Concurrency - Java 8 Recipes - page 373

private int getArea() { return (endRow
- startRow) * (endCol - startCol); }
}
How It Works
The Fork / Join framework can be used for breaking down tasks into discrete units of
work. The first part of the solution creates a ForkJoinPool object. The default con-
structor provides reasonable defaults (such as creating as many threads as there are
CPU cores) and sets up an entry point to submit divide-and-conquer work. While the
ForkJoinPool inherits from ExecutorService , it is best suited to handle tasks
that extend from RecursiveAction . The ForkJoinPool object has the in-
voke(RecursiveAction) method, which will take a RecursiveAction ob-
ject and apply the divide-and-conquer strategy.
The second part of the solution creates the GameOfLifeAdvancer class, which
extends the RecursiveAction class. By extending the RecursiveAction class,
the work can be split. The GameOfLifeAdvancer class advances the Game of Life
board to the next generation. The constructor takes a two-dimensional Boolean array
(which represents a Game of Life board), a start row/column, an end row/column, and
a destination two-dimensional Boolean array, on which the result of advancing the
Game of Life for one generation is collected.
The GameOfLifeAdvancer is required to implement the compute() method.
In this method, determine how much work there is to be completed. If the work is
small enough, the work is completed directly (achieved by calling the computeDir-
ectly() method and returning). If the work is not small enough, the method splits
the work by creating two GameOfLifeAdvancer instances that process only half of
the current GameOfLifeAdvancer work. This is done by either splitting the num-
ber of rows to be processed into two chunks or by splitting the number of columns into
two chunks. The two GameOfLifeAdvancer instances are then passed to the
ForkJoin pool by calling the invokeAll() method of the RecursiveAction
class. The invokeAll() method takes the two instances of GameOfLifeAdvan-
cer (it can take as many as needed) and waits until they both are finished executing
(that is, the meaning of the -all postfix in the invokeAll() method name; it waits
for all of the tasks submitted to be completed before returning control).