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

Instead of chaining the collectors, you should build an abstraction which is an aggregator of collectors: implement the Collector interface with a class which accepts a list of collectors and delegates each method invocation to each of them. Then, in the end, you return new Data() with all the results the nested collectors produced.

You can avoid creating a custom class with all the method declarations by making use of Collector.of(supplier, accumulator, combiner, finisher, Collector.Characteristics... characteristics) The finisher lambda will call the finisher of each nested collector, then return the Data instance.