Tag Archives: Comparator

Java: Collections Utility Class Methods and Examples


java.util.Collections class inherits Object class. The Collections utility class is used exclusively with static methods that operate on or return collections.

Points to remember

  • Collections class supports the polymorphic to operate on collections.
  • Collections class throws a NullPointerException if passing objects are null.

See Also:

Collections Declaration

public class Collections extends Object 

Collections Utility Class Methods

Method Descriptions
static boolean addAll() It is used to adds all of the specified element to the specified collection.
static Queue asLifoQueue() It returns a view of a Deque as a Last-in-first-out (LIFO) Queue.
static int binarySearch() It searches the list for the specified object and returns its position in a sorted list.
static Collection checkedCollection() It is used to returns a dynamically typesafe view of the specified collection.
static List checkedList() It is used to returns a dynamically typesafe view of the specified list.
static Map checkedMap() It is used to returns a dynamically typesafe view of the specified map.
static NavigableMap checkedNavigableMap() It is used to returns a dynamically typesafe view of the specified navigable map.
static NavigableSet checkedNavigableSet() It is used to returns a dynamically typesafe view of the specified navigable set.
static Queue checkedQueue() It is used to returns a dynamically typesafe view of the specified queue.
static Set checkedSet() It is used to returns a dynamically typesafe view of the specified set.
static SortedMap checkedSortedMap() It is used to returns a dynamically typesafe view of the specified sorted map.
static SortedSet checkedSortedSet() It is used to returns a dynamically typesafe view of the specified sorted set.
static void copy() It is used to copy all the elements from one list into another list.
static boolean disjoint() It returns true if the two specified collections have no elements in common.
static Enumeration emptyEnumeration() It is used to get an enumeration that has no elements.
static Iterator emptyIterator() It is used to get an Iterator that has no elements.
static List emptyList() It is used to get a List that has no elements.
static ListIterator emptyListIterator() It is used to get a List Iterator that has no elements.
static Map emptyMap() It returns an empty map that is immutable.
static NavigableMap emptyNavigableMap() It returns an empty navigable map that is immutable.
static NavigableSet emptyNavigableSet() It is used to get an empty navigable set which is immutable in nature.
static Set emptySet() It is used to get the set that has no elements.
static SortedMap emptySortedMap() It returns an empty sorted map which is immutable.
static SortedSet emptySortedSet() It is used to get the sorted set that has no elements.
static Enumeration enumeration() It is used to get the enumeration over the specified collection.
static void fill() It is used to replace all of the elements of the specified list with the specified elements.
static int frequency() It is used to get the number of elements in the specified collection equal to the specified object.
static int indexOfSubList() It is used to get the starting position of the first occurrence of the specified target list within the specified source list. It returns -1 if there is no such occurrence in the specified list.
static int lastIndexOfSubList() It is used to get the starting position of the last occurrence of the specified target list within the specified source list. It returns -1 if there is no such occurrence in the specified list.
static ArrayList list() It is used to get an array list containing the elements returned by the specified enumeration in the order in which they are returned by the enumeration.
static > T max() It is used to get the maximum value of the given collection, according to the natural ordering of its elements.
static > T min() It is used to get the minimum value of the given collection, according to the natural ordering of its elements.
static List nCopies() It is used to get an immutable list consisting of n copies of the specified object.
static Set newSetFromMap() It is used to return a set backed by the specified map.
static boolean replaceAll() It is used to replace all occurrences of one specified value in a list with the other specified value.
static void reverse() It is used to reverse the order of the elements in the specified list.
static Comparator reverseOrder() It is used to get the comparator that imposes the reverse of the natural ordering on a collection of objects which internally implement the Comparable interface.
static void rotate() It is used to rotate the elements in the specified list by a given distance.
static void shuffle() It is used to randomly reorders the specified list elements using default randomness.
static Set singleton() It is used to get an immutable set that contains only the specified object.
static List singletonList() It is used to get an immutable list that contains only the specified object.
static Map singletonMap() Use to get an immutable map, mapping only the specified key to the specified value.
static >void sort() It is used to sort the elements present in the specified list of the collection in ascending order.
static void swap() It is used to swap the elements at the specified positions in the specified list.
static Collection synchronizedCollection() It is used to get a synchronized (thread-safe) collection backed by the specified collection.
static List synchronizedList() It is used to get a synchronized (thread-safe) collection backed by the specified list.
static Map synchronizedMap() It is used to get a synchronized (thread-safe) map backed by the specified map.
static NavigableMap synchronizedNavigableMap() It is used to get a synchronized (thread-safe) navigable map backed by the specified navigable map.
static NavigableSet synchronizedNavigableSet() It is used to get a synchronized (thread-safe) navigable set backed by the specified navigable set.
static Set synchronizedSet() It is used to get a synchronized (thread-safe) set backed by the specified set.
static SortedMap synchronizedSortedMap() It is used to get a synchronized (thread-safe) sorted map backed by the specified sorted map.
static SortedSet synchronizedSortedSet() It is used to get a synchronized (thread-safe) sorted set backed by the specified sorted set.
static Collection unmodifiableCollection() It is used to get an unmodifiable view of the specified collection.
static List unmodifiableList() It is used to get an unmodifiable view of the specified list.
static Map unmodifiableMap() It is used to get an unmodifiable view of the specified map.
static NavigableMap unmodifiableNavigableMap() It is used to get an unmodifiable view of the specified navigable map.
static NavigableSet unmodifiableNavigableSet() It is used to get an unmodifiable view of the specified navigable set.
static Set unmodifiableSet() It is used to get an unmodifiable view of the specified set.
static SortedMap unmodifiableSortedMap() It is used to get an unmodifiable view of the specified sorted map.
static SortedSet unmodifiableSortedSet() It is used to get an unmodifiable view of the specified sorted set.

