Java 8 Stream: groupingBy with multiple Collectors

Question

I want to use a Java 8 Stream and Group by one classifier but have multiple Collector functions. So when grouping, for example the average and the sum of one field (or maybe

Answer 1

You could chain them,

A collector can only produce one object, but this object can hold multiple values. You could return a Map for example where the map has an entry for each collector you are returning.

You can use Collectors.of(HashMap::new, accumulator, combiner);

Your accumulator would have a Map of Collectors where the keys of the Map produced matches the name of the Collector. Te combiner would need a way to combine multiple result esp when this is performed in parallel.

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Generally the built in collectors use a data type for complex results.

From Collectors

public static 
Collector summarizingDouble(ToDoubleFunction mapper) {
    return new CollectorImpl(
            DoubleSummaryStatistics::new,
            (r, t) -> r.accept(mapper.applyAsDouble(t)),
            (l, r) -> { l.combine(r); return l; }, CH_ID);
}

and in its own class

public class DoubleSummaryStatistics implements DoubleConsumer {
    private long count;
    private double sum;
    private double sumCompensation; // Low order bits of sum
    private double simpleSum; // Used to compute right sum for non-finite inputs
    private double min = Double.POSITIVE_INFINITY;
    private double max = Double.NEGATIVE_INFINITY;