Collecting Into a List, a Set and a Map

A pipeline has to end somewhere, and most of the time the somewhere is a collection. collect is the terminal operation that builds one, and the Collectors class supplies the recipes.

java

List<String> refs   = stream.map(Invoice::reference).toList();
Set<String> unique  = stream.map(Invoice::customer).collect(Collectors.toSet());
Map<String, Long> byCustomer =
        stream.collect(Collectors.groupingBy(Invoice::customer, Collectors.counting()));

The collectors worth knowing by name
Collector	Produces
`toList()` on the stream	An unmodifiable list, in encounter order
`Collectors.toSet()`	A set, with duplicates gone and no promised order
`Collectors.toMap(key, value)`	A map, throwing if two elements produce one key
`Collectors.groupingBy(key)`	A map from key to a list of the elements with it
`Collectors.joining(", ")`	One string, with a separator

groupingBy is the one that replaces the most code. A second collector inside it says what to do with each group, so counting, summing and collecting to a set are all the same call with a different second argument.

So the end of a pipeline is a choice about shape rather than about work. Decide what you want out, and the collector is usually already written.