要映射的Java 8组列表

Question

I have a Employee and a EmployeeJob class with the following signatures:

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;
    }
}

I want a Map<Integer, List<EmployeeJob>> from a List<EmployeeJob> where the key would be the employeeId from the Employee class. How can I can I use java 8 streams to group that list by its employee->employeeId ?.

It is possible to solve in a single stream or should I split it somehow or go for the classical solution?

Answer 1

Use Collectors.groupingBy :

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

Answer 2

groupingBy() does exactly this:

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

Unfortunately, you cannot use a method reference since you have to go through the Employee class.

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
}