Comparable and Comparator

[Nik Arora]

Hi All,
Can anybody explain me in detail regarding how to work with the comparable and comparator in collections?



Thanks All

[Chandra Bhatt]

Hi Nik,

1- Comparable interface is implemented my classes like String, Integer, Double... Date etc. Comparable enables the objects of the class to be stored in the TreeSet or TreeMap like collection. There should be any way the collection could decide how to place the objects of that class in sorted order that we tell in the compare(...) method of the Comparable interface.

2- Comparator is useful when you need more that one sorting sequence; because you can only create one sorting sequence using Comparable; use can define various classes that implement Comparator interface and define the method compareTo() that takes two arguments.

You can be little specific to your question.

Thank God it is raining here!

Regards,
[ May 01, 2007: Message edited by: Chandra Bhatt ]

[Nik Arora]

Hi Chandra,
My doubt is how can we create multiple sorting sequence using comparator and only one sorting sequence using comparable. Can you explain me with one example.

[Louis Moloney]

when an class implements comparable you give that class a 'natural ordering'. And it wouldn't make sense for a class to have more than one natural ordering.

eg
list.add(new Integer(2));
list.add(new Integer(3));
list.add(new Integer(1));
Collections.sort(list);

the list is now in this order 1,2,3 this is the natural order of Integer.

to get other orderings eg reverse order you can define a comparator and use that.

[Chandra Bhatt]

Hi Nik,

See the code below:

package mycollection;
 
import java.util.*;
 
class Employee implements Comparable<Employee>{

Employee(int emp_id, String firstname,String surname) {
this.emp_id=emp_id;
this.firstname=firstname;
this.surname=surname;
}

public int compareTo(Employee e1) {
return emp_id-e1.emp_id;
}

public String toString() {
return "\nEmp id : " + emp_id + "\nfirstname : " + firstname +
"\nsurname : " + surname;
}
private int emp_id;
String firstname;
String surname;
}
 
public class DemoEmployee {
 
public static void main(String... args) {
Employee[] employee = new Employee[5];

employee[0] = new Employee(232,"Wikky","Jordon");
employee[1] = new Employee(43,"Aikoo","Zombie");
employee[2] = new Employee(2,"Phakkad","Singh");
employee[3] = new Employee(12,"Jhakki","Antonio");
employee[4] = new Employee(93,"Peter","England");

System.out.println("Before sorting :");
printArray(employee);

Arrays.sort(employee);
System.out.println("After natural order sorting :");
printArray(employee);

Arrays.sort(employee,new SortOnFirstName());
System.out.println("Sorting on firstname :");
printArray(employee);

Arrays.sort(employee,new SortOnSurName());
System.out.println("Sorting on second name :");
printArray(employee);

Arrays.sort(employee,new ReverseSortOnFirstName());
System.out.println("Reverse sorting on firstname :");
printArray(employee);

}

//Helper method to print the array
static void printArray(Employee[] employee) {
for(Employee e1 : employee) {
System.out.println(e1);
}
}

static class SortOnFirstName implements Comparator<Employee> {
public int compare(Employee e1,Employee e2) {
return e1.firstname.compareTo(e2.firstname);
}
}

static class SortOnSurName implements Comparator<Employee> {
public int compare(Employee e1,Employee e2) {
return e1.surname.compareTo(e2.surname);
}
}

static class ReverseSortOnFirstName implements Comparator<Employee> {
public int compare(Employee e1,Employee e2) {
return e2.firstname.compareTo(e1.firstname);
}
}
}

[Nik Arora]

Thanks chandra and louis