如何使用 Java 流在 groupby 之后应用排序和限制

Question

我有以下员工数据列表，我需要根据员工部门对这些数据进行分组，然后我想找到每个部门中薪酬最高的 2 名员工。

public class Employee {

    private int id;
    private String name;
    private String dept;
    private int salary;

    //setters and getters
}

List<Employee> listOfEmp = new ArrayList<>();

listOfEmp.add(new Employee (1, "A", "IT",100));
listOfEmp.add(new Employee (2, "B", "IT",200));
listOfEmp.add(new Employee (3, "C", "SUPPORT",100));
listOfEmp.add(new Employee (4, "D", "SUPPORT",200));
listOfEmp.add(new Employee (5, "E", "SUPPORT",300));
listOfEmp.add(new Employee (6, "F", "SUPPORT",400));
listOfEmp.add(new Employee (7, "G", "IT",500));
listOfEmp.add(new Employee (8, "H", "IT",600));
listOfEmp.add(new Employee (9, "I", "IT",700));

以下是我写给按部门分组员工的查询

Map<String, List<String>> departmentWiseEmployees  = listOfEmp.stream().
        collect(Collectors.groupingBy(Employee::getDept, Collectors.mapping(Employee::getName, toList())));

我正在尝试使用以下方法使用以下方法查找每个部门中薪酬最高的 2 名员工。但它是

int limit = 2;
    Map<String, List<String>> groupByTeachers =
            listOfEmp.stream()
                    .collect(
                            Collectors.groupingBy(
                                    Employee::getDept,
                                    Collectors.collectingAndThen(
                                            Collectors.toList(),
                                            e -> e.stream().sorted().limit(limit).collect(toList() ) ) ) );

这里编译器抱怨为collectingAndThen提供的lambda参数出现以下错误，它说e的类型是List<Employee>，但它必须是List<Object>

有人可以帮我理解这里出了什么问题吗？

Answer 1

这部分代码：

e -> e.stream().sorted().limit(limit).collect(Collectors.toList())

返回List<Employee> 的列表而不是List<String>，所以要么将结果类型更改为：

Map<String, List<Employee>> groupByTeachers = 
                // ...The rest of your code

或者如果您期望Map<String, List<String>>，则更改collectingAndThen 以获得期望的字段，例如getName 或getDep：

e -> e.stream().sorted().limit(limit)
        .map(Employee::getDep) // for example getDep
        .collect(Collectors.toList())

Answer 2

这里是有兴趣的人的最终答案。

Map<String, List<String>> groupByTeachers =
        listOfEmp.stream()
                .collect(
                        Collectors.groupingBy(
                                Employee::getDept,
                                Collectors.collectingAndThen(
                                        Collectors.toList(),
                                        e -> e.stream().sorted(Comparator.comparingLong(Employee::getSalary).reversed()).limit(limit).map(Employee::getName).collect(toList() ) ) ) );

Answer 3

您正在寻找的是 1. 按部门分组 2. 然后从分组的条目中，对值进行流式处理，根据属性对它们进行排序，最后限制它的 N 个值。这可以转换为代码：

Map<String, List<String>> highestPaidEmployeesGroupByDepartment = listOfEmp.stream()
        .collect(Collectors.collectingAndThen(Collectors.groupingBy(Employee::getDept),
                // part 2 starting here
                    m -> m.entrySet().stream()
                        .map(e -> Map.entry(e.getKey(), e.getValue().stream()
                                .sorted(Comparator.comparing(Employee::getSalary).reversed())
                                .limit(limit)
                                .map(Employee::getName)
                                .collect(Collectors.toList())))
                        .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue))));

或者没有collectingAndThen 的替代方案是

Map<String, List<String>> collect = listOfEmp.stream()
        .collect(Collectors.groupingBy(Employee::getDept))
        .entrySet().stream()
        .collect(Collectors.toMap(Map.Entry::getKey, e -> e.getValue().stream()
                .sorted(Comparator.comparing(Employee::getSalary).reversed())
                .limit(limit)
                .map(Employee::getName)
                .collect(Collectors.toList())));