Stream API: Collectors

In this section, you will learn to collect streams into other kinds of data structures.

ND079 JPND C2 L01 A12 Stream API Collectors

What is a Collector?

A Collector is a terminal stream operation that accumulates stream elements into a container.

`Collector` Code Examples

The collect() method is a terminal operation that aggregates streams of elements. Collectors can be passed to collect() to determine what kind of collection is created.

Set<String> s = stringList.stream().collect(Collectors.toSet());

Here, the collector aggregates the elements into a Set. There are collectors for all the common data structures such as lists, sets, and maps.

Collectors can be used to perform reduction operations such as adding or counting.

Map<Year, Long> graduatingClassSizes = studentList.stream()
    .collect(Collectors.groupingBy(
        Student::getGraduationYear, Collectors.counting());

Here, groupingBy() is used to collect elements into a Map. Collectors.counting() counts the number of values for each key, so, in this example, it will count how many students there are for each graduation year.

SOLUTION:

Aggregating stream elements into a data structure such as list, a map or a set.
Performing reduction operations like summing or counting elements.