Java溪流集团与汇合

Question

我有一张班友名单

class foo(){
  private String location;
  private int total;

  //constructor, getters, and setters
}

我想分组名单的foo根据他们的位置和总和，我如何实现它与Java流？

expected output
[
  {
    "location" = "locationA",
    "total" = 123
  },
  {
    "location" = "locationB",
    "total" = 167
  }
]

我所取得的成就：

{locationA=123, locationB=167}

我的代码：

static Function<List<Case>, Object> totalPerYear = cases -> cases.stream()
                                                   .collect(Collectors.groupingBy((Case c)->c.getYear(), 
                                                   Collectors.summingInt((Case c)->c.getNumber())));

Answer 1

我假设您希望将案例列表映射到Foos列表：

class Main {

    private static List<Integer> costsList;

    public static void main(String args[]) throws Exception {
        List<Case> list = new ArrayList<>();
        list.add(new Case("2020", 4));
        list.add(new Case("2021", 2));
        list.add(new Case("2020", 2));
        list.add(new Case("2022", 3));
        List<Foo> foos = totalPerYear.apply(list);
        foos.forEach(System.out::println);

    }


    static Function<List<Case>, List<Foo>> totalPerYear = cases -> cases.stream()
            .collect(Collectors.groupingBy(Case::getYear, Collectors.summingInt(Case::getNumber)))
            .entrySet()
            .stream()
            .map(entry -> new Foo(entry.getKey(), entry.getValue()))
            .collect(Collectors.toList());


    static class Foo {
        private String year;
        private int total;

        public Foo(String year, int total) {
            this.year = year;
            this.total = total;
        }

        public String getYear() {
            return year;
        }

        public void setYear(String year) {
            this.year = year;
        }

        public int getTotal() {
            return total;
        }

        public void setTotal(int total) {
            this.total = total;
        }

        @Override
        public String toString() {
            return "Foo{" +
                    "year='" + year + '\'' +
                    ", total=" + total +
                    '}';
        }
    }

    static class Case {
        private String year;
        private int number;

        public Case(String year, int number) {
            this.year = year;
            this.number = number;
        }

        public String getYear() {
            return year;
        }

        public void setYear(String year) {
            this.year = year;
        }

        public int getNumber() {
            return number;
        }

        public void setNumber(int number) {
            this.number = number;
        }
    }
}

输出：

Foo{year='2022', total=3} 
Foo{year='2021', total=2} 
Foo{year='2020', total=6}

Answer 2

如果我正确理解了您的要求，您希望为每个location找到属性值的总和total。如果是，您可以按照以下方式进行：

Map<String, Integer> map = 
    list.stream()
    .collect(Collectors.groupingBy(Foo::getLocation, Collectors.summingInt(Foo::getTotal)));

演示：

public class Main {
    public static void main(String[] args) {
        List<Foo> list = List.of(new Foo("x", 10), new Foo("y", 5), new Foo("x", 15), new Foo("y", 10),
                new Foo("z", 10), new Foo("a", 10));

        Map<String, Integer> map = 
                list.stream()
                .collect(Collectors.groupingBy(Foo::getLocation, Collectors.summingInt(Foo::getTotal)));

        System.out.println(map);
    }
}

输出：

{a=10, x=25, y=15, z=10}