See Also:

Collections Example : Add elements in list

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class CollectionsExample1 {

	public static void main(String[] args) {
		 List list = new ArrayList();
	        list.add("C");
	        list.add("C++");
	        list.add("Python");
	        list.add("Lisp");
	        list.add("Java Script");
	        System.out.println("Initial collection value:"+list);
	        //add some more element in collection
	        Collections.addAll(list, "Servlet","JSP");
	        System.out.println("After adding elements collection value:"+list);
	        String[] strArr = {"C#", "Closure",".Net"};
	        //add some more elements
	        Collections.addAll(list, strArr);
	        System.out.println("After adding array collection value:"+list);
	}

}

Output :


Initial collection value:[C, C++, Python, Lisp, Java Script]
After adding elements collection value:[C, C++, Python, Lisp, Java Script, Servlet, JSP]
After adding array collection value:[C, C++, Python, Lisp, Java Script, Servlet, JSP, C#, Closure, .Net]

Collections Example :max()

import java.util.*;

public class CollectionsExample2 {
	public static void main(String a[]) {
		List list = new ArrayList();
		list.add(15);
		list.add(50);
		list.add(3);
		list.add(90);
		list.add(2);
		list.add(16);
		System.out.println("Max element from the collection: " + Collections.max(list));
	}
}

Output :


Max element from the collection: 90

Output:

Collections Example :min()

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class CollectionsExample3 {
	public static void main(String a[]) {
		List list = new ArrayList();
		list.add(15);
		list.add(50);
		list.add(3);
		list.add(90);
		list.add(2);
		list.add(16);
		System.out.println("Min element from the collection: " + Collections.min(list));
	}
}

Output :


Min element from the collection: 2

Java : Sort Date Time Object and Text Format


In Java classes java.util.Date, java.util.Calendar, java.time.LocalDateTime, java.time.LocalDate and java.time.LocalTime  implements comparable interface for natural ordering. We can sort these objects implements comparable interface as below


Collections.sort(List <Date> list);
Collections.sort(List <Calendar> list);
Collections.sort(List <LocalDateTime> list);
Collections.sort(List <LocalDate> list);
Collections.sort(List <LocalTime> list);

Problem occurred when date in String format and following different format pattern as MM/dd/yyyy ‘@’hh:mm a or some more patterns then you have to implement Comparator to sort this date text patterns as below


Collections.sort(dateStrList, new Comparator() {
DateFormat f = new SimpleDateFormat("MM/dd/yyyy '@'hh:mm a"); 

@Override public int compare(String o1, String o2) {
 try { 
   return f.parse(o1).compareTo(f.parse(o2)); 
} catch (ParseException e) { 
throw new IllegalArgumentException(e); 
} 
} }); 

To learn Comparator and Comparable check below link.

