EmployeeStorer.java

import java.util.Comparator;
import java.util.NavigableSet;
import java.util.TreeSet;

class Employee {
    private final String name;

    Employee(String name) {
        this.name = name;
    }

    public String getName() {
        return this.name;
    }
}

class NameComparator implements Comparator<Employee> {
    @Override
    public int compare(Employee x, Employee y) {
        return x.getName().compareTo(y.getName());
    }
}

public class EmployeeStorer {
    public static void main(String... args) {

        NavigableSet<Employee> employees = new TreeSet<>(new NameComparator());
        employees.add(new Employee("Kanako"));
        employees.add(new Employee("Shiori"));
        employees.add(new Employee("Ayaka"));
        employees.add(new Employee("Momoka"));
        employees.add(new Employee("Reni"));

        employees.forEach(e -> System.out.println(e.getName()));
    }
}

LambdaEmployeeStorer.java

import java.util.NavigableSet;
import java.util.TreeSet;

class Employee {
    private final String name;

    Employee(String name) {
        this.name = name;
    }

    public String getName() {
        return this.name;
    }
}

public class LambdaEmployeeStorer {

    public static void main(String... args) {
        NavigableSet<Employee> employees = new TreeSet<>(
                (x, y) -> x.getName().compareTo(y.getName())
        );
        employees.add(new Employee("Kanako"));
        employees.add(new Employee("Shiori"));
        employees.add(new Employee("Ayaka"));
        employees.add(new Employee("Momoka"));
        employees.add(new Employee("Reni"));

        employees.forEach(e -> System.out.println(e.getName()));
    }
}