java.lang.Comparator Vs java.lang.Comparable

What are Java Comparators and Comparables?
As both names suggest (and you may have guessed), these are used for comparing objects in Java. Using these concepts; Java objects can be
sorted according to a predefined order.

Two of these concepts can be explained as follows.
Comparable
A comparable object is capable of comparing itself with another object. The class itself must implements the java.lang.Comparable interface in order to be able to compare its instances.

Comparator
A comparator object is capable of comparing two different objects. The class is not comparing its instances, but some other class’s instances. This comparator class must implement the java.lang.Comparator interface.


How to use these?
There are two interfaces in Java to support these concepts, and each of these has one method to be implemented by user.
Those are;

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


java.lang.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 o1


java.util.Collections.sort(List) and java.util.Arrays.sort(Object[]) methods can be used to sort using natural ordering of objects.
java.util.Collections.sort(List, Comparator) and java.util.Arrays.sort(Object[], Comparator) methods can be used if a Comparator is available for comparison.

The above explained Employee example is a good candidate for explaining these two concepts. First we’ll write a simple Java bean to represent the Employee.

 
package com.champ.model;
public class Employee {
   private int empId;
   private String name;
   private int age;

   public Employee(int empId, String name, int age) {
       // set values on attributes
   }
   // getters & setters
}

Next we’ll create a list of Employees for using in different sorting requirements. Employees are added to a List without any specific order in the following class.

import java.util.*;

public class Util {
  
   public static List<employee> getEmployees() {
      
       List<employee> col = new ArrayList<employee>();
      
       col.add(new Employee(5, "Diptirmaya", 28));
       col.add(new Employee(1, "Puneet", 19));
       col.add(new Employee(6, "Yogesh", 34));
       col.add(new Employee(3, "Rohit", 10));
       col.add(new Employee(7, "Chinmayee", 8));
       col.add(new Employee(4, "Rahul",16 ));
       col.add(new Employee(8, "Vishal", 40));
       col.add(new Employee(2, "Vikas", 30));
      
       return col;
   }
}

Sorting in natural ordering
Employee’s natural ordering would be done according to the employee id. For that, above Employee class must be altered to add the comparing ability as follows.

    public class Employee implements Comparable<employee> {
   private int empId;
   private String name;
   private int age;
  
   /**
    * Compare a given Employee with this object.
    * If employee id of this object is
    * greater than the received object,
    * then this object is greater than the other.
    */
   public int compareTo(Employee o) {
       return this.empId - o.empId ;
   }
   ….
}

  

The new compareTo() method does the trick of implementing the natural ordering of the instances. So if a collection of Employee objects is sorted using Collections.sort(List) method; sorting happens according to the ordering done inside this method.

We’ll write a class to test this natural ordering mechanism. Following class use the Collections.sort(List) method to sort the given list in natural order.

import java.util.*;

public class TestEmployeeSort {
  
   public static void main(String[] args) {    
       List coll = Util.getEmployees();
       Collections.sort(coll); // sort method
       printList(coll);
   }
  
   private static void printList(List<employee> list) {
       System.out.println("EmpId\tName\tAge");
       for (Employee e: list) {
           System.out.println(e.getEmpId() + "\t" + e.getName() + "\t" + e.getAge());
       }
   }
}
Find Out Answer..

Sorting by other fields
If we need to sort using other fields of the employee, we’ll have to change the Employee class’s compareTo() method to use those fields. But then we’ll loose this empId based sorting mechanism. This is not a good alternative if we need to sort using different fields at different occasions. But no need to worry; Comparator is there to save us.

By writing a class that implements the java.lang.Comparator interface, you can sort Employees using any field as you wish even without touching the Employee class itself; Employee class does not need to implement java.lang.Comparable or java.lang.Comparator interface.
Sorting by name field
Following EmpSortByName class is used to sort Employee instances according to the name field. In this class, inside the compare() method sorting mechanism is implemented. In compare() method we get two Employee instances and we have to return which object is greater.

public class EmpSortByName implements Comparator<employee>{

   public int compare(Employee o1, Employee o2) {
       return o1.getName().compareTo(o2.getName());
   }
}

Watch out: Here, String class’s compareTo() method is used in comparing the name fields (which are Strings).

Now to test this sorting mechanism, you must use the Collections.sort(List, Comparator) method instead of Collections.sort(List) method. Now change the TestEmployeeSort class as follows. See how the EmpSortByName comparator is used inside sort method.

import java.util.*;

public class TestEmployeeSort {
  
   public static void main(String[] args) {
      
       List coll = Util.getEmployees();
       //Collections.sort(coll);
       //use Comparator implementation
       Collections.sort(coll, new EmpSortByName());
       printList(coll);
   }
  
   private static void printList(List<employee> list) {
       System.out.println("EmpId\tName\tAge");
       for (Employee e: list) {
           System.out.println(e.getEmpId() + "\t" + e.getName() + "\t" + e.getAge());
       }
   }
}
Find out the answer.