Java : Comparable Vs Comparator

In below example you will see sorting of date  in text format and objects also.

Example

package com.date;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class SortingDateTime {
public static void main(String[] args) {
//java 8
sortDateTimeObject();
sortDateTimeText();
}

private static void sortDateTimeObject() {
List<LocalDateTime>dateTimeList=new ArrayList<LocalDateTime>();
dateTimeList.add(LocalDateTime.of(2012, 05, 12, 5, 16));
dateTimeList.add(LocalDateTime.of(2014, 03, 23, 11, 26));
dateTimeList.add(LocalDateTime.of(2011, 02, 13, 9, 36));
dateTimeList.add(LocalDateTime.of(2013, 11, 12, 5, 16));
dateTimeList.add(LocalDateTime.of(2017, 8, 11, 21, 26));
dateTimeList.add(LocalDateTime.of(2016, 9, 05, 19, 36));

DateTimeFormatter dateTimeFormatter = DateTimeFormatter
				.ofPattern("MM/dd/yyyy '@'hh:mm a");
System.out.println("---> Date & Time Object List Before Sort (MM/dd/yyyy '@'hh:mm a)");
for(LocalDateTime dateTime:dateTimeList)
{
System.out.println(dateTime.format(dateTimeFormatter));
}
// LocalDateTime  internally having Comparable interface no need additional Comparator
Collections.sort(dateTimeList);

System.out.println("---> Date & Time Object List After Sort (MM/dd/yyyy '@'hh:mm a)");
for(LocalDateTime dateTime:dateTimeList)
{
System.out.println(dateTime.format(dateTimeFormatter));
}

}

private static void sortDateTimeText()
{
List<String>dateStrList=new ArrayList<String>();
// MM/dd/yyyy '@'hh:mm a
dateStrList.add("01/21/2014 @03:13 PM");
dateStrList.add("01/21/2011 @04:37 PM");
dateStrList.add("01/21/2012 @10:41 AM");
dateStrList.add("01/21/2013 @10:48 AM");
dateStrList.add("01/22/2015 @06:16 AM");
dateStrList.add("01/22/2013 @06:19 AM");
dateStrList.add("01/21/2018 @05:19 PM");
dateStrList.add("01/21/2013 @05:19 PM");

System.out.println("---> Date & Time List Before Sort (MM/dd/yyyy '@'hh:mm a)");
for(String dateStr:dateStrList)
System.out.println(dateStr);

//Sort String Date
Collections.sort(dateStrList, new Comparator<String>() {
DateFormat f = new SimpleDateFormat("MM/dd/yyyy '@'hh:mm a");
@Override
public int compare(String o1, String o2) {
try {
  return f.parse(o1).compareTo(f.parse(o2));
    } catch (ParseException e) {
      throw new IllegalArgumentException(e);
    }
}
});

System.out.println("---> Date & Time List After Sort (MM/dd/yyyy '@'hh:mm a)");
for(String dateStr:dateStrList)
System.out.println(dateStr);
 }
}

Output


---> Date & Time Object List Before Sort (MM/dd/yyyy '@'hh:mm a)
05/12/2012 @05:16 AM
03/23/2014 @11:26 AM
02/13/2011 @09:36 AM
11/12/2013 @05:16 AM
08/11/2017 @09:26 PM
09/05/2016 @07:36 PM
---> Date & Time Object List After Sort (MM/dd/yyyy '@'hh:mm a)
02/13/2011 @09:36 AM
05/12/2012 @05:16 AM
11/12/2013 @05:16 AM
03/23/2014 @11:26 AM
09/05/2016 @07:36 PM
08/11/2017 @09:26 PM
---> Date & Time List Before Sort (MM/dd/yyyy '@'hh:mm a)
01/21/2014 @03:13 PM
01/21/2011 @04:37 PM
01/21/2012 @10:41 AM
01/21/2013 @10:48 AM
01/22/2015 @06:16 AM
01/22/2013 @06:19 AM
01/21/2018 @05:19 PM
01/21/2013 @05:19 PM
---> Date & Time List After Sort (MM/dd/yyyy '@'hh:mm a)
01/21/2011 @04:37 PM
01/21/2012 @10:41 AM
01/21/2013 @10:48 AM
01/21/2013 @05:19 PM
01/22/2013 @06:19 AM
01/21/2014 @03:13 PM
01/22/2015 @06:16 AM
01/21/2018 @05:19 PM

