A simple Java program to keep track of employees in a company which implements HashMap and TreeSet data sructures.

Employee.java

//Employee.java - Jimmy Kurian

public class Employee implements Comparable<Employee>
{
	private String ssn;
	private String lastName;
	private String firstName;


	public int compareTo(Employee emp)
	{
		int compLasts = lastName.compareTo(emp.lastName);
		int compFirsts = firstName.compareTo(emp.firstName);
		int compSSN = ssn.compareTo(emp.ssn);

		if ((compSSN != 0) && (compLasts != 0) && (compFirsts != 0))
		{
			return compLasts;
		}
		else
		{
			throw new IllegalArgumentException("Empolyee already exists with that SSN.");
		}
	}


	public Employee (String socialSN, String ln, String fn)
	{
		ssn = socialSN;
		lastName = ln;
		firstName = fn;
	}

	public String toString()
	{
		return "Employee[SSN= " + ssn + ", Last Name= " + lastName + ", First Name= " + firstName + "]";
	}

	public String getLastName()
	{
		return lastName;
	}

	public String getFirstName()
	{
		return firstName;
	}

	public String getSSN()
	{
		return ssn;
	}

	public void setLastName(String setLN)
	{
		lastName = setLN;
	}

	public void setFirstName(String setFN)
	{
		firstName = setFN;

	}

	public void setSSN(String setSSN)
	{
		ssn = setSSN;
	}
}

HMandTS_Tester.java

//HMandTS_Tester - Jimmy Kurian

import java.util.Scanner;
import java.util.*;

public class HMandTS_Tester
{
  public static int menu()
  {
    System.out.println("Choose one option from following : ");
  	System.out.println("1) Add a new employee.");
  	System.out.println("2) Delete an employee via SSN.");
  	System.out.println("3) Retrive an employee via SSN.");
  	System.out.println("4) List all employees.");
  	System.out.println("5) Exit.");
  	Scanner sc = new Scanner(System.in);
  	return sc.nextInt();
  }

  public static void main(String[] args)
  {
    HashMap<String, Employee> employeesByKey = new HashMap<String, Employee>();
    TreeSet<Employee> employeesByName = new TreeSet<Employee>();

	int menuStore = menu();
	while(menuStore != 5)
	{
	  System.out.println();
	  switch (menuStore)
	  {
	    case 1:
	      Scanner input1 = new Scanner(System.in);

 	      System.out.println("Enter a SSN: ");
 	      String str1 = input1.nextLine();
	      System.out.println("Enter Last Name: ");
	      String str2 = input1.nextLine();
	      System.out.println("Enter First Name: ");
	      String str3 = input1.nextLine();

	      Employee newEmp = new Employee(str1, str2, str3);

	      employeesByKey.put(newEmp.getSSN(), newEmp);
	      employeesByName.add(newEmp);

	      System.out.println();
	      break;

	    case 2:
	      Scanner input2 = new Scanner(System.in);

	      System.out.println("Please enter the SSN of the employee you wish to delete: ");
	      String delStr = input2.nextLine();

              if(employeesByKey.containsKey(delStr))
              {
		      employeesByKey.remove(delStr);

		      Iterator<Employee> iterator;
		      iterator = employeesByName.iterator();

		      while (iterator.hasNext())
		      {
				  Employee iterEmp = iterator.next();
				  if (delStr.compareTo(iterEmp.getSSN()) == 0)
				  {
					  iterator.remove();
				  }
		      }

		      System.out.println("Employee removed.");
		      System.out.println("");
		  }

		  else
		  {
			  System.out.println("No such SSN.");
			  System.out.println("");
		  }

	      break;

	    case 3:
	      Scanner input3 = new Scanner(System.in);

	      System.out.println("Please enter the SSN of the employee you wish to retrive: ");
	      String retStr = input3.nextLine();

              if(employeesByKey.containsKey(retStr))
              {
			Employee recievedEmployee = employeesByKey.get(retStr);
			System.out.println(recievedEmployee);
			System.out.println("");

	      }

	      else
	      {
		        System.out.println("No such SSN.");
		        System.out.println("");
	      }


	      break;


        case 4:
          Iterator iterator;
	  iterator = employeesByName.iterator();

          if (employeesByName.isEmpty())
          {
		    System.out.print("Tree Set is empty.");
	  }

	  else
	  {
	            while (iterator.hasNext())
		    {
			  System.out.print(iterator.next() + " ");
		    }
	  }
          System.out.println("");
	  break;

	  case 5:
	    break;
	}
	  menuStore = menu();
     }
   }
}

Hello there jimmy, I was examining your above code about Employee record management. Since I am working on java, I executed your code and got some error in your above code. I want you to resolve the problem which I am going to mention below.
1.Why did you use compareTo() method in Employee Class.
2.While adding employee detail for second time it is giving me an error, where you thrown an exception.

Same error was encountered by me.