All grouping collectors have a classification function (the function that classifies the elements of the stream into different groups). Mainly, this is an instance of the Function<T, R> functional interface.
Each element of the stream (of the T type) is passed through this function, and the return will be a classifier object (of the R type). All the returned R types represent the keys (K) of a Map<K, V>, and each group is a value in this Map<K, V>.
In other words, the key (K) is the value returned by the classification function, and the value (V) is a list of elements in the stream that have this classified value (K). So, the final result is of the Map<K, List<T>> type.
Let's look at an example to bring some light to ...
