Lambda Comparator Sorting List

Question

I have the following task:

Implement the following functionality in CompanyTest:

Create a collection list of all employees from all departments named allEmployees. Sort the list using the sort method (Comparator c), which inherits allEmployees from the List interface. Implement the comparator with a lambda expression.

public class CompanyTest1 {
    public static void main(String[]args) {

        Sequence<Integer> MNR = new Sequence<Integer>(){
            int id = 0;
            public Integer next() {
                return id++;
            }
        };

        List<String> AllEmployees = new ArrayList<>();


        Department dep1 = new Department("Verwaltung", MNR);
        Department dep2 = new Department("Lager", MNR);
        dep1.addEmployee("Chris", "Schmidt"); 
        dep1.addEmployee("Max", "Mustermann");
        dep2.addEmployee("Müller", "Wagner");
        System.out.println("Department: " +dep1.getName());
        dep1.forEach(Employee -> System.out.println(Employee));
        System.out.println("Department: " +dep2.getName());
        dep2.forEach(Employee -> System.out.println(Employee));
        AllEmployees.sort((Comparator<? super E> c) (Employee e1, Employee e2)->e1.getLastName()-e2.getFirstname()); 
    }
}

I do not know what i am doing wrong. Can someone help me correct my code. I am still a java noob and all help is appreciated.

import java.util.Iterator;

import java.util.*;

public class Department implements Iterable<Employee> {
    private String name;
    private Map<Integer, Employee> employees;
    private Sequence<Integer> employeeIdSequence;


    public Department(String name, Sequence<Integer> employeeIdSequence){
        this.name = name;
        employees = new HashMap<Integer, Employee>();
        this.employeeIdSequence = employeeIdSequence;
    }


    public String getName() {
        return name;
    }

    public Sequence<Integer>  addEmployee(String firstName, String lastName) {
        int neueMitarbeiterNummer = employeeIdSequence.next();
        employees.put(neueMitarbeiterNummer, new Employee(firstName, lastName));
        return employeeIdSequence;


//      Employee a = new Employee(firstName, lastName);
//      int id = employeeIdSequence.next();
//      employees.put(id, a);

    }


    @Override
    public Iterator<Employee> iterator() {
        return employees.values().iterator();
    }

}
public class Employee {
    private String firstName;
    private String lastName;

    public Employee(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public String getFirstname() {
        return firstName;
    }
    public String getLastName() {
        return lastName;
    }

    public String toString() {
        return "Vorname: " + firstName + " " + "Nachname: " + lastName;
    }



}

"e1.getLastName()-e2.getFirstname()" what is "Turner" - "Andy"? What do you think you are doing if you subtract strings? — Andy Turner
– Andy Turner, Commented May 31, 2018 at 11:02
AllEmployees is an empty List of Strings. And you try to sort it with a Comparator or Employees. — Eran
– Eran, Commented May 31, 2018 at 11:04

ernest_k · Accepted Answer · 2018-05-31 11:39:58Z

The problem is a signature mismatch between the comparator required by your list and the comparator passed to sort on that list.

Your variable declaration:

List<String> AllEmployees

Says that the list contains instances of String. And the generic sort method in List will take a comparator of the generic type of elements, which means:

AllEmployees.sort

Accepts a comparator of Strings (Comparator<String>).

The problem is that your lambda expression for that comparator has the signature of a comparator of Employee:

(Employee e1, Employee e2)->e1.getLastName()-e2.getFirstname()

Can only be used as a Comparator<Employee>. And that's why the compiler prevents it from being used in the place of a Comparator<String>, which your AllEmployees list requires.

You probably meant to add all employee entries to your allEmployees list, then sort that list:

List<Employee> allEmployees = new ArrayList<>();
dep1.forEach(employee -> allEmployees.add(employee));
dep2.forEach(employee -> allEmployees.add(employee));

//And run sort
allEmployees.sort(
(Employee e1, Employee e2) ->  e1.getLastName().compareTo(e2.getLastName()));

Note that you need to properly implement the logic of the comparator that dictates the sorting order. The above example just compares last names.

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