Functional Programming in Java, 2nd Edition

Counting Occurrences

In ​Using the collect Method and the Collectors Class​, you saw the amazing capabilities of the groupingBy function and the mapping function to split a collection into different groups. We got a taste of the recursive nature of the Collector there. The collect method takes a Collector as a parameter. We can pass the response from calling groupingBy, for example. But we saw a variation where we passed the result of a call to the mapping method as the second argument to groupingBy. Then again, the mapping function itself expects a Collector as its second argument. This recursive structure is mind-blowing:

//pseudocode
...stream()
.collect(groupingBy(...)) //collect takes a Collector

or

...stream()
.collect(groupingBy(..., ...