Java8：对列表中对象的特定字段的值求和

Question

Suppose to have a class Obj

class Obj{

  int field;
}

and that you have a list of Obj instances, ie List<Obj> lst .

Now, how can I find in Java8 with streams the sum of the values of the int fields field from the objects in list lst under a filtering criterion (eg for an object o , the criterion is o.field > 10 )?

Answer 1

You can do

int sum = lst.stream().filter(o -> o.getField() > 10).mapToInt(o -> o.getField()).sum();

or (using Method reference)

int sum = lst.stream().filter(o -> o.getField() > 10).mapToInt(Obj::getField).sum();

Answer 2

You can try

int sum = list.stream().filter(o->o.field>10).mapToInt(o->o.field).sum();

Like explained here

Answer 3

You can also collect with an appropriate summing collector like Collectors#summingInt(ToIntFunction)

Returns a Collector that produces the sum of a integer-valued function applied to the input elements. If no elements are present, the result is 0.

For example

Stream<Obj> filtered = list.stream().filter(o -> o.field > 10);
int sum = filtered.collect(Collectors.summingInt(o -> o.field));

Answer 4

尝试：

int sum = lst.stream().filter(o -> o.field > 10).mapToInt(o -> o.field).sum();

Answer 5

In Java 8 for an Obj entity with field and getField() method you can use:

List<Obj> objs ...

Stream<Obj> notNullObjs =
  objs.stream().filter(obj -> obj.getValue() != null);

Double sum = notNullObjs.mapToDouble(Obj::getField).sum();

Answer 6

您可以执行此方法：“IntSummaryStatistics”

IntSummaryStatistics insum = li.stream().filter(v-> v%2==0).mapToInt(mapper->mapper).summaryStatistics();