else if(comparator.compare(val, max)>0)
            max = val;

        count++;
    }

    public Stats<T> combine(Stats<T> that)
    {
        if(this.count==0) return that;
        if(that.count==0) return this;

        this.count += that.count;
        if(comparator.compare(that.min, this.min)<0)
            this.min = that.min;
        if(comparator.compare(that.max, this.max)>0)
            this.max = that.max;

        return this;
    }

    public static <T> Collector<T, Stats<T>, Stats<T>> collector(Comparator<? super T> comparator)
    {
        return Collector.of(
            ()->new Stats<>(comparator),
            Stats::accept,
            Stats::combine,
            Collector.Characteristics.UNORDERED, Collector.Characteristics.IDENTITY_FINISH
        );
    }

    public static <T extends Comparable<? super T>> Collector<T, Stats<T>, Stats<T>> collector()
    {
        return collector(Comparator.naturalOrder());
    }
}

Answer 3

Map each element of the stream to a pair, where the two elements represent the min and the max; and then reduce the pairs by taking the min of the mins, and the max of the maxes.

For example, using some Pair class and some Comparator<T>:

Comparator<T> comparator = ...;
Optional<Pair<T, T>> minMax = list.stream()
    .map(i -> Pair.of(i /* "min" */, i /* "max" */))
    .reduce((a, b) -> Pair.of(
        // The min of the min elements.
        comparator.compare(a.first, b.first) < 0 ? a.first : b.first,
        // The max of the max elements.
        comparator.compare(a.second, b.second) > 0 ? a.second : b.second));

Answer 4

starting with Java 12 you can obtain two result or more in a single pass by using Collectors::teeing:

class Movie {
    String title;
    double rating;
    //...
}

class Pair<T1, T2> {
    T1 left;
    T2 right;
    //...
}

@Test
void shouldFindWorstAndBestMovie() {
    var m1 = new Movie("Groundhog Day", 8);
    var m2 = new Movie("Stop! Or My Mom Will Shoot", 4.4);
    var m3 = new Movie("Forrest Gump", 8.8);

    var ratingComparator = Comparator.comparing(Movie::getRating);

    Pair<Movie, Movie> result = Stream.of(m1, m2, m3)
            .collect(Collectors.teeing(
                    Collectors.minBy(ratingComparator),
                    Collectors.maxBy(ratingComparator),
                    (min, max) -> new Pair<>(min.orElse(null), max.orElse(null))
            ));

    assertEquals(m2, result.getLeft(), "min does not match");
    assertEquals(m3, result.getRight(), "max does not match");
}

You can find more details and example in this article.

Answer 5

I think you need that

IntStream myIntStream = IntStream.rangeClosed(1, 100);
IntSummaryStatistics intStatistic = myIntStream.summaryStatistics();

System.out.println("Max: " + intStatistic.getMax() + " Min: " + intStatistic.getMin());

Answer 6

For a pure Java solution that's fairly concise, you can use .peek(). This is not truly Functional, as anything that .peek() does is a side-effect. But this does do it all in one pass, doesn't require sorting and isn't too verbose. There is a "temp" Object, the AtomicRef, but you'll probably allocate a local var/ref to hold the min and max anyway.

Comparator<T> cmp = ...
Stream<T> source = ...
final AtomicReference<T> min = new AtomicReference<T>();
Optional<T> max = source.peek(t -> {if (cmp.compare(t,min.get()) < 0) min.set(t);})
    .max(cmp);
//Do whatever with min.get() and max.get()

Answer 7

A straightforward approach using any mutable Pair class:

final Pair<T, T> pair = new Pair<>();
final Comparator<T> comparator = ...;
Stream.of(...).forEachOrdered(e -> {
    if(pair.first == null || comparator.compare(e, pair.first) < 0){
        pair.first = e;
    }
    if(pair.second == null || comparator.compare(e, pair.second) > 0){
        pair.second = e;
    }
});