Java 8 Comparator Example using Lambda Expressions

                             Lambda Expressions with  Comparator Example

1. Main purpose of Comparator is to sort  a Collection of Objects which can be compared with each other.
2. Comparison can be done using Comparable interface as well, but it restrict you compare these objects in a single particular way only.
3. If you want sort this collection, based on multiple fields , then you  have to use Comparator only.
4. I am sharing some knowledge about lambda expressions

    Demo with Employee with four attributes, will see various use cases:

    class Employee {

private Integer id;

public Integer getId() {

return id;

}

public void setId(Integer id) {

this.id = id;

}

public String getFirstName() {

return firstName;

}

public void setFirstName(String firstName) {

this.firstName = firstName;

}

public String getLastName() {

return lastName;

}

public void setLastName(String lastName) {

this.lastName = lastName;

}

public Integer getAge() {

return age;

}

public void setAge(Integer age) {

this.age = age;

}

private String firstName;

private String lastName;

private Integer age;

public Employee(Integer id, String firstName, String lastName, Integer age)

 {

this.id = id;

this.firstName = firstName;

this.lastName = lastName;

this.age = age;

 }

@Override

public String toString() 

{

return "\n[" + this.id + "," + this.firstName + "," + this.lastName + "," + this.age + "]";

}

@Override

public boolean equals(Object obj) {

if (obj == this) {

return true;

}

if (!(obj instanceof Employee)) {

return false;

}

Employee empObj = (Employee) obj;

return this.age == empObj.age && this.firstName.equalsIgnoreCase(empObj.firstName)

&& this.lastName.equalsIgnoreCase(empObj.lastName);

}

@Override

public int hashCode() {

int hash = 1;

hash = hash * 17 + this.firstName.hashCode();

hash = hash * 17 + this.lastName.hashCode();

hash = hash * 31 + this.age;

return hash;

}

}

class Employee {

private Integer id;

public Integer getId() {

return id;

}

public void setId(Integer id) {

this.id = id;

}

public String getFirstName() {

return firstName;

}

public void setFirstName(String firstName) {

this.firstName = firstName;

}

public String getLastName() {

return lastName;

}

public void setLastName(String lastName) {

this.lastName = lastName;

}

public Integer getAge() {

return age;

}

public void setAge(Integer age) {

this.age = age;

}

private String firstName;

private String lastName;

private Integer age;

public Employee(Integer id, String firstName, String lastName, Integer age) {

this.id = id;

this.firstName = firstName;

this.lastName = lastName;

this.age = age;

}

@Override

public String toString() {

return "\n[" + this.id + "," + this.firstName + "," + this.lastName + "," + this.age + "]";

}

@Override

public boolean equals(Object obj) {

if (obj == this) {

return true;

}

if (!(obj instanceof Employee)) {

return false;

}

Employee empObj = (Employee) obj;

return this.age == empObj.age && this.firstName.equalsIgnoreCase(empObj.firstName)

&& this.lastName.equalsIgnoreCase(empObj.lastName);

}

@Override

public int hashCode() {

int hash = 1;

hash = hash * 17 + this.firstName.hashCode();

hash = hash * 17 + this.lastName.hashCode();

hash = hash * 31 + this.age;

return hash;

}

}

Main Class:

package com.coding.core.sort;

import java.util.ArrayList;

import java.util.Comparator;

import java.util.List;

public class ComparatorWithLamda {

public static void main(String[] args) {

// TODO Auto-generated method stub

// 1) Sort all employees by first name

List<Employee> employees = getEmployees();

// Sort all employees by first name

employees.sort(Comparator.comparing(e -> e.getFirstName()));

// OR you can use below

employees.sort(Comparator.comparing(Employee::getFirstName));

System.out.println("*** sorted emps by First Name*********");

// Let's print the sorted list

System.out.println(employees);

System.out.println();

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

System.out.println("Sorted All emp's by Fname with reverse Order");

// Sort all employees by first name; And then reversed

Comparator<Employee> comparator = Comparator.comparing(e -> e.getFirstName());

employees.sort(comparator.reversed());

// Let's print the sorted list

System.out.println(employees);

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

}

private static List<Employee> getEmployees() {

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

employees.add(new Employee(1, "Lokesh", "Gupta", 32));

employees.add(new Employee(2, "Rajesh", "Guru", 28));

employees.add(new Employee(3, "Avinash", "Rijesh", 52));

employees.add(new Employee(4, "Nirlam", "Tracy", 72));

employees.add(new Employee(5, "David", "Kameron", 19));

employees.add(new Employee(6, "Yashod", "kumar", 25));

employees.add(new Employee(7, "Karan", "Johar", 59));

return employees;

}
//Case3 :

/*4) Sort By first name and then by last name (sort on multiple fields)

Here we are sorting list of employees first by their first name, and then sort again the list of last name. Just as we apply sorting on SQL statements. It’s actually a very good feature.

Now you don’t need to always use sorting on multiple fields in SQL select statements, you can sort them in java as well.*/

// Sorting on multiple fields; Group by.

Comparator<Employee> groupByComparator = Comparator.comparing(Employee::getFirstName)

.thenComparing(Employee::getLastName);

employees.sort(groupByComparator);

System.out.println(employees);

}
Output:
***********Sorted All Employees by First Name*********
[
[3,Avinash,Rijesh,52], 
[5,David,Kameron,19], 
[7,Karan,Johar,59], 
[1,Lokesh,Gupta,32], 
[4,Nirlam,Tracy,72], 
[2,Rajesh,Guru,28], 
[6,Yashod,kumar,25]]

****************************************************
Sorted All Employees by First Name with reverse Order
[
[6,Yashod,kumar,25], 
[2,Rajesh,Guru,28], 
[4,Nirlam,Tracy,72], 
[1,Lokesh,Gupta,32], 
[7,Karan,Johar,59], 
[5,David,Kameron,19], 
[3,Avinash,Rijesh,52]]

*****************************************************