How to implement ReverseOrderComparator

Java.Util.Comparator is an interface used for sorting collections. In previous example, we used a MultyField Comparator to sort a list in ascending order by multiple fields. In this example, we will implement a reverse order multy field Comparator in order to achieve a sorting in descending order or in other words in reverse order of whatever is implemented in the Comparator. This can be achieved just by a single line of code as

Comparator c= Collections.reverseOrder(new mployeeMultyFieldComparator());

 This example is almost a replica of the previous example. It consists of the following 3 classes.

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;

}

}

EmployeeMultyFieldComparator.java:

package com.soft.model;

import java.util.Comparator;

public class EmployeeMultyFieldComparator implements Comparator

            @Override

            public int compare(Object obj1, Object obj2) {

                        Employee e1=(Employee)obj1;

                        Employee e2=(Employee)obj2;

                        if(e1.getName().compareTo(e2.getName())==0){

                                    if(e1.getDepartment().compareTo(e2.getDepartment())==0){

                                                return e1.getAge()-e2.getAge();

                                    }else{

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

                                    }

                        }else{

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

                        }

            }

}

MultyFieldComparatorExample.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.EmployeeMultyFieldComparator;

public class MultyFieldComparatorExample {

            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);

                        e=new Employee();

                        e.setName("Anil Kumar");

                        e.setDepartment("Information Technology");

                        e.setAge(26);

                        list.add(e);

                        e=new Employee();

                        e.setName("Anil Kumar");

                        e.setDepartment("Information Technology");

                        e.setAge(25);

                        list.add(e);

                        e=new Employee();

                        e.setName("Anil Kumar");

                        e.setDepartment("Project Management");

                        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, Department, Age Sorting through Comparator ***********");

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

                        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, Department, Age Sorting through Comparator In Descending Order**");

Collections.sort(list,Collections.reverseOrder(new EmployeeMultyFieldComparator()));

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

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

}

                       

            }

}

Output:

************ Before Sorting ***********

Danial Fritz   ----- Field Based Development   ----- 36

Denis Khoo     ----- Project Management        ----- 30

Joe Kum        ----- Adminstration             ----- 40

Anil Kumar     ----- Information Technology    ----- 26

Anil Kumar     ----- Information Technology    ----- 26

Anil Kumar     ----- Information Technology    ----- 25

Anil Kumar-----Project Management-----26

****** Name, Department, Age Sorting through Comparator *******

Anil Kumar     ----- Information Technology     ----- 25

Anil Kumar     ----- Information Technology     ----- 26

Anil Kumar     ----- Information Technology     ----- 26

Anil Kumar     ----- Project Management         ----- 26

Danial Fritz   ----- Field Based Development    ----- 36

Denis Khoo     ----- Project Management         ----- 30

Joe Kum        ----- Adminstration              ----- 40

** Name, Department, Age Sorting through Comparator in Descending Order**

Joe Kum        ----- Adminstration                     ----- 40

Denis Khoo   ----- Project Management           ----- 30

Danial Fritz   ----- Field Based Development   ----- 36

Anil Kumar   ----- Project Management           ----- 26

Anil Kumar   ----- Information Technology     ----- 26

Anil Kumar   ----- Information Technology     ----- 26

Anil Kumar   ----- Information Technology     ----- 25

An example of a simple MultyFieldComparator can be found at

How to implement MultyFieldComparator

You may also find the following articles helpful.

How to implement ReverseOrderComparable

How to implement MultyFieldComparable

How to implement a java.lang.Comparable

How to implement a java.UtilComparator

Differences between Comparator and Comparable  

How to implement a java.lang.Comparable

Java.lang.Comparable is an interface used for sorting collections. The classes implementing Comparables must implement the compareTo() method. Here is a simple example of how a comparable can be used. The example sorts a list of employees on the basis of their age. In this example we have a simple pojo named Employee.java implementing a java.lang.Comparable interface by overriding the compareTo method and a main class to demonstrate the functionality.

Employee.java:

package com.soft.model;

public class Employee implements Comparable

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;

}

@Override

public int compareTo(Object o) {

            if(this.age>((Employee)o).getAge()){

                        return 1;

            } else if(this.age<((Employee)o).getAge()){

                        return -1;

            } else{

                        return 0;

            }

}

}

ComparableExample:

package com.soft.examples;

import java.util.ArrayList;

import java.util.Collections;

import java.util.List;

import com.soft.model.Employee;

public class ComparableExample {

            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("************ Age Sorting through Comparable ***********");

                        Collections.sort(list);

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

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

                        }

                       

            }

}

This functionality can also be achieved with a java.util.Comparator, the implementation of which is given at

 How to implement a comparator

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

 Differences between Comparator and Comparable

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