How to implement a java.UtilComparator

Java.Util.Comparator is an interface used for sorting collections. The classes implementing Comparators must implement the compare() method. Here is a simple example of how a comparator can be used. In this example we have a simple pojo named Employee.java, 3 simple Comparator classes and a main class to demonstrate the functionality.

Employee.java:

package com.soft.model;

public class Employee {

private String name;

private String department;

private int age;

public String getName() {

            return name;

}

public void setName(String name) {

            this.name = name;

}

public String getDepartment() {

            return department;

}

public void setDepartment(String department) {

            this.department = department;

}

public int getAge() {

            return age;

}

public void setAge(int age) {

            this.age = age;

}

}

EmployeeNameComparator.java:

package com.soft.model;

import java.util.Comparator;

public class EmployeeNameComparator implements Comparator

            public int compare(Object obj1, Object obj2) {

                        Employee e1=(Employee)obj1;

                        Employee e2=(Employee)obj2;

                        return e1.getName().compareTo(e2.getName());

            }

}

EmployeeDepartmentComparator.java

package com.soft.model;

import java.util.Comparator;

public class EmployeeDepartmentComparator implements Comparator

            public int compare(Object obj1, Object obj2) {

                        Employee e1=(Employee)obj1;

                        Employee e2=(Employee)obj2;

                        return e1.getDepartment().compareTo(e2.getDepartment());

            }

}

EmployeeAgeComparator.java:

package com.soft.model;

import java.util.Comparator;

public class EmployeeAgeComparator implements Comparator

            public int compare(Object obj1, Object obj2) {

                        Employee e1=(Employee)obj1;

                        Employee e2=(Employee)obj2;

                        if(e1.getAge()>e2.getAge()) {

                                    return 1;

                        }else if(e1.getAge()

                                    return -1;

                        } else{

                                    return 0;

                        }

            }

}

ComparatorExample.java:

package com.soft.examples;

import java.util.ArrayList;

import java.util.Collections;

import java.util.List;

import com.soft.model.Employee;

import com.soft.model.EmployeeAgeComparator;

import com.soft.model.EmployeeDepartmentComparator;

import com.soft.model.EmployeeMultipleFieldsComparator;

import com.soft.model.EmployeeNameComparator;

public class ComparatorExample {

public static void main(String[] args) {

List<Employee> list= new ArrayList<Employee>();

Employee e=new Employee();

e.setName("Danial Fritz");

e.setDepartment("Field Based Development");

e.setAge(36);

list.add(e);

e=new Employee();

e.setName("Denis Khoo");

e.setDepartment("Project Management");

e.setAge(30);

list.add(e);

e=new Employee();

e.setName("Joe Kum");

e.setDepartment("Adminstration");

e.setAge(40);

list.add(e);

e=new Employee();

e.setName("Anil Kumar");

e.setDepartment("Information Technology");

e.setAge(26);

list.add(e);

           

                       

System.out.println("************ Before Sorting ***********");

for (int i=0;i< list.size();i++){

System.out.println(list.get(i).getName()+"-----"+list.get(i).getDepartment()+"-----"+list.get(i).getAge());

}

System.out.println("************ Name Sorting ***********");

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

for (int i=0;i< list.size();i++){

System.out.println(list.get(i).getName()+"-----"+list.get(i).getDepartment()+"-----"+list.get(i).getAge());

}

System.out.println("************ Department Sorting ***********");

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

for (int i=0;i< list.size();i++){

System.out.println(list.get(i).getName()+"-----"+list.get(i).getDepartment()+"-----"+list.get(i).getAge());

}

System.out.println("************ Age Sorting ***********");

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

for (int i=0;i< list.size();i++){

System.out.println(list.get(i).getName()+"-----"+list.get(i).getDepartment()+"-----"+list.get(i).getAge());

}

}

}

This sorting can also be done through java.lang.Comparable, the implementation of which is given at
How to implement a java.lang.Comparable

The differences between java.util.Comparator and java.lang.Comparable are given at 

Differences between Comparator and Comparable