Sorting in java - Comparable and Comparator example

Comparable and Comparator are interfaces used during sorting of Java collections. Where Comparable is used to define default / natural ordering, Comparator is mainly used to define customized ordering. Here java.util.Collections class comes into picture with 2 important methods, sort(List list) and sort(List list,Comparator c). The best place to read about them is Javadoc.

/**
 * Student class implements comparable interface 
 * and applies comparison logic in compareTo method.
 * This logic is default logic for ordering students.
 */
class Student implements Comparable<Student>{
 private String firstName;
 private String lastName;
 private Integer rollNo;
 
 public Student(Integer rollNo, String firstName, String lastName) {
  super();
  this.rollNo = rollNo;
  this.firstName = firstName;
  this.lastName = lastName;
 }
 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 int getRollNo() {
  return rollNo;
 }
 public void setRollNo(int rollNo) {
  this.rollNo = rollNo;
 }
 @Override
 public String toString() {
  return rollNo+"\t"+firstName+"\t"+lastName;
 }
 @Override
 public int compareTo(Student s) { //method Comparable interface
  return rollNo.compareTo(s.getRollNo());
 }
}

/**
 * Following program demonstrates use of Comparable and Comparator in java
 */
 
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
 
/**
 * @author dinesh
 *
 */
public class CollectionSortDemo {
 
 /**
  * @param args
  */
 public static void main(String[] args) {
  List<Student> students = new ArrayList<Student>();
  students.add(new Student(4, "Tom", "Peterson"));
  students.add(new Student(1, "Dick", "Loyd"));
  students.add(new Student(5, "Harry", "Ken"));
  students.add(new Student(2, "James", "Powel"));
  students.add(new Student(3, "Dan", "Brown"));
  
  System.out.println("Unsorted List");
  printList(students);
  
  Collections.sort(students);
  System.out.println("Sorted List - Natural Ordering by roll number in ascending way");
  printList(students);
  
  Comparator<Student> fnameComparator = new Comparator<Student>() { // Customized Comparator 
   @Override
   public int compare(Student s1, Student s2) {
    return s1.getFirstName().compareTo(s2.getFirstName());
   }
  };
  Collections.sort(students, fnameComparator);
  System.out.println("Sorted List - Customized Ordering by first name in ascending way");
  printList(students);
 }
 
 private static void printList(List<Student> students) {
  for (Student student : students) {
   System.out.println(student);
  }
 }
}

Output:

Unsorted List
4 Tom Peterson
1 Dick Loyd
5 Harry Ken
2 James Powel
3 Dan Brown
Sorted List - Natural Ordering by roll number in ascending way
1 Dick Loyd
2 James Powel
3 Dan Brown
4 Tom Peterson
5 Harry Ken
Sorted List - Customized Ordering by first name in ascending way
3 Dan Brown
1 Dick Loyd
5 Harry Ken
2 James Powel
4 Tom Peterson