Friday, August 16, 2013

How to Sort List in reverse Order in Java Collection Framework with Example

Java Collection Framework provides a convenient reverse comparator, to sort List of objects in reverse order.  You can obtain reverse Comparator, by calling Collections.reverseOrder(), which by default sort List of Integer in reverse numeric order and List of String in reverse alphabetic order. It actually reverse natural ordering of objects imposed by Comparable interface. Apart from this method, Collections class also provides an overloaded method Collections.reverseOrder(Comparator cmp), which takes a Comparator, and sort List on reverse order of that Comparator. So next time if you need to sort your Collection in reverse order, you don’t need to write any extra comparator by yourself, you can directly leverage reverse comparator provided by java.util.Collections class. It is as simple as calling Collections.sort() method providing comparator wrapped into Collections.reverseOrder() method. By using these two methods, you can sort any List implementation e.g. ArrayList, LinkedList or Vector in Java.


Java Program to sort ArrayList in reverse Order

Java program to sort List in reverse orderHere is complete code for sorting List in reverse order in Java. In this program we have two list, List<Integer> and List<String> and we are sorting them first in natural order and than in there reverse order by using Collections.reverseOrder() method.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
  * Java Program to sort List of object in reverse order. We will see examples of
  * sorting Array List in reverse order of both default ordering as well as any
  * custom ordering imposed by Compartor.
  *
  * @author Javin Paul
  */
public class CollectionSorting {
 
    private static final Logger logger = LoggerFactory.getLogger(CollectionSorting.class);
  
    public static void main(String args[]) {
      
        // Creating and initializing Collection to sort in reverse order
        // Integer will be sorted in reverse numeric order
        List<Integer> collection = new ArrayList<Integer>();
        collection.add(101);
        collection.add(201);
        collection.add(105);
        collection.add(302);
      
        logger.debug("Current order in Collection : " + collection);
      
        // Sorting Collection in reverse order
        Collections.sort(collection, Collections.reverseOrder());
      
        logger.debug("Sorted on Reverse order in Collection : {}", collection);
      
        // Sorting List of String in reverse Order
        List<String> alphabets = Arrays.asList("A", "B", "C", "D");
      
        logger.debug("Current order in List of String : {}", alphabets);
      
        // Sorting List in reverse Order
        Collections.sort(alphabets, Collections.reverseOrder());
      
        logger.debug("List of String sorted in reverse order {}", alphabets);
    }
 
}

Output
 [main] DEBUG CollectionSorting  - Current order in Collection : [101, 201, 105, 302]
 [main] DEBUG CollectionSorting  - Sorted on Reverse order in Collection : [302, 201, 105, 101]

[main] DEBUG CollectionSorting  - Current order in List of String : [A, B, C, D]
[main] DEBUG CollectionSorting  - List of String sorted in reverse order [D, C, B, A]

That's all on How to sort List of Strings or Integers in to revrese order. You can also use same technique to sort any List into reverse order of there natural ordering or custom ordering imposed by any Comparator in Java.

2 comments :

Anonymous said...

Curly brackets are mistyped in the listing (as per the output).

Anonymous said...

For the sake of your life, stop using static fields, unless REALLY required

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