Sum up ArrayList<Double> via Java Stream [duplicate]

问题

This question already has answers here:

Is mapToDouble() really necessary for summing a List<Double> with Java 8 streams? (3 answers)

Closed 4 years ago.

Im trying to Figure out how to Sum up all the elements of an 'ArrayList'.

Ive tried already:

 double totalevent = myList.stream().mapToDouble(f -> f).sum();

while myList is a ArrayList<Double>.

is there a way to do it without the useless mapToDouble function?

回答1:

The mapToDouble call is not useless: it performs an implicit unboxing. Actually it's the same as

double totalevent = myList.stream().mapToDouble(f -> f.doubleValue()).sum();

Or

double totalevent = myList.stream().mapToDouble(Double::doubleValue).sum();

Alternatively you can use summingDouble collector, but it's not a big difference:

double totalevent = myList.stream().collect(summingDouble(f -> f));

In my StreamEx library you can construct a DoubleStream directly from Collection<Double>:

double totalevent = DoubleStreamEx.of(myList).sum();

However internally it also uses mapToDouble.

来源：https://stackoverflow.com/questions/31038180/sum-up-arraylistdouble-via-java-stream