Java : Sort Date Object and Text Format


In Java classes java.util.Date, java.util.Calendar, java.time.LocalDateTime, java.time.LocalDate and java.time.LocalTime  implements comparable interface for natural ordering. We can sort these objects implements comparable interface as below


Collections.sort(List <Date> list);
Collections.sort(List <Calendar> list);
Collections.sort(List <LocalDateTime> list);
Collections.sort(List <LocalDate> list);
Collections.sort(List <LocalTime> list);

Problem occurred when date in String format and following different format pattern as dd/MM/yyyy or some more patterns then you have to implement Comparator to sort this date text patterns as below


Collections.sort(dateStrList, new Comparator() {
DateFormat f = new SimpleDateFormat("dd/MM/yyyy"); 

@Override public int compare(String o1, String o2) {
 try { 
   return f.parse(o1).compareTo(f.parse(o2)); 
} catch (ParseException e) { 
throw new IllegalArgumentException(e); 
} 
} }); 

To learn Comparator and Comparable check below link.

Java : Comparable Vs Comparator

In below example you will see sorting of date  in text format and objects also.

Example

package com.date;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class SortingDate {

public static void main(String[] args) {
 //java 8
sortDateObject();
sortDateText();
}

private static void sortDateText() {
List<String>dateStrList=new ArrayList<String>();
// Date format dd/MM/YYYY
dateStrList.add("03/04/2016");
dateStrList.add("03/01/2016");
dateStrList.add("02/06/2016");
dateStrList.add("02/09/2016");
dateStrList.add("22/09/2016");
dateStrList.add("16/09/2016");
dateStrList.add("03/04/2016");
dateStrList.add("01/08/2017");
dateStrList.add("02/06/2017");
dateStrList.add("02/09/2014");
dateStrList.add("22/09/2018");
dateStrList.add("16/09/2015");

System.out.println("---> Date List Before Sort (dd/MM/yyyy)");
for(String dateStr:dateStrList)
System.out.println(dateStr);

//Sort String Date
Collections.sort(dateStrList, new Comparator<String>() {
DateFormat f = new SimpleDateFormat("dd/MM/yyyy");
    @Override
    public int compare(String o1, String o2) {
    try {
	 return f.parse(o1).compareTo(f.parse(o2));
	} catch (ParseException e) {
	       throw new IllegalArgumentException(e);
	   }
	   }
	});

    System.out.println("---> Date List After Sort (dd/MM/yyyy)");
    for(String dateStr:dateStrList)
    System.out.println(dateStr);

	}

private static void sortDateObject() {
List<LocalDate>dateList=new ArrayList<LocalDate>();
dateList.add(LocalDate.of(2012, 05, 12));
dateList.add(LocalDate.of(2014, 03, 23));
dateList.add(LocalDate.of(2011, 02, 13));
dateList.add(LocalDate.of(2013, 11, 12));
dateList.add(LocalDate.of(2017, 8, 11));
dateList.add(LocalDate.of(2016, 9, 05));

DateTimeFormatter dateTimeFormatter = DateTimeFormatter
				.ofPattern("MM/dd/yyyy");
System.out.println("---> Date  Object List Before Sort (MM/dd/yyyy)");
for(LocalDate localDate:dateList)
{
  System.out.println(localDate.format(dateTimeFormatter));
}
// LocalDate  internally having Comparable interface no need additional Comparator
Collections.sort(dateList);

System.out.println("---> Date Object List After Sort (MM/dd/yyyy)");
for(LocalDate localDate:dateList)
{
System.out.println(localDate.format(dateTimeFormatter));
}

