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

Question

I want to determine the minimum area required to display a collection of points. The easy way is to loop through the collection like this:

int minX = Integer.MAX_VALUE;
int maxX = Integer.MIN_VALUE;
int minY = Integer.MAX_VALUE;
int maxY = Integer.MIN_VALUE;
for (Point point: points) {
    if (point.x < minX) {
        minX = point.x;
    }
    if (point.x > maxX) {
        maxX = point.x;
    }
    if (point.y < minY) {
        minY = point.y;
    }
    if (point.y > maxY) {
        maxY = point.y;
    }
}

I am getting to know streams. To do the same, you can do the following:

int minX = points.stream().mapToInt(point -> point.x).min().orElse(-1);
int maxX = points.stream().mapToInt(point -> point.x).max().orElse(-1);
int minY = points.stream().mapToInt(point -> point.y).min().orElse(-1);
int maxY = points.stream().mapToInt(point -> point.y).max().orElse(-1);

Both give the same result. However, although the streams approach is elegant, it is much slower (as expected).

Is there a way to get minX, maxX, minY and maxY in a single stream operation?

Answer 1

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.

Answer 2

By analogy with IntSummaryStatistics, create a class PointStatistics which collects the information you need. It defines two methods: one for recording values from a Point, one for combining two Statistics.

class PointStatistics {
    private int minX = Integer.MAX_VALUE;
    private int maxX = Integer.MIN_VALUE;

    private int minY = Integer.MAX_VALUE;
    private int maxY = Integer.MIN_VALUE;

    public void accept(Point p) {
        minX = Math.min(minX, p.x);
        maxX = Math.max(maxX, p.x);

        minY = Math.min(minY, p.y);
        maxY = Math.max(minY, p.y);
    }

    public void combine(PointStatistics o) {
        minX = Math.min(minX, o.minX);
        maxX = Math.max(maxX, o.maxX);

        minY = Math.min(minY, o.minY);
        maxY = Math.max(maxY, o.maxY);
    }

    // getters
}

Then you can collect a Stream<Point> into a PointStatistics.

class Program {
    public static void main(String[] args) {
        List<Point> points = new ArrayList<>();

        // populate 'points'

        PointStatistics statistics = points
                    .stream()
                    .collect(PointStatistics::new, PointStatistics::accept, PointStatistics::combine);
    }
}

UPDATE

I was completely baffled by the conclusion drawn by OP, so I decided to write JMH benchmarks.

Benchmark settings:

# JMH version: 1.21
# VM version: JDK 1.8.0_171, Java HotSpot(TM) 64-Bit Server VM, 25.171-b11
# Warmup: 1 iterations, 10 s each
# Measurement: 10 iterations, 10 s each
# Timeout: 10 min per iteration
# Benchmark mode: Average time, time/op

For each iteration, I was generating a shared list of random Points (new Point(random.nextInt(), random.nextInt())) of size 100K, 1M, 10M.

The results are

100K

Benchmark                        Mode  Cnt  Score   Error  Units

customCollector                  avgt   10  6.760 ± 0.789  ms/op
forEach                          avgt   10  0.255 ± 0.033  ms/op
fourStreams                      avgt   10  5.115 ± 1.149  ms/op
statistics                       avgt   10  0.887 ± 0.114  ms/op
twoStreams                       avgt   10  2.869 ± 0.567  ms/op

1M

Benchmark                        Mode  Cnt   Score   Error  Units

customCollector                  avgt   10  68.117 ± 4.822  ms/op
forEach                          avgt   10   3.939 ± 0.559  ms/op
fourStreams                      avgt   10  57.800 ± 4.817  ms/op
statistics                       avgt   10   9.904 ± 1.048  ms/op
twoStreams                       avgt   10  32.303 ± 2.498  ms/op

10M

Benchmark                        Mode  Cnt    Score     Error  Units

customCollector                  avgt   10  714.016 ± 151.558  ms/op
forEach                          avgt   10   54.334 ±   9.820  ms/op
fourStreams                      avgt   10  699.599 ± 138.332  ms/op
statistics                       avgt   10  148.649 ±  26.248  ms/op
twoStreams                       avgt   10  429.050 ±  72.879  ms/op