Java 8 组列表映射

Question

我有一个 Employee 和一个 EmployeeJob 类，其签名如下:

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;
    }
}

我想要一个Map<Integer, List<EmployeeJob>>来自List<EmployeeJob>其中键是 Employee 类中的 employeeId。如何使用 java 8 流按 employee->employeeId 对该列表进行分组？

可以在单个流中解决问题，还是应该以某种方式将其拆分或采用经典解决方案？

Answer 1

使用Collectors.groupingBy:

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