Teeing Collector is the new collector utility introduced in the Streams API.
This collector has three arguments – Two collectors and a Bi-function.
All input values are passed to each collector and the result is available in the Bi-function.
double mean = Stream.of(1, 2, 3, 4, 5)
summingDouble(i -> i),
(sum, n) -> sum / n));
The output is 3.0.