Creating a Frequency Map

The groupingBy(classifier, downstream) collector allows the collection of Stream elements into a Map by classifying each element in a group and performing a downstream operation on the elements classified in the same group.

A classic example of this principle is to use a Map to count the occurrences of elements in a Stream. In this example, the classifier is simply the identity function, which returns the element as-is. The downstream operation counts the number of equal elements, using counting().

Stream.of("apple", "orange", "banana", "apple")
      .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
      .entrySet()
      .forEach(System.out::println);

The downstream operation is itself a collector (Collectors.counting()) that operates on elements of type String and produces a result of type Long. The result of the collect method call is a Map<String, Long>.

This would produce the following output:

banana=1 orange=1 apple=2