Java Comparable Vs. Comparator

package com.stackoverflow.questions.Q1440134;

import org.jetbrains.annotations.NotNull;

import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

/**
* @see [http://stackoverflow.com/questions/1440134/when-should-a-class-be-comparable-and-or-comparator]
* @see [http://www.digizol.com/2008/07/java-sorting-comparator-vs-comparable.html]
* @see [http://java-journal.blogspot.in/2011/01/when-to-use-comparable-and-when-to-use.html]
* <p>
* If we have only one single sorting criteria to sort our elements then we can use 'Comparable'
* but if we have more than one sorting criterias then we have to go for 'Comparator' "also".
* <p>
* Said differently, it's more that we use 'Comparable' if there is only one single obvious thing to compare on.
* And even then we can just write a 'Comparator'. If we have two different programs using the same class,
* and one wants to sort by just age and the other by just name, at least one of them will need to use a
* Comparator.
* <p>
* java.lang.Comparable: int compareTo(Object o1)
* ----------------------------------------------
* This method compares this object with o1 object. Returned int value has the following meanings.
* 1. positive – this object is greater than o1
* 2. zero – this object equals to o1
* 3. negative – this object is less than o1
* <p>
* java.util.Comparator: int compare(Object o1, Objecto2)
* ------------------------------------------------------
* This method compares o1 and o2 objects. Returned int value has the following meanings.
* 1. positive – o1 is greater than o2
* 2. zero – o1 equals to o2
* 3. negative – o1 is less than o2
*/
public class ComparableVsComparator {
    public static void main(String[] args) {
        List<Employee> employeeList = createEmployeesAndAddThemToAList();
        System.out.println(employeeList.size() + " employees created!");
        System.out.println("EMPLOYEE LIST(UNSORTED):");
        employeeList.forEach(System.out::println);

        System.out.println("@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@");

        /**
        * Sorting in natural ordering
        */
        Collections.sort(employeeList);
        System.out.println("EMPLOYEE LIST(SORTED AS PER NATURAL ORDER):");
        employeeList.forEach(System.out::println);

        System.out.println("@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@");

        /**
        * Sorting Employee types by other fields(for example: sort by 'age'
        * See 'AgeComparator' which sorts Employees by their age by implementing
        * 'java.util.Comparator'
        */
        System.out.println("EMPLOYEE LIST(SORTED BY AGE):");
        Collections.sort(employeeList, new AgeComparator());
        employeeList.forEach(System.out::println);

        /**
        * Even the ordering by 'empId' (previously done using Comparable) can be implemented using 'Comparator';
        * by thefollowing class
        *<code>
        *     class IdComparator implements Comparator<Employee>{
        *          public int compare(Employee o1, Employee o2) {
        *              return o1.getEmpId() - o2.getEmpId();
        *          }
        *     }
        * </code>
        *
        * and then
        *
        * Collections.sort(employeeList, new IdComparator());
        */
    }

    private static List<Employee> createEmployeesAndAddThemToAList() {
        return Arrays.asList(
        new Employee(5, "Frank", 28),
        new Employee(1, "Jorge", 19),
        new Employee(6, "Bill", 34),
        new Employee(3, "Michel", 10),
        new Employee(7, "Simpson", 8),
        new Employee(4, "Clerk", 16),
        new Employee(8, "Lee", 40),
        new Employee(2, "Mark", 30));
    }
}


class Employee implements Comparable<Employee> {
    protected int empId;
    protected String name;
    protected int age;

    /**
    * @param empId Employee Id
    * @param name  Employee Name
    * @param age   Employee Age
    */
    public Employee(int empId, String name, int age) {
        this.empId = empId;
        this.name = name;
        this.age = age;
    }

    @Override
    public String toString() {
        return "Employee{" +
            "empId=" + empId +
            ", name='" + name + '\'' +
            ", age=" + age +
        '}';
    }

    /**
    * Sorting in natural ordering
    *
    * @param employee an Employee instance
    * @return compared value
    */
    @Override
    public int compareTo(@NotNull Employee employee) {
        return this.empId - employee.empId;
    }
    /* Accesors and mutators */
}

@SuppressWarnings("unused")
class AgeComparator implements Comparator<Employee> {
    @Override
    public int compare(Employee employee1, Employee employee2) {
        return employee1.age < employee2.age ? -1 : employee1.age == employee2.age ? 0 : 1;
    }
}