}

}

Output


---> Date  Object List Before Sort (MM/dd/yyyy)
05/12/2012
03/23/2014
02/13/2011
11/12/2013
08/11/2017
09/05/2016
---> Date Object List After Sort (MM/dd/yyyy)
02/13/2011
05/12/2012
11/12/2013
03/23/2014
09/05/2016
08/11/2017
---> Date List Before Sort (dd/MM/yyyy)
03/04/2016
03/01/2016
02/06/2016
02/09/2016
22/09/2016
16/09/2016
03/04/2016
01/08/2017
02/06/2017
02/09/2014
22/09/2018
16/09/2015
---> Date List After Sort (dd/MM/yyyy)
02/09/2014
16/09/2015
03/01/2016
03/04/2016
03/04/2016
02/06/2016
02/09/2016
16/09/2016
22/09/2016
02/06/2017
01/08/2017
22/09/2018

Java : Comparable Vs Comparator


Comparable and Comparator both are interface used for compare and sorting collections of objects. Where  Comparable used for natural ordering of objects while Comparator used for sorting on user specific value on different type objects. In both the cases if objects are not equate or compatible will through ClassCastException.

In below links, I have explained about in depth detail about Comparable and Comparator implementation and cases where to use. Here mainly focus on some points for technical, implementation and functional points.

Comprable :Sort ArrayList in Ascending or Descending Order or Natural or Chronological Order

How to Sort By Comparable Interface in Ascending and Descending Order : Java

How to sort object by Comparator interface in ascending and descending order : JAVA

Difference between Comparable and Comparator

Comparable Comparator
Comparable is in java lang package as java.lang.Comparable. Comparator is in java util package as java.util.Comparator.
Method used int compareTo(Object t); Method used int compare(Object t1 , Object t2);

Comparable is used to compare itself by using with another object.

Here modify the class whose instance you want to sort. So that only one sort sequence can be created per class.

Comparator is used to compare two datatypes are objects.

Here build a class separate from class whose instance you want to sort. So that multiple sort sequence can be created per class.

 A comparable used for default and natural ordering of objects.  A comparator represents the ordering itself for specific use.

Some java classes having build in Comparable interface like String, Date, Wrapper Classes etc.

Some classes actually provide Comparators for common cases; for instance, Strings are by default case-sensitive when sorted, but there is also a static Comparator called CASE_INSENSITIVE_ORDER.
 For sorting use methods like Collections.Sort(List) or Array.sort().  For Sorting use method Collections.sort(List, Comparator).

Reference :

https://docs.oracle.com/javase/tutorial/collections/interfaces/order.html

How to sort object by Comparator interface in ascending and descending order : JAVA


As in previous posts shown like wrapper of primitive type , String and Date classes having  implements in-built comparable interface and also sort user-defined classes by implements  Comparable interface which provide natural or chronological or alphabetical sorting of elements.

Sort ArrayList in Ascending or Descending Order or Natural or Chronological Order

How to Sort By Comparable Interface in Ascending and Descending Order : Java

Java : Comparable Vs Comparator

What is Comparator Interface?

java.util.Comparator  interface imposes user define sorting on different class fields  based on comparing values from different objects.  It provide compare() method which compares two objects  and returns a negative integer, 0, or a positive integer depending on whether the receiving object is less than, equal to, or greater than the specified object. If the specified object cannot be compared to the receiving object, the method throws a ClassCastException.

How to use Comparator Interface?

Below is syntax of compare method:

public interface Comparator {
    public int compare(T o1, T o2);

}

compare() method compare two objects  fields values and return negative integer, 0 or a positive integer and Collections.sort() method will sort based on this return value.

Lists (and arrays) of objects that implement this comparator interface can be sorted automatically by Collections.sort (and Arrays.sort). Objects that implement this comparator  interface can be used as keys in a sorted map or as elements in a sorted set, without the need to specify a comparable.

Note :  Below example is base on List of Employee objects it cannot be used to order a sorted collection, such as TreeSet because it generates an ordering that is not compatible with equals. It means that Comparator equates objects that the equals method does not.

