Streams - Beginning Java 8 Language Features - page 628

(a, b) -> {
double combined = a + b;
System.out.println(Thread.currentThread().getName() +
" - Combiner: a = " + a + ", b = " + b +
", combined = " + combined);
return combined;
});
System.out.println(sum);
main - Accumulator: partialSum = 0.0, person = (1, Ken, MALE, 1970-05-04, 6000.00),
accumulated = 6000.0
main - Accumulator: partialSum = 6000.0, person = (2, Jeff, MALE, 1970-07-15, 7100.00),
accumulated = 13100.0
main - Accumulator: partialSum = 13100.0, person = (3, Donna, FEMALE, 1962-07-29, 8700.00),
accumulated = 21800.0
main - Accumulator: partialSum = 21800.0, person = (4, Chris, MALE, 1993-12-16, 1800.00),
accumulated = 23600.0
main - Accumulator: partialSum = 23600.0, person = (5, Laynie, FEMALE, 2012-12-13, 0.00),
accumulated = 23600.0
main - Accumulator: partialSum = 23600.0, person = (6, Li, MALE, 2001-05-09, 2400.00),
accumulated = 26000.0
26000.0
The output shows that the accumulator was sufficient to produce the result and the combiner was never called.
Notice that there was only one thread named main that processed all people in the stream.
Let's turn the stream into a parallel stream keeping all the debugging messages. The following code uses a
parallel stream to get the sum of the incomes of all people. You may get a different output containing a different
message, but the sum value would be the same as 26000.0.
double sum = Person.persons()
.parallelStream()
.reduce(0.0,
(Double partialSum, Person p) -> {
double accumulated = partialSum + p.getIncome();
System.out.println(Thread.currentThread().getName() +
" - Accumulator: partialSum = " +
partialSum + ", person = " + p +
", accumulated = " + accumulated);
return accumulated;
},
(a, b) -> {
double combined = a + b;
System.out.println(Thread.currentThread().getName() +
" - Combiner: a = " + a + ", b = " + b +
", combined = " + combined);
return combined;
});