Contents

1. 1 Introduction :
2. 2 Problem :
3. 3 Solution :
4. 4 Program :
   1. 4.1 Step 1 - Coding
   2. 4.2 Step 2 - Deployment
   3. 4.3 Step 3 - Testing
   4. 4.4 Output :
5. 5 FAQ
   1. 5.1 Q.1 What is Comparator and what it is used for?
   2. 5.2 Q.2 When to use Comparator ?
   3. 5.3 Q.3 How many Comparators you can create for a single class say "Employee" ?

Introduction :

A Comparator object is capable of comparing two different objects. It should implement  java.util.Comparator interface.

Comparator interface has one method  Compare(Object o1, Object o2). The return values of the compare() method of Comparator are similar to the compareTo() method of Comparable.

• if the first element comes before the second element in the ordering, a negative value is returned.
• If the first element comes after, then a positive value is returned. Otherwise, zero is returned.
  
• Similar to Comparable, a zero return value does not signify equality of elements.

Problem :

Solution :

Program :

Step 1 - Coding

(3)CompareEmployeeByIDDesc Class

/*
* Compare two employees by their ID using Comparator interface implementation.
* Used in Collections.sort() to sort Employees' list in descending order.
*/

(4)CompareEmployeeByName Class 

import java.util.Comparator;

* Compare Employee Names and returns
* -1 : If Employee One Name is less than Employee Name Two
* 0 : If both Employee Names are equal
* 1 : If Employee One Name
* is greater than Employee Name Two
*/
public int compare(Employee emp1, Employee emp2) {

return emp1.getName().compareTo(emp2.getName());
}

(5)SortEmployeeListByComparator Class

* A program to compare and sort Employee objects using multiple Comparator implementation.
* A List contains 3 Employee objects and sort it by Employee ID Ascending/ Descending,
* and by Employee Name
*/

public class SortEmployeeListByComparator {

Employee e1 = new Employee(3, "Ram lal");

Employee e2 = new Employee(1, "Shanti Lal");

Employee e3 = new Employee(2, "Chunni Lal");

ArrayList list = new ArrayList();

list.add(e1);
list.add(e2);
list.add(e3);

// Sort Employee list by Employee ID in ascending order

Collections.sort(list, new CompareEmployeeByID());

Iterator<Employee> itr = list.iterator();
Employee emp = null;

System.out.println("****Employee List By ID ( Ascending )****");

System.out.println("ID\tName");

while (itr.hasNext()) {

emp = itr.next();
System.out.println(emp.getId() + "\t" + emp.getName());

}

// Sort Employees by Employee IDin descending order

Collections.sort(list, new CompareEmployeeByIDDesc());

itr = list.iterator();

System.out.println("****Employee List By ID ( Descending )****");

System.out.println("ID\tName");

while (itr.hasNext()) {

emp = itr.next();
System.out.println(emp.getId() + "\t" + emp.getName());

}

// Sort Employees by Name in ascending order

Collections.sort(list, new CompareEmployeeByName());

itr = list.iterator();

System.out.println("****Employee List BY Name ( Ascending )****");

System.out.println("ID\tName");

while (itr.hasNext()) {

emp = itr.next();
System.out.println(emp.getId() + "\t" + emp.getName());

}

}

Step 2 - Deployment

Create a folder 'c:\sunilos'.

Create or copy Employee.java,CompareEmployeeID.java,CompareEmployeeIDDesc.java,CompareEmployeeByName.java,SortEmployeeListByComparator. into 'c:\sunilos' folder.

Open your command prompt and go to 'c:\sunilos'

Compile Employee.java,CompareEmployeeID.java,CompareEmployeeIDDesc.java,CompareEmployeeByName.java,SortEmployeeListByComparator.java with help of

javac *.java java command. Command will create class file in the same folder.

Congratulations!! your Java program is ready to serve.

Step 3 - Testing

Make sure you are on Command Prompt under c:\sunilos directory

Now start your java program from command prompt with help of

java SortEmployeeListByComparator command.

Output :

1 Shanti Lal
2 Chunni Lal
3 Ram lal

3 Ram lal
2 Chunni Lal
1 Shanti Lal

2 Chunni Lal
3 Ram lal
1 Shanti Lal

FAQ

Q.1 What is Comparator and what it is used for?

A : Comparator : A Comparator object is capable of comparing two different objects. instances. This comparator class must implement the java.util.Comparator interface . Comparator interface is used when an extra logic is required to sort the objects. One need to override compare(Object obj1, Object obj2) method.

Q.2 When to use Comparator ?

A : By using Comparator, you could instead make different comparators, one that compares Employee objects by name, and another one that compares them by ID. If you want to sort by name, you use the first Comparator object, and if you want to sort by ID you use the other Comparator object.