In particular, any two employees who were joined on the same date will compare as equal. When you’re sorting a List this doesn’t matter; but when you’re using the Comparator to order a sorted collection, it’s fatal. If you use this Comparator to insert multiple employees joined on the same date into a TreeSet only the first one will be added to the set; the second will be seen as a duplicate element and will be ignored.

Example :

Below is simple Employee class which is having fields and getter/setter methods and also constructor to create object.

package sorting
public class Employee {
private int id;
private String firtsName;
private String lastName;
private String designation;
private double salary;
private int age;

//Default Constructor
public Employee()
{
}
//Parametrize Constructor
public Employee(int id, String firtsName, String lastName, String designation, double salary, int age) {
	super();
	this.id = id;
	this.firtsName = firtsName;
	this.lastName = lastName;
	this.designation = designation;
	this.salary = salary;
	this.age = age;
}

public int getId() {
	return id;
}
public void setId(int id) {
	this.id = id;
}
public String getFirtsName() {
	return firtsName;
}
public void setFirtsName(String firtsName) {
	this.firtsName = firtsName;
}
public String getLastName() {
	return lastName;
}
public void setLastName(String lastName) {
	this.lastName = lastName;
}
public String getDesignation() {
	return designation;
}
public void setDesignation(String designation) {
	this.designation = designation;
}
public double getSalary() {
	return salary;
}
public void setSalary(double salary) {
	this.salary = salary;
}
public int getAge() {
	return age;
}
public void setAge(int age) {
	this.age = age;
}

@Override
public String toString() {
	return "Employee [id=" + id + ", firtsName=" + firtsName + ", lastName=" + lastName + ", designation=" + designation
			+ ", salary=" + salary + ", age=" + age + "]";
}
}

Here we have created EmpComparator class and implements compare() method  of java.util.Comparator interface which will compare object fields values based on selected fields on time of sorting. ex: sortingField value.

For example : sortingField value as firstName then compare() method of  java.util.Comparator  interface will compare firstName value of passing two object on compare() method and return a integer value like (negative integer, 0 or positive integer) depend on compare result.

package sorting;
import java.util.Comparator;

public class EmpComparator implements Comparator {
	public static final String ID = "id";
	public static final String FIRST_NAME = "firstName";
	public static final String LAST_NAME = "lastName";
	public static final String DESIGNATION = "designation";
	public static final String SALARY = "salary";
	public static final String AGE = "age";
	private String sortingField;

	@Override
	public int compare(Employee emp1, Employee emp2) {
		int diff = 0;
		switch (sortingField) {
		case ID:
			diff=emp1.getId()-emp2.getId();
			break;
		case FIRST_NAME:
			diff=emp1.getFirtsName().compareTo(emp2.getFirtsName());
			break;
		case LAST_NAME:
			diff=emp1.getLastName().compareTo(emp2.getLastName());
			break;
		case DESIGNATION:
			diff=emp1.getDesignation().compareTo(emp2.getDesignation());
			break;
		case SALARY:
			diff=(int)(emp1.getSalary()-emp2.getSalary());
			break;
		case AGE:
			diff=emp1.getAge()-emp2.getAge();
			break;

		}
		return diff;
	}

	public String getSortingField() {
		return sortingField;
	}

	public void setSortingField(String sortingField) {
		this.sortingField = sortingField;
	}
}

Here in below example creating List of Employee objects for sorting in ascending and descending order.

For sorting by firstName assigning value of sortingField as “firstName” in empComparator object. First we are using Collections.sort(List, Comparator) sorting method to sort list in ascending order by firstName because current value of sortingField is firstName. For descending order we will just reverse order of objects by Collections.resverse() method. Similarly using for lastName and also try with other fields of Employee class.

package sorting;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class SortComparator {

	public static void main(String[] args) {
		Employee[] empArr = {
				new Employee(1, "Saurabh", "Gupta", "Sr Project Lead", 60000, 35),
				new Employee(2, "Gaurav", "Gupta", "Developer", 50000, 32),
				new Employee(3, "Shailesh", "Nagar", "Manager", 100000, 36),
				new Employee(4, "Ankur", "Mehrotra", "Lead", 55000, 30),
				new Employee(5, "Ranjith", "Ranjan", "Tester", 35000, 45),
				new Employee(6, "Ramesh", "Bhardwaj", "Support", 25000, 35)
				};
		// Convert Array to LIst
		List empList = Arrays.asList(empArr);
		// Print Assigned Values Before Sort;
		System.out.println("********Print Employee List Before Sort********");
		printArrayList(empList);

