How to get multiple values from an object using a single stream operation?

by

JDK 12 and above has Collectors.teeing (webrev and CSR), which collects to two different collectors and then merges both partial results into a final result.

You could use it here to collect to two IntSummaryStatistics for both the x coordinate and the y coordinate:

List<IntSummaryStatistics> stats = points.stream()
    .collect(Collectors.teeing(
             Collectors.mapping(p -> p.x, Collectors.summarizingInt()),
             Collectors.mapping(p -> p.y, Collectors.summarizingInt()),
             List::of));

int minX = stats.get(0).getMin();
int maxX = stats.get(0).getMax();
int minY = stats.get(1).getMin();
int maxY = stats.get(1).getMax();

Here the first collector gathers statistics for x and the second one for y. Then, statistics for both x and y are merged into a List by means of the JDK 9 List.of factory method that accepts two elements.

An aternative to List::of for the merger would be:

(xStats, yStats) -> Arrays.asList(xStats, yStats)

If you happen to not have JDK 12 installed on your machine, here’s a simplified, generic version of the teeing method that you can safely use as a utility method:

public static <T, A1, A2, R1, R2, R> Collector<T, ?, R> teeing(
        Collector<? super T, A1, R1> downstream1,
        Collector<? super T, A2, R2> downstream2,
        BiFunction<? super R1, ? super R2, R> merger) {

    class Acc {
        A1 acc1 = downstream1.supplier().get();
        A2 acc2 = downstream2.supplier().get();

        void accumulate(T t) {
            downstream1.accumulator().accept(acc1, t);
            downstream2.accumulator().accept(acc2, t);
        }

        Acc combine(Acc other) {
            acc1 = downstream1.combiner().apply(acc1, other.acc1);
            acc2 = downstream2.combiner().apply(acc2, other.acc2);
            return this;
        }

        R applyMerger() {
            R1 r1 = downstream1.finisher().apply(acc1);
            R2 r2 = downstream2.finisher().apply(acc2);
            return merger.apply(r1, r2);
        }
    }

    return Collector.of(Acc::new, Acc::accumulate, Acc::combine, Acc::applyMerger);
}

Please note that I’m not considering the characteristics of the downstream collectors when creating the returned collector.

More Related Contents:

  1. How to convert to Java 8 map grouping by Map [closed]
  2. Java 8 List into Map
  3. Collection to stream to a new collection
  4. How to convert a Java 8 Stream to an Array?
  5. In Java, what are the advantages of streams over loops? [closed]
  6. How to sum a list of integers with java streams?
  7. Java 8 stream’s .min() and .max(): why does this compile?
  8. Why does Stream not implement Iterable?
  9. How can I create a stream from an array?
  10. In Java 8, is there a ByteStream class?
  11. Sum values from specific field of the objects in a list
  12. Files.walk(), calculate total size
  13. Does Stream.forEach respect the encounter order of sequential streams?
  14. Most efficient way to get the last element of a stream
  15. Why does findFirst() throw a NullPointerException if the first element it finds is null?
  16. Sort map by value using lambdas and streams
  17. Java 8 Stream IllegalStateException: Stream has already been operated on or closed
  18. Equivalent of Scala dropWhile
  19. Why does Stream.allMatch() return true for an empty stream?
  20. Modifying Objects within stream in Java8 while iterating
  21. Take every nth element from a Java 8 stream
  22. How to sum values in a Map with a stream?
  23. Static context cannot access non-static in Collectors
  24. What Java 8 Stream.collect equivalents are available in the standard Kotlin library?
  25. Java 8, how can I implement a switch statement using streams?
  26. In Java 8 how do I transform a Map to another Map using a lambda?
  27. Is it possible to get next element in the Stream?
  28. Visualization of Java Stream parallelization
  29. Get last element of Stream/List in a one-liner
  30. Java Stream API – count items of a nested list

Leave a Comment