按文件分组并计算Java8[duplicate]中的非空字段

Question

我尝试使用Java8特性。

我有课

@Data
@NoArgsConstructor
@AllArgsConstructor
class User {
    private String pId;
    private String uId;
    private String price;
}

我有一个列表，我尝试按PID分组，并计算非空的UID和Price。示例：

List<User> list =
Arrays.asList(
    new User ("p1", "u1", null),
    new User ("p1", "u2", "a"),
    new User ("p2", null, "b"),
    new User ("p2", null, "c"),
    new User ("p3", "u4", "d")
);

我的预期输出是

[
    { pId:"p1", uCount:2, priceCount:1 },
    { pId:"p2", uCount:0, priceCount:2 },
    { pId:"p3", uCount:1, priceCount:1 }
]

我试着跟在后面

Map<String, Map<Object, Long>> collect =
    list.stream()
        .collect(
            Collectors.groupingBy(
                User ::getPId,
                Collectors.groupingBy(f -> f.getUId(), Collectors.counting())));

@Data
@NoArgsConstructor
@AllArgsConstructor
class Stat {
    private String pId;
    private Integer uCount;
    private Integer priceCount;
}

因为我是刚到java的，我面临着要完成它的斗争，我尽了最大的努力。是否可以删除空文件和计数？

Answer 1

你能试试下面的解决方案吗？它将提供正确的输出：

import java.util.Arrays;
import java.util.List;
import java.util.Objects;
import java.util.Set;
import java.util.stream.Collectors;

public class Main {

    public static void main(String[] args) {
        List<User> list =
                Arrays.asList(
                    new User ("p1", "u1", null),
                    new User ("p1", "u2", "a"),
                    new User ("p2", null, "b"),
                    new User ("p2", null, "c"),
                    new User ("p3", "u4", "d")
                );

        Set<Stat> set = list.stream().map(s -> new Stat(s.getPId(), 0, 0)).collect(Collectors.toSet());
        set = set.stream().map(s -> updateCounts(list, s.getPId())).collect(Collectors.toSet());

        System.out.println(set);
    }

    public static Stat updateCounts(List<User> list, String pId) {
        Long uCount = list.stream().filter(s -> pId.equals(s.getPId()) && Objects.nonNull(s.getUId()))
            .collect(Collectors.counting());

        Long priceCount = list.stream().filter(s -> pId.equals(s.getPId()) && Objects.nonNull(s.getPrice()))
                .collect(Collectors.counting());
        return new Stat(pId, Integer.valueOf(uCount+""), Integer.valueOf(priceCount+""));
    }
}