		// Sort List in Ascending order by first Name
		EmpComparator empComparator = new EmpComparator();
		empComparator.setSortingField("firstName");
		Collections.sort(empList, empComparator);
		System.out.println("\n********Print Employee List in Ascending Order********");
		printArrayList(empList);

		// Sort List in Descending order by FirtName
		Collections.reverse(empList);
		System.out.println("\n********Print Employee List in Descending Order********");
		printArrayList(empList);

		// Sort List in Ascending order by lastName

		empComparator.setSortingField("lastName");
		Collections.sort(empList, empComparator);
		System.out.println("\n********Print Employee List in Ascending Order********");
		printArrayList(empList);

		// Sort List in Descending order by last Name
		Collections.reverse(empList);
		System.out.println("\n********Print Employee List in Descending Order********");
		printArrayList(empList);

	}

	private static void printArrayList(List empList) {
		for (Employee emp : empList) {
			System.out.println(emp);<span id="mce_SELREST_start" style="overflow:hidden;line-height:0;"></span>
		}
	}
}

Output :

********Print Employee List Before Sort********
Employee [id=1, firtsName=Saurabh, lastName=Gupta, designation=Sr Project Lead, salary=60000.0, age=35]
Employee [id=2, firtsName=Gaurav, lastName=Gupta, designation=Developer, salary=50000.0, age=32]
Employee [id=3, firtsName=Shailesh, lastName=Nagar, designation=Manager, salary=100000.0, age=36]
Employee [id=4, firtsName=Ankur, lastName=Mehrotra, designation=Lead, salary=55000.0, age=30]
Employee [id=5, firtsName=Ranjith, lastName=Ranjan, designation=Tester, salary=35000.0, age=45]
Employee [id=6, firtsName=Ramesh, lastName=Bhardwaj, designation=Support, salary=25000.0, age=35]

********Print Employee List in Ascending Order********
Employee [id=4, firtsName=Ankur, lastName=Mehrotra, designation=Lead, salary=55000.0, age=30]
Employee [id=2, firtsName=Gaurav, lastName=Gupta, designation=Developer, salary=50000.0, age=32]
Employee [id=6, firtsName=Ramesh, lastName=Bhardwaj, designation=Support, salary=25000.0, age=35]
Employee [id=5, firtsName=Ranjith, lastName=Ranjan, designation=Tester, salary=35000.0, age=45]
Employee [id=1, firtsName=Saurabh, lastName=Gupta, designation=Sr Project Lead, salary=60000.0, age=35]
Employee [id=3, firtsName=Shailesh, lastName=Nagar, designation=Manager, salary=100000.0, age=36]

********Print Employee List in Descending Order********
Employee [id=3, firtsName=Shailesh, lastName=Nagar, designation=Manager, salary=100000.0, age=36]
Employee [id=1, firtsName=Saurabh, lastName=Gupta, designation=Sr Project Lead, salary=60000.0, age=35]
Employee [id=5, firtsName=Ranjith, lastName=Ranjan, designation=Tester, salary=35000.0, age=45]
Employee [id=6, firtsName=Ramesh, lastName=Bhardwaj, designation=Support, salary=25000.0, age=35]
Employee [id=2, firtsName=Gaurav, lastName=Gupta, designation=Developer, salary=50000.0, age=32]
Employee [id=4, firtsName=Ankur, lastName=Mehrotra, designation=Lead, salary=55000.0, age=30]

********Print Employee List in Ascending Order********
Employee [id=6, firtsName=Ramesh, lastName=Bhardwaj, designation=Support, salary=25000.0, age=35]
Employee [id=1, firtsName=Saurabh, lastName=Gupta, designation=Sr Project Lead, salary=60000.0, age=35]
Employee [id=2, firtsName=Gaurav, lastName=Gupta, designation=Developer, salary=50000.0, age=32]
Employee [id=4, firtsName=Ankur, lastName=Mehrotra, designation=Lead, salary=55000.0, age=30]
Employee [id=3, firtsName=Shailesh, lastName=Nagar, designation=Manager, salary=100000.0, age=36]
Employee [id=5, firtsName=Ranjith, lastName=Ranjan, designation=Tester, salary=35000.0, age=45]

