Sort Dates in Java

The sort() method that is a part of Comparator mechanism of Collection class that sorts the data in decreasing order.

If we want to achieve the aim in generic while considering the boundary condition where the objects to be sorted are user-defined, we can use Comparator interface. Both ways are discussed below, where the user-defined type is also used to generate the object.

There are several ways in Java for sorting things in an ArrayList by date. It can be accomplished either by using the Comparable<> interface or the Collections.sort() method; we can use any of the following ways.

Comparator interface
sort() method

1. Comparator interface

Java Programs to sort elements of ArrayList by Date using Comparator interface.

SortDate1.java

  import java.util.*;  class DateI  {  String date;   /* Constructor */  DateI(String date)  {  /* This keyword is used to refer current object */  this.date = date;  }  }  class sortCompare implements Comparator   {  // Method of this class  @Override  public int compare(DateI a, DateI b)  {  /* Returns sorted data in ascending order */  return a.date.compareTo(b.date);  }  }  public class SortDate1   {  /* Driver Code */  public static void main(String args[])  {  ArrayList dates = new ArrayList<>();  /* Adding data to the ArrayList */  dates.add(new DateI(“2021-05-14”));  dates.add(new DateI(“2019-06-24”));  dates.add(new DateI(“1998-02-20”));  dates.add(new DateI(“1999-04-26”));  /* Sort the ArrayList with Collections.sort() method */  Collections.sort(dates, new sortCompare());  System.out.println(“Dates sorted in Ascending Order”);  for (DateI d : dates)   {  System.out.println(d.date);  }  }  }  

Output:

Dates sorted in Ascending Order  1998-02-20  1999-04-26  2019-06-24  2021-05-14

Let’s see another example.

SortDate2.java

  import java.util.*;  class DateI   {  String date;  /* Constructor */  DateI(String date)  {          /* This keyword is used to refer current object */  this.date = date;  }  }  class sortCompare implements Comparator   {  @Override  // Method of this class  public int compare(DateI a, DateI b)  {  /* Returns sorted data in Descending order */  return b.date.compareTo(a.date);  }  }  public class SortDate2 {  /* Driver Code */  public static void main(String args[])  {  ArrayList dates = new ArrayList<>();  /* Adding data to the ArrayList */  dates.add(new DateI(“2021-05-14”));  dates.add(new DateI(“2019-06-24”));  dates.add(new DateI(“1998-02-20”));  dates.add(new DateI(“1999-04-26”));          /* Sort the ArrayList with Collections.sort() method */  Collections.sort(dates, new sortCompare());  System.out.println(“Dates sorted in Descending Order”);        for (DateI d : dates)   {  System.out.println(d.date);  }  }  }  

Output:

Dates sorted in Descending Order  2021-05-14  2019-06-24  1999-04-26  1998-02-20

2. sort() method

Java Programs to sort elements of ArrayList by Date using Collections.sort() method

SortDate3.java

  import java.util.*;  public class SortDate3   {  /* Driver Code */  public static void main(String[] args)  {  /* Creating an ArrayList of String to store the Dates */  ArrayList datesList = new ArrayList<>();  /* Adding date to ArrayList using standard add() method */  datesList.add(“2020-03-25”);  datesList.add(“2019-01-27”);  datesList.add(“2020-03-26”);  datesList.add(“2020-02-26”);  // Display message only  System.out.println(  “Dates Object before sorting : “);  /* Iterating in the ArrayList using for each loop */  for (String dates : datesList) {  // Printing the data from the list  System.out.println(dates);  }  /* Sorting the ArrayList using Collections.sort() method */  Collections.sort(datesList);  // Display message only  System.out.println(“Dates Object after sorting : “);  /* Iterating in the ArrayList using for-each loop */  for(String dates : datesList) {  /* Printing the data from the list */  System.out.println(dates);  }  }  }  

Output:

Dates Object before sorting:   2020-03-25  2019-01-27  2020-03-26  2020-02-26  Dates Object after sorting:   2019-01-27  2020-02-26  2020-03-25  2020-03-26

SortDate4.java

  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 SortDate4  {  /* Driver Code */  public static void main(String[] args)   {  //java 8  sortDateTimeObject();  sortDateTimeText();  }     private static void sortDateTimeObject() {  ListdateTimeList=new ArrayList();  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()  {  ListdateStrList=new ArrayList();  // 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() {  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

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