Java Streams API - Impose Ordering while collecting with groupingBy()

Question

I am trying to group the following list of Foo objects by getName() and count occurrences while ordering by getNumber() in ascending order.

Input:

List<Foo> fooList = List.of( new Foo("Alpha", 1.1f), new Foo("Delta", 1.2f), new Foo("Charlie", 3.1f), new Foo("Alpha", 2.1f), new Foo("Charlie", 4.1f), new Foo("Delta", 2.2f) );

Map Output:

{Alpha=2, Delta=2, Charlie=2}

My current solution is as follows:

Map<String, Integer> map = new LinkedHashMap<>(); fooList .stream() .sorted(Comparator.comparingDouble(Foo::getNumber)) .forEach(foo -> { int val = map.getOrDefault(foo.getName(), 0); map.put(foo.getName(), val + 1); });

Is there any better way with Collectors.groupingBy()?

Alexander Ivanchenko · Accepted Answer · 2022-08-01 16:29:45Z

Is there any better way with Collectors.groupingBy()?

You need the flavor of groupingBy(classifier,mapFactory,downstream) which allows to specify a mapFactory.

As a downstream collector, we need to apply combination of collectingAndThen() and counting() because the resulting type produced by counting() is Long and we need to turn it into Integer.

Map<String, Integer> fooNameByCount = fooList.stream() .sorted(Comparator.comparing(Foo::getNumber)) .collect(Collectors.groupingBy( Foo::getName, LinkedHashMap::new, Collectors.collectingAndThen(Collectors.counting(), Long::intValue) ));

We can achieve the same result by using a single collector with the following version of toMap(keyMapper,valueMapper,mergeFunction,mapFactory):

Map<String, Integer> fooNameByCount = fooList.stream() .sorted(Comparator.comparing(Foo::getNumber)) .collect(Collectors.toMap( Foo::getName, foo -> 1, Integer::sum, LinkedHashMap::new ));

Also note that using collector is always a preferred way to accumulate the result.

Usage of forEach() such cases when you generate the result by the means of collect(), reduce(), etc. is discouraged by the API documentation

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