我是java的新手8.我正在尝试下面的场景。
class Numbers{
private Long userId;
private Long number1;
private Long number2;
}
List<Numbers> list = new ArrayList();
Input == {
"userId":1,
"number1":10,
"number2":20
}
{
"userId":1,
"number1":20,
"number2":40
}
输出::地图
{
"userId":1,
"sum":90 // addition of all numbers associated with userId
}
我想使用java 8 stream api。
我这样想,但它给了我错误,
Map<Long, Long> hashMap =results.stream().collect(Collectors.toMap(Numbers::userId,Collectors.summingLong(?));
所以我的问题,我可以从Collectors.toMap()
调用下面的方法
Long addNumbers(Long number1,Long number2){
return number1+number2;
}
使用Collectors::groupingBy
和下游收集器有一种更简单的方法:
results.stream().collect(
Collectors.groupingBy(
Numbers::getUserId, Collectors.summingLong(x -> x.getNumber1() + x.getNumber2())
)
);
你仍然可以用Collectors.toMap
做到这一点:
results.stream().collect(
Collectors.toMap(
Numbers::getUserId,
x -> x.getNumber1() + x.getNumber2(),
Long::sum
)
);
或者没有溪流:
Map<Long, Long> all = new HashMap<>();
numbers.forEach(x -> all.merge(
x.getUserId(),
x.getNumber1() + x.getNumber2(),
Long::sum);