要映射的Java 8組列表

Question

我有一個帶有以下簽名的Employee和EmployeeJob類：

public class Employee {

    public Integer employeeId;

    public Integer getEmployeeId() {
        return employeeId;
    }

    public void setEmployeeId(Integer employeeId) {
        this.employeeId = employeeId;
    }
}

public class EmployeeJob {

    private Integer employeeJobId;
    private Employee employee;
    private String jobName;
    public Integer getEmployeeJobId() {
        return employeeJobId;
    }
    public void setEmployeeJobId(Integer employeeJobId) {
        this.employeeJobId = employeeJobId;
    }
    public Employee getEmployee() {
        return employee;
    }
    public void setEmployee(Employee employee) {
        this.employee = employee;
    }
    public String getJobName() {
        return jobName;
    }
    public void setJobName(String jobName) {
        this.jobName = jobName;
    }
}

我想要一個List<EmployeeJob>中的Map<Integer, List<EmployeeJob>> ，其中鍵將是Employee類中的employeeId 。 我如何使用Java 8流按其employee-> employeeId將該列表分組？

可以在單個流中求解，還是應該以某種方式拆分它還是采用經典解決方案？

Answer 1

使用Collectors.groupingBy ：

Map<Integer, List<EmployeeJob>> map =
    list.stream()
        .collect(Collectors.groupingBy(ej -> ej.getEmployee().getEmployeeId()));

Answer 2

groupingBy()正是這樣做的：

Map<Integer, List<EmployeeJob>> map = jobs.stream().collect(groupingBy(job -> job.getEmployee().getEmployeeId()));

不幸的是，由於必須經過Employee類，因此不能使用方法引用。

Answer 3

public static void main(String[] args) {

List<EmployeeJob> listEmployees= new ArrayList<>();
System.out.println(listEmployees);//previous list

Map<Integer, List<EmployeeJob>> newMap =
    listEmployees.stream()
        .collect(Collectors.groupingBy(employee->employee.getEmployee().getEmployeeId()));
System.out.println(newMap); // new expected map
}