********Print Employee List in Descending Order********
Employee [id=5, firtsName=Ranjith, lastName=Ranjan, designation=Tester, salary=35000.0, age=45]
Employee [id=3, firtsName=Shailesh, lastName=Nagar, designation=Manager, salary=100000.0, age=36]
Employee [id=4, firtsName=Ankur, lastName=Mehrotra, designation=Lead, salary=55000.0, age=30]
Employee [id=2, firtsName=Gaurav, lastName=Gupta, designation=Developer, salary=50000.0, age=32]
Employee [id=1, firtsName=Saurabh, lastName=Gupta, designation=Sr Project Lead, salary=60000.0, age=35]
Employee [id=6, firtsName=Ramesh, lastName=Bhardwaj, designation=Support, salary=25000.0, age=35]
<span id="mce_SELREST_start" style="overflow:hidden;line-height:0;"></span>

Reference :

https://docs.oracle.com/javase/tutorial/collections/interfaces/order.html

Sort ArrayList in Ascending or Descending Order or Natural or Chronological Order


Collections framework provide Collections.sort(List) method which sort String elements in natural or chronological order or ascending order.

For descending order use Collections.reverse(List) method to reverse order of complete list elements.

Related Topics:

How to Sort By Comparable Interface in Ascending and Descending Order : Java

How to sort object by Comparator interface in ascending and descending order : JAVA

Java : Comparable Vs Comparator

Points to Remember:

  • Below are classes  implements  Comparable interface which provide a natural ordering for a class  sorted automatically.
  • If sorting a list of the elements which do not  implement Comparable , Collections.sort(list) method will throw ClassCastException. Similarly, Collections.sort(list, comparator ) will throw ClassCastException if you try to sort a list whose elements cannot be compared to another using Comparator.
                                 Classes Implementing Comparable
Class Natural Ordering
Byte Signed numerical
Character Unsigned numerical
Long Signed numerical
Integer Signed numerical
Short Signed numerical
Double Signed numerical
Float Signed numerical
BigInteger Signed numerical
BigDecimal Signed numerical
Boolean Boolean.FALSE < Boolean.TRUE
File System-dependent lexicographic on path name
String Lexicographic
Date Chronological
CollationKey Locale-specific lexicographic

Example :
In below example sorting list of names in ascending order by Collections.sort(list). As shown above String Class by default having Comparable interface which will sort elements in Ascending or Chronological order.

For Descending order used Collections.reverse(List) reverse method which will reverse list of sorted list.

package sorting;

import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class SortData {

	public static void main(String[] args) {
		String [] empArr={"Saurabh","Gaurav","Shailesh","Ankur","Ranjith","Ramesh"};
		//Convert Array to LIst
		List empList=Arrays.asList(empArr);
		//Print Assigned Values Before Sort;
		System.out.println("********Print Employee List Before Sort********");
		printArrayList(empList);

		//Sort List in Ascending order by collections api
		Collections.sort(empList);
		System.out.println("\n********Print Employee List in Ascending Order********");
		printArrayList(empList);

		//Sort List in Descending order by collections api
		Collections.reverse(empList);
		System.out.println("\n********Print Employee List in Descending Order********");
		printArrayList(empList);

	}

	private static void printArrayList(List empList)
	{
		for(String emp:empList)
		{
			System.out.println(emp);
		}
	}

}

Output:

********Print Employee List Before Sort********
Saurabh
Gaurav
Shailesh
Ankur
Ranjith
Ramesh

********Print Employee List in Ascending Order********
Ankur
Gaurav
Ramesh
Ranjith
Saurabh
Shailesh

********Print Employee List in Descending Order********
Shailesh
Saurabh
Ranjith
Ramesh
Gaurav
Ankur

Reference :

https://docs.oracle.com/javase/tutorial/collections/interfaces/order.html