Java8：对列表中对象的特定字段的值求和
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Java8: sum values from specific field of the objects in a list
提问by mat_boy
Suppose to have a class Obj
假设有一个类 Obj
class Obj{
int field;
}
and that you have a list of Objinstances, i.e. List<Obj> lst.
并且您有一个Obj实例列表，即List<Obj> lst.
Now, how can I find in Java8 with streams the sum of the values of the int fields fieldfrom the objects in list lstunder a filtering criterion (e.g. for an object o, the criterion is o.field > 10)?
现在，我如何在 Java8 中使用流找到过滤条件下field列表中对象的 int 字段值的总和lst（例如，对于对象o，条件是o.field > 10）？
采纳答案by Aniket Thakur
You can do
你可以做
int sum = lst.stream().filter(o -> o.getField() > 10).mapToInt(o -> o.getField()).sum();
or (using Method reference)
或（使用方法参考）
int sum = lst.stream().filter(o -> o.getField() > 10).mapToInt(Obj::getField).sum();
回答by JeanValjean
Try:
尝试：
int sum = lst.stream().filter(o -> o.field > 10).mapToInt(o -> o.field).sum();
回答by Pawe? ?wik
回答by Sotirios Delimanolis
You can also collectwith an appropriate summing collector like Collectors#summingInt(ToIntFunction)
您还可以collect使用合适的求和收集器，例如Collectors#summingInt(ToIntFunction)
Returns a
Collectorthat produces the sum of a integer-valued function applied to the input elements. If no elements are present, the result is 0.
返回一个
Collector产生应用于输入元素的整数值函数的总和。如果不存在元素，则结果为 0。
For example
例如
Stream<Obj> filtered = list.stream().filter(o -> o.field > 10);
int sum = filtered.collect(Collectors.summingInt(o -> o.field));
回答by Zon
In Java 8 for an Objentity with fieldand getField() method you can use:
在 Java 8 中，对于Obj带有fieldgetField() 方法的实体，您可以使用：
List<Obj> objs ...
Stream<Obj> notNullObjs =
objs.stream().filter(obj -> obj.getValue() != null);
Double sum = notNullObjs.mapToDouble(Obj::getField).sum();
