How to implement comparator

Question

I was given the code below as Homework. I am being asked to implement comparator for employee objects. The compare method returns an int. However, none of the methods that I have in my employee class return an int if you compare them. Can anyone give me some guidance on how the compare method is supposed to work? Thank you

import java.util.*;
import java.io.*;
public class EmployeeClient
{
//A Comparator for Employees
// Primary key   : Employee category - Salaried > Weekly > Daily
// Secondary key : Employee gross pay
private static class EmployeeComparator implements Comparator
{
  public int compare(Object one, Object two)
  {   
      Employee uno = (Employee) one;
      Employee dos = (Employee) two;

  }
}

---

public abstract class Employee {

private String idNumber;
private double payRate;

//Accessor: Return the id number of employee
public String getidNumber()
{
    return idNumber;
}

//Accessor: Return the payrate of the employee
public double getpayRate()
{
    return payRate;
}

public String toString()
{
    return getidNumber()+" "+getpayRate();
}

public abstract double grossPay();

}

Answer 1

I would suggest checking out this link.

The goal of a custom comparator is to take two objects and determine how they relate to one another. Consider it an ordering problem. Say I have two bags with red and blue marbles in them. If I want to do a comparison of the bags in terms of blue marbles, it is the same as asking which bag has more/less blue marbles in it.

The return value of the compare function determines which object is valued higher than the other. let ret be the value after comparing the two objects.

• ret > 0 means the left object is valued higher than the right
• ret < 0 means the right object is valued higher than the left
• ret == 0 means the two objects are valued the same

Hope this helps.

Edit:

In the case you are not allowed or unable to modify the Employee class, do the comparisons in your compare function in your first set of code. For example, if you wanted to compare two integers int a and int b, you would do:

a.compareTo(b)

or

Integer.compare(a, b)

• If a > b : either method would return a value greater than 0
• If a < b : either method would return a value less than 0
• If a == b : either method would return 0

Simply apply this idea to your code

Answer 2

Actually, all of the fields in your Employee class return ints when compared.

Also, each Employee method returns an object that can be compared using compareTo() to return an int.

Your Employee class has two fields:

String idNumber;
double payRate;

Both String and Double (the object form of double) have a compareTo() method that returns an int.

So... Your EmployeeComparator.compare() method could simply return:

one.getidNumber().compareTo(two.getidNumber());

---

Here's how you could rewrite your Comparator class:

private static class EmployeeComparator implements Comparator<Employee>
{
  public int compare(Employee empOne, Employee empTwo)
  {   
      return empOne.getidNumber().compareTo(empTwo.getidNumber());
  }
}

The comparator above bases comparison strictlly on employee id number.

But... It should be obvious how to modify it to consider other fields.

Answer 3

Well it always depends on what property you want to sort. For generic entities like these I would suggest its internal id. BUT: The Employee class is actually poorly designed. getIdNumber() should return an int not a String. If you can redesign the class, do it! If not parse the string using Integer.parseInt(), then youll have an int for comparsion.