Parallel data processing and performance - Java 8 in Action: Lambdas, Streams, and Functional-Style Programming - page 206

Figure 7.3. The fork/join process
As you might have noticed, this is nothing more than the parallel version of the well-known
divide-and-conquer algorithm. To demonstrate a practical example of how to use the fork/join
framework and to build on our previous examples, let's try to calculate the sum of a range of
numbers (here represented by an array of numbers long[]) using this framework. As explained,
you need to first provide an implementation for the RecursiveTask class, as shown by the
ForkJoinSumCalculator in the following listing.