Sum values from specific field of the objects in a list

Question

Suppose to have a class Obj:

class Obj {   int field; } 

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

Now, how can I find with streams the sum of the values of the int fields field from the objects in list lst under a filtering criterion (e.g. 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 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));