Performing multiple operations in parallel on a stream - Java 8 in Action: Lambdas, Streams, and Functional-Style Programming - page 453

Listing C.1. Defining a StreamForker to execute multiple operations on a
stream
Here the fork method accepts two arguments:
 A Function , which transforms the stream into a result of any type representing one of these
operations
 A key, which will allow you to retrieve the result of that operation and accumulates these key/function
pairs in an internal Map
The fork method returns the StreamForker itself; therefore, you can build a pipeline by forking
several operations. Figure C.1 shows the main ideas behind